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#1
Posted to rec.audio.tubes
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HP333A examination and impressions
I have examined my HP33A for about 5 hours armed with the HP service
manuals just to understand some of the basics involved with this instrument, and to try to find out why the 333A distortion measurements were plain wrong compared to my own design of THD measurer built about 12 years ago. But even in voltmeter mode, the use of my 333A was became pointless because after it had been turned on for 1/2 an hour the voltage measurement strated to drift about 15% high, and then If I tried to use it to measure THD while monitoring on my CRO, and inevitably overloading the circuits when changing range settings the voltmeter would become unstable and give readings which were up to 3 times what they should have been. The instrument is fairly immune failures if Vin is only 10Vrms which I used, 1kHz, for my circuit testing and probing. Nothing became overheated. But really bad drift occurs. There are many electrolytic coupling caps. The meter amp appears to be faulty. Now the meter amp is a damn strange concoction of discrete transistors with both positive and negative loops of FB and which have different effects at different F, so that it can operate from 5Hz to several MHz, with a very flat response **providing** all the vari-caps and gain pots and the biasing pot are all set just right. I found the bias pot was somewhat out, but when adjusted right according to the schematic Vdc voltages the bad meter drift still occurred. It would have been shortly after the 333A was developed by HP that opamps were invented and VOILA, all the efforts HP went to became immediately obsolete. I began to think the HP meter circuit was like a horse and buggy compared to a good motot car, and the circuit working seems to rely on all resistors never changing in value, and on many other things like cap leakage, and perhaps bjts and other bits becoming dodgy and almost unidentifiable in the years since 1967. Anyway, methinks blind freddy could have told HP that they had little chance of making the meter amp reliable and precise over a long time. The meter amp needs only approximately 1mVrms to develop 100mV of output signal which will swing the meter full scale. Whatever is drifting to cause the meter reading to wander about the workshop when it wants to remains difficult to fathom, but I can see drift could be very easy with all that open loop gain and with an absurd closed loop gain of about 100. If the meter amp gain was 10, and there was less attenuation by switching and resistors before amplifying a signal back up again for metering, methings reliability would be better. Before the drift began, I set the unit going in Voltmeter function with a known 10Vrms input and measured all Vac and Vdc all over the circuit boards. There was less than 1mV of input to the meter amp, and the NFB at the emitter of the first bjt also measured the same, so differential input between base and emitter was a lot less than 1mV,and not easy to measure. So open loop gain at 1khz was very high, like an opamp, but all devices are set up in SE mode, something one does not see in many opamps because its so difficult to get it to work well compared to using differential pairs and a PP output stage. OPamps are good to use in DIY test gear and with sockets so that if an opamp dies, just unplug&chuckit, and in goes another for $3. I've rarely ever replaced any opamps in the many bits of test gear I made. But the ordinary sockets for opamps are more likely to stay TIGHT than the kind of wonky HP lead to pin connectors. I found the meter readings were difficult to interpret, and I have tried a few times to read the manual but it is bloody obtuse, and just what is going on and exactly what is being measured remains vague to me There were plenty of early generation GEEKS who managed to stay employed at companies like HP but who could not write manuals to be easily be understood. Now my own THD meter is FAR SIMPLER, with 1/20 of the parts, and far less likely to go wrong, yet it measured THD easily to 0.002% from a sample audio signal at 1kHz as low as 0.5Vrms. I have a low THD signal which is fed to an amp. The amp output can be applied to a dummy load aross which is a 1k switched attenuator to reduce the signal to below 10Vrms if it is a high level signal without attenuation, which could be the case of a 300W amp feeding 8 ohms. There is a following LC bridged T notch filter which blocks the 1 kHz signal to a depth of -90dB, approx, and all other F can go past where they can be applied to a meter and CRO. So %THD is calculated as 100 x Vdistortion / Vsample, and there are few attenuators in the signal path, except a hum filter to remove content below 300Hz to be able to see and measure harmonics above 1 khz better. After the bridged T LC filter I have a HPF with an opamp 1.5kHz to 10kHz BW. Gain is 10 or +20dB and this lowers 1khz down another 20dB while amplifying the distortion to examine low distortion signals on the CRO's lowest and most sensitive range. For amps where I know the result will be nearly all above 0.05%, just the passive bridged T is enough and the following opamp and HPF is not used. When I use my own THD meter I know exactly what I am measuring. When using the 333A for THD, I set the input to Distortion and adjust the meter range to a low level to display the THD on the CRO from the outpt terminals of the 333A and to be able to measure the 333A output voltage. But the meter reading has no corelation to what I measure at the output with an external meter. Its only when I compare the use of my own simple reliable gear to this vastly over complex and old fashioned HP junk that I began to feel some despair, and concluded that I could be wasting days farnarkling around with something that's better abandoned than restored, or seriously modified with replacement opamps. Now I know why I was given the 333A and the 331 and mabye the previous owner couldn't make out what he was measuring, and as sure as God made lil' apples, he must have had fun swearing and cursing the crazy behaviour. I have not fully decided to junk the 333A. The bridged nulling circuit with wien bridge with precision transistors and a 4 gang tuning cap is an engineering masterpeice. But HP have light dependant resistors and lamps that light them up and a complex kind of circuit that defies easy understanding. While it works, it is a continuing miracle. And the working voltages are very low, and there's lots of high impedance circuitry with wide BW and if one tries to test the unit without covers on there is terrible noise problems. But all I need is one meter which tells me the voltage input, and another to tell me the distortion voltage after the fundemental tone has been either filtered out with a tunable nulling circuit or filtered out with a tunable bandstop filter. I always use two meters and a CRO to measure THD, so I can plot the THD easily against output voltage because we need to know how THD increases between low DUT levels and high DUT levels, right up to clipping and maybe beyond. I've got plenty of meters. The construction of a narrow bandwidth bandstop filter which is infinitely tunable across a decade of frequencies means one has to vary C, or R, or pairs of Cs or Rs, anf the tuning gang is actually very stable and good compared to any pot. But switched caps and switched resistors are also good but then one has to tune the input test F to suit the meter filter F settings. I've found the wien bridge oscillators with C tuning gangs to be the most long lasting and stable and with a nice spread of F on the dial. But for very low F, the R associated with a given C of say 1,200pF max from a parallel 3 gang radio cap has to be above 20Megohms to get the F below 7Hz for amp testing. A variable R + fixed C wien bridge oscillator is a nice srable thing if the range of R variation is small and you just want to test audio amps with a single fixed F, say 1kHz, and you have a "broad tune range pot for +/- 50Hz with a second adjust pot for +/- 5Hz to get the bandstop filter to react perfectly and in phase with the amp output signal which is being fed with the slightly adjustable single F oscillator. This careful tuning gives the deep rejection of 1,000Hz. But one may use a fixed notch filter based around the RC parallel T and some FB around two opamps. Usually this has limitations to the depth of the notch, maybe only -55dB. But if you cascade a couple of such easy simple things then the depth of the notch can become enormous, and there is nothing to adjust except the oscillator F. I soon learnt the LC bridged T notch filter was a real marvel because it was passive, and could easily take the full 70Vrms applied from any amp without needing any precision attenuators and worries about frying delicate solid state active parts, and what came out of the bridged T could be easily measured and viewed on a CRO, metered, and used for calculations without having any confusions. I found most wisdom about audio signal testing in the pages of Wireless World between the years 1950 and 1980 which was a UK magazine which attracted articles from many of the brightest stars in analog electronics over many years. I doubt HP contributed anything, and methinks the British boffins may have smiled at HP's valiant efforts which always seemed to use many more devices and be about unaffordable. But the Yanks were first to the moon in 1969, so despite the complexity of their gear they could do wonders with it while it worked, and if the dudes spent enough time to learn how to use it. I think I saw the price of a 333A for 1967 at $1,250.00, and in today's money its maybe 20 grand, just for a little box gadget. I guess the wondrous metalwork and stainless steel screws cost a bomb. To conclude, If I find the time and can't improve the existing 333A performance I may well change the meter amp to an opamp or two with enough bandwidth and fit some diode voltage clamping to prevent the worst effects of internal lingering Vdc changes due to the C coupling caps all charging up then discharging slowly after an over load. I just don't know enough to improve on the HP wien bridge nulling circuit electronics. But perhaps a slower acting fine tune adjust mechanism for the tuning cap would be in order. There is a also a buffer stage to drive the meter amp, and it is unity gain, and that also could be replaced with a far simpler opamp arangement. The meter range switching circuit is a veritable wonder though which seems to work perfectly in 10dB steps and which has no active circuitry. Switch quality seems to be fair even though my 333A has had a very hard life judging my external appearance and some mild corrosion. The internal wiring has many wires crimped onto lead end grippers which then slide onto what look like gold plated pcb pins. many came loose, some were intermitent and what is needed is to cut off the lead grippers and solder the wires to the pcb pins and restrain them from movement by dressing wire positions and using cable ties to avoid joint failure. The more I look at old electronics gear, the more I see how to make it perform a little better, and I do like my gear to be easier to live with than my present old HP333A. One multi band variable wien bridge oscillator I use all the time with just one opamp has now got intermittent pot problems. I have a couple of box fulls of old radio tuning gangs, maybe several identical 3 gang types with each gang 20pF to 400pF which can make 60pF to 1,200pF when paralleled. Two such 3 gangers made to turn together in a wien bridge can be made to get from 5Hz to 60Hz, and give 5 ranges with the top one at 50k to 600kHz. With the variable C, one needs a 5 position two pole rotary switch and 1% fixed resistors of 22Mohms, 2.2M, 220k, 22k and 2k2. I used to have a Topward function generator with sine, square and triangular waves and with AM and FM and a horrible complex circuit on a tiny board, horrid plasic case, and a noisy little mains tranny. I blew its output amp up about 3 times by accidental contact with tube amp rail voltages. The press button range switches began to be unreliable, the single gang pot for varying Vdc which controlled the variable F wore out, twice, and I swapped that pot for a 12 pos switched attenuator crammed in, but even that gave troubles after time and the F would drift. The THD was bad, and worse than any of my wien bridge oscillators, including the tube one I use. Eventually after several fixes, The Topward output lead came into accidental contact with some quite high tube amp B+. There was a fizzling sound with puff of smoke, and the high Vdc pushed way too much current deep into the many ICs in the unit, and all I could do was put the cooked mess into my rubbish bin. Patrick Turner. |
#3
Posted to rec.audio.tubes
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HP333A examination and impressions
As a general rule, HP analog test equipment is the best designed of
its era, excepting only its oscilloscopes which are generally considered not quite as good as Tektronix. I have not used the 333, but the similar 334 is still considered a first rate piece of equipment . I have used them but never worked on them. If you are not happy with it I think you should pass it on to someone else. J L Hood was a competent engineer and I was just reading his "Valve and Transistor Audio Amplifiers" book yesterday. That said I do not believe his designs would have satisfied the requirements that HP was given at the time they were given them. Keep in mind when this box was designed and the fact that it was years later that IC op amps were of instrumentation grade. Some HP boxes were better than others to be sure. HP's most successful products were its 200xx audio generators, its tube and solid state RF generators culminating in the 8640B,its frequenc counters and its digital products which are now obsolete for the most part such as logic analyzers and such. The VTVMs are also quite good. |
#4
Posted to rec.audio.tubes
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HP333A examination and impressions
Patrick Turner wrote:
But even in voltmeter mode, the use of my 333A was became pointless because after it had been turned on for 1/2 an hour the voltage measurement strated to drift about 15% high how are the power supply capacitors and/or voltage regulators doing? My first impression is a drifting power supply voltage (probably drifting due to component thermal breakdown). I used to have a projector TV that had thermal problems in the power supply (I eventually replaced it with a better TV) and I left the back off and pointed a fan directly at the affected components, which worked for quite a while until things got unmanageable (and I bought a new TV). Now the meter amp is a damn strange concoction of discrete transistors with both positive and negative loops of FB and which have different effects at different F, so that it can operate from 5Hz to several MHz, with a very flat response **providing** all the vari-caps and gain pots and the biasing pot are all set just right. I found the bias pot was somewhat out, but when adjusted right according to the schematic Vdc voltages the bad meter drift still occurred. if your bias power supply voltage drifts 'in relation to' other voltages that matter, it's pretty much the same thing as changing its setting. I've done quite a bit with direct coupled transistor circuits and I know how dependent on regulated voltages they can be. If you've got some 'liquid cold' (or one of those "canned air" spray cans held upside down) you can freeze certain compoments to see if you can alter the behavior. |
#5
Posted to rec.audio.tubes
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HP333A examination and impressions
On 2/12/2010 11:27, Big Bad Bob wrote:
Patrick Turner wrote: But even in voltmeter mode, the use of my 333A was became pointless because after it had been turned on for 1/2 an hour the voltage measurement strated to drift about 15% high how are the power supply capacitors and/or voltage regulators doing? My first impression is a drifting power supply voltage (probably drifting due to component thermal breakdown). This could be, but Patrick is competent enough to have checked that... I used to have a projector TV that had thermal problems in the power supply (I eventually replaced it with a better TV) and I left the back off and pointed a fan directly at the affected components, which worked for quite a while until things got unmanageable (and I bought a new TV). If you found out it was the psu, why not fix it instead of buying a new one. Unless it wasn't worth it. Now the meter amp is a damn strange concoction of discrete transistors with both positive and negative loops of FB and which have different effects at different F, so that it can operate from 5Hz to several MHz, with a very flat response **providing** all the vari-caps and gain pots and the biasing pot are all set just right. I found the bias pot was somewhat out, but when adjusted right according to the schematic Vdc voltages the bad meter drift still occurred. if your bias power supply voltage drifts 'in relation to' other voltages that matter, it's pretty much the same thing as changing its setting. I've done quite a bit with direct coupled transistor circuits and I know how dependent on regulated voltages they can be. That's true If you've got some 'liquid cold' (or one of those "canned air" spray cans held upside down) you can freeze certain compoments to see if you can alter the behavior. Spraying "Kalte Spray" can help but in these circuits a drop in temperature will surely make some components and settings drift. So what's the cause then: Thermally instable components or the sudden drop in environment temperature? I do have some older testgear and some are out of spec or broken. I wonder if it's worth the effort to fix them. Some components are specially made for or by the manufacturer and are no longer available. Maybe it's better to Ebay them and with the money from it buy a more recent(and hopefully) better one... Cheers, Zilog |
#6
Posted to rec.audio.tubes
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HP333A examination and impressions
On Dec 2, 6:27*pm, Bret L wrote:
*As a general rule, HP analog test equipment is the best designed of its era, excepting only its oscilloscopes which are generally considered not quite as good as Tektronix. *I have not used the 333, but the similar 334 is still considered a first rate piece of equipment . I have used them but never worked on them. The 334 is the same as 333 but has an RF detector so that the AF signal used to modulate an AM RF or IF wave can be checked for THD. When brand new, presumably they all worked well, but now they are old, some 45 years, and I'm sure that if you were alive in 1965, you are worse now and probably need a couple of things fixed...... *If you are not happy with it I think you should pass it on to someone else. Nobody I know wants this stuff now. *J L Hood was a competent engineer and I was just reading his "Valve and Transistor Audio Amplifiers" book yesterday. That said I do not believe his designs would have satisfied the requirements that HP was given at the time they were given them. Keep in mind when this box was designed and the fact that it was years later that IC op amps were of instrumentation grade. Well, seems to me the Linsley hood design was for the hobbyist with restricted F range, and standard value resistors used which alone limits accuracy of voltage measure. But you can get good enough measure with standard R values for hobbyists. If you have to read an analog meter then you get visual errors probably greater than circuit inaccuracies. HP used a pile of non standard precision R to get accuracy, and although the properties of the j-fets and bjts they used would have varied considerably, all their active circuitry has large amounts of negative feedback to keep everything working linearly so that their circuit THD stayed below the lowest anticipated distortion in signals. Quite a few companies made gear to basic recipes found in magazines. The idea of the wien bridge used as a tunable nulling element for fundemental F has been around for a long time and may be used with a few tubes instead of bjts. * Some HP boxes were better than others to be sure. HP's most successful products were its 200xx audio generators, its tube and solid state RF generators culminating in the 8640B,its frequenc counters and its digital products which are now obsolete for the most part such as logic analyzers and such. The VTVMs are also quite good. I have a HP 606A - not bad for a tubed oscillator, and I have a 302A analyser, all SS which I have yet to try out properly. Much old stuff is very useful - providing it is not falling to bits and suffering aging problems which of course are nowhere to be seen described in the FAQ sections of manuals. I have no time to become an HP expert. The service manuals for the 333A and my similar simpler 331A don't have working Vac signal voltages listed, and I found it impossible to re-draw up the circuit as it is used when switched to measure say a 10Vrms input voltage and show all the working voltages along the circuit. Such detail in service manuals make it easy to service the darn things because you do what the manual says and then measure the voltages at the output and if any part of the circuit works wrongly, you will spot it. Patrick Turner. |
#7
Posted to rec.audio.tubes
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HP333A examination and impressions
On Dec 2, 9:27*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote: Patrick Turner wrote: But even in voltmeter mode, the use of my 333A was became pointless because after it had been turned on for 1/2 an hour the voltage measurement strated to drift about 15% high how are the power supply capacitors and/or voltage regulators doing? *My first impression is a drifting power supply voltage (probably drifting due to component thermal breakdown). *I used to have a projector TV that had thermal problems in the power supply (I eventually replaced it with a better TV) and I left the back off and pointed a fan directly at the affected components, which worked for quite a while until things got unmanageable (and I bought a new TV). After playing with this 333A for a day and switching on/off many times, the +/- 25V rails never varied, and had low hum. Now the meter amp is a damn strange concoction of discrete transistors with both positive and negative loops of FB and which have different effects at different F, so that it can operate from 5Hz to several MHz, with a very flat response **providing** all the vari-caps and gain pots and the biasing pot are all set just right. I found the bias pot was somewhat out, but when adjusted right according to the schematic Vdc voltages the bad meter drift still occurred. if your bias power supply voltage drifts 'in relation to' other voltages that matter, it's pretty much the same thing as changing its setting. *I've done quite a bit with direct coupled transistor circuits and I know how dependent on regulated voltages they can be. The 333A uses dc feedback loops to stabilise bias as well as copius positive and negative FB loops to control circuit gains and compensate for droop in performance at HF. HP knew what they were doing alright,and when new, the gear all worked well. But like old humans, such gear becomes almost impossible to fix completely. I have too many other more important priorities, and I can't afford the time, and its uneconomic to repair. If you've got some 'liquid cold' (or one of those "canned air" spray cans held upside down) you can freeze certain compoments to see if you can alter the behavior. Indeed, but meters drifting up 200% high and then back down again and with intermittencies somewhere which always evades analysis when you hunt for the cause may not be affected by temporary cooling because the NFB loops ensure drift in component behaviour is very much corrected. The bias settings could be altered to allow +/- 33% change in idle currents of transistors and while things did seem to work a bit without drift, the bias change made virtually no difference to working signal voltages which were all very much lower than the maximum possible without clipping. For example, the meter amp had 4 bjts and uses +/-25V rails. It has only to make 0.1vrms to make the meter give full swing. The closed loop gain of the meter amp is nominally 100, but open loop gain is many times more. Only 1mV of signal at the input of the meter amp is needed to swing the meter fully. The NFB resistors seemed stable to me, and the gain adjust pot in the FB loop. Even if I spent years fixing the 331A and 333A meters, they don't actually do what I want a distortion meter to do, ie, give a reliable easy to see measurement of THD down to 0.005% like the simple pile of **** I designed and built so many years ago in about 1995 because I could never find any cheap used but working gear then. Most techs was still using analog stuff and almost none had made the switch to much more modern methods and gear. When the PC became mainstream and new gear became cheap and many old techs expired, died or retired, then there was a lot of their old gear being auctioned or sold. But I never found anything worth aquiring. One old radio ham gave me a pile of stuff including a couple of CROs made well before WW2 and these were truly bloody awful and I chucked them out - real time guzzling monsters which defined what a boat anchor was. Patrick Turner. |
#8
Posted to rec.audio.tubes
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HP333A examination and impressions
On Dec 2, 10:34*pm, Zilog wrote:
On 2/12/2010 11:27, Big Bad Bob wrote: Patrick Turner wrote: But even in voltmeter mode, the use of my 333A was became pointless because after it had been turned on for 1/2 an hour the voltage measurement strated to drift about 15% high how are the power supply capacitors and/or voltage regulators doing? My first impression is a drifting power supply voltage (probably drifting due to component thermal breakdown). This could be, but Patrick is competent enough to have checked that... I used to have a projector TV that had thermal problems in the power supply (I eventually replaced it with a better TV) and I left the back off and pointed a fan directly at the affected components, which worked for quite a while until things got unmanageable (and I bought a new TV). If you found out it was the psu, why not fix it instead of buying a new one. Unless it wasn't worth it. Now the meter amp is a damn strange concoction of discrete transistors with both positive and negative loops of FB and which have different effects at different F, so that it can operate from 5Hz to several MHz, with a very flat response **providing** all the vari-caps and gain pots and the biasing pot are all set just right. I found the bias pot was somewhat out, but when adjusted right according to the schematic Vdc voltages the bad meter drift still occurred. if your bias power supply voltage drifts 'in relation to' other voltages that matter, it's pretty much the same thing as changing its setting. I've done quite a bit with direct coupled transistor circuits and I know how dependent on regulated voltages they can be. That's true Yes but HP knew all about drift in transistor and tube circuits. Their experts knew just how difficult it always was to make a partially DC coupled and partially CR coupled circuit full of discretes work with good stability. 1965 wasn't long after analog computer circuits had been used, with caluclations and so on reliant on the NFB loop accuracy. If you've got some 'liquid cold' (or one of those "canned air" spray cans held upside down) you can freeze certain compoments to see if you can alter the behavior. Spraying "Kalte Spray" can help but in these circuits a drop in temperature will surely make some components and settings drift. So what's the cause then: Thermally instable components or the sudden drop in environment temperature? I do have some older testgear and some are out of spec or broken. I wonder if it's worth the effort to fix them. Some components are specially made for or by the manufacturer and are no longer available. Maybe it's better to Ebay them and with the money from it buy a more recent(and hopefully) better one... I might rationalise the metering circuit with a couple of opamps to at least get the meter circuit to work far better than it ever was designed. This is very easy these days, and I have already done it in another metering circuit I made which powers a 4" wide meter where the needle is centred with no signals. The meter amp is logarithmic and I measure +/- 20dB each side of centre. This makes recording and plotting the acoustic response af a speaker easy. The opamps can be made to function as very linear modulation detectors driving diodes which charge up caps and the DC is fed back to the opamp input so that the opamp has not got to handle linear wide bandwidth voltage amplification and then make a dc signal at the output. If the opamps function as linear detectors, voila, simple and stable. The Vdc outputs of the opamps follow the peak levels of the wave forms. Both positive and negative going Vdc voltages from two opamp outputs which are firmly biased for 0V can be made to provide 2 Vdc signals with the meter slung between the + and -. Many better ways of doing things have been dreamed up since 1965, so rather than spend a month of sundays repairing to the old ingenious standards of 1965 which have serious limitations, its better to remove some 1965 ingenuity and replace with later invented simple brilliance. Some things, like the HP meters do indeed seem like very nice kit and they are a pleasure to read, although a light inside would be nice. The 1972 Linsley Hood distortion measuring circuit could be used with the nicely made HP wien bridge. This would allow a 0.01% THD measurement, using about 1/4 of the parts HP thought appropriate. The box quality and mechanical parts HP had made were a lot better than anything Linsley Hood might have had access to. In 1965, the US was a real industrial powerhouse with a huge number of highly skilled tradesmen and women beavering away to make fabulous gear that cost a kings ransom. Britain was still hurting after WW2. But Britain didn't lack ideas. Patrick Turner. Cheers, Zilog |
#9
Posted to rec.audio.tubes
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HP333A examination and impressions
On 12/02/10 04:54, Patrick Turner so witilly quipped:
Indeed, but meters drifting up 200% high and then back down again and with intermittencies somewhere which always evades analysis when you hunt for the cause may not be affected by temporary cooling because the NFB loops ensure drift in component behaviour is very much corrected. The bias settings could be altered to allow +/- 33% change in idle currents of transistors and while things did seem to work a bit without drift, the bias change made virtually no difference to working signal voltages which were all very much lower than the maximum possible without clipping. direct coupled bipolar circuits are inherently vulnerable to weird instabilities. If it's a 'discrete built' op-amp circuit, you might end up with significant changes in forward transconductance over time, which is likely to cause LOTS of bizarre problems, all of which would be correctable by restoring the circuit to its design specs. Thermal breakdown and aging of transistors might be just enough to create these kinds of problems. Sometimes a little 'liquid cold' might at least verify which component(s) are likely culprits, but from your description you might have to replace all of the transistors to get it to work again. One old radio ham gave me a pile of stuff including a couple of CROs made well before WW2 and these were truly bloody awful and I chucked them out - real time guzzling monsters which defined what a boat anchor was. in the navy we had a term "float test". It's what you do to said equipment to see if it's "still good". If it floats, it's ok (but you'll have to throw it out because of the seawater contamination). If it sinks, it needed replacing anyway. I only heard of one instance where 'float testing' actually happened, and it was totally by accident. And the divers had to go find it (and it ended up being replaced). Ah, well. That 'reality' thing getting in the way. For THD measurements I would be inclined to use a 24-bit sound card to generate a perfect sine wave at the desired frequency, then capture the audio on the output with the same sound card and do a discrete fourier transform on the waveform and calculate the THD as a geometric sum of the harmonic amplitudes divided by the fundamental (I think that's the right calculation). It's been a while since I've done wave analysis. DFT is better than FFT since it lets you have sample counts of "not a power of 2" for a single cycle, so ANY frequency can be used. And we all have fast computers nowadays so a DFT for 50 harmonics would take less time than the audio capture. |
#10
Posted to rec.audio.tubes
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HP333A examination and impressions
On Dec 5, 8:32*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote: On 12/02/10 04:54, Patrick Turner so witilly quipped: Indeed, but meters drifting up 200% high and then back down again and with intermittencies somewhere which always evades analysis when you hunt for the cause may not be affected by temporary cooling because the NFB loops ensure drift in component behaviour is very much corrected. The bias settings could be altered to allow +/- 33% change in idle currents of transistors and while things did seem to work a bit without drift, the bias change made virtually no difference to working signal voltages which were all very much lower than the maximum possible without clipping. direct coupled bipolar circuits are inherently vulnerable to weird instabilities. *If it's a 'discrete built' op-amp circuit, you might end up with significant changes in forward transconductance over time, which is likely to cause LOTS of bizarre problems, all of which would be correctable by restoring the circuit to its design specs. *Thermal breakdown and aging of transistors might be just enough to create these kinds of problems. *Sometimes a little 'liquid cold' might at least verify which component(s) are likely culprits, but from your description you might have to replace all of the transistors to get it to work again. During a few days of poor health, I managed to cobble together the copies of the HP333A and HP331A schematics and to study how they work in more detail. I could find no explanations why voltages I measured led me be quite bamboozled when I should not have been, and when both 333A and 331A should have given identical consistent outcomes. From the manuals I now have a list of voltages and the procedure in mind required to check the circuit working of this gear. The 331A looks like being the most redeemable of the two units, and most easily fixed. The add on functional features of the 333A are utterly excessive to my needs, and so I will endeavour to make the 333A into a wien bridge oscillator plus a working voltmeter using my own simpler designs. I did build a voltmeter in about 1993 using an old but good meter plus $20 worth of cheap SS parts. Its BW is 1MHz, and has ranges between 10mV and 1,000Vac, with Rin at about 1M. I made a simple discrete SS voltage amp with NFB to give the BW, but I didn't include the voltmeter and its diodes in the NFB path. It meant the meter scale I drew looked a little non linear right above 0.0V for each range so measuring below 1mV was dodgy. I also didn't have the intermediate ranges of 0V to 3.2V, which allow better readings of voltages less than 10% on a 0V to 10V scale. The HP meter is a very nice peice of kit for a meter. The wien bridge in the 333A should make a fine oscillator with 5 ranges and 5Hz to 600kHz. The tuning caps have 1,060pF for the 2 gangs used for the parallel and series wien C elements. The largest value resistance needed is 29Megohms with 1060pF to get down to the 5Hz, and just low enough for most of my testing. I already have a TLO72 opamp in several wien bridge oscilators I have built. The last one I made gets up to 200kHz with C = 470pF and R about 1k7. It makes 2.7Vrms output. If I make another discrete transistor amp with maybe 4 x 2n2222 plus high Rin j-fet the HF should reach the 600kHz capability of the HP wien bridge now used as a THD nuller. The existing minimum R value = 2k9, and for 600kHz, C must be 91pF, and methinks the tuning gang used plus its trimmer caps would have total minimum C 100pF. One old radio ham gave me a pile of stuff including a couple of CROs made well before WW2 and these were *truly bloody awful and I chucked them out - real time guzzling monsters which defined what a boat anchor was. in the navy we had a term "float test". *It's what you do to said equipment to see if it's "still good". *If it floats, it's ok (but you'll have to throw it out because of the seawater contamination). *If it sinks, it needed replacing anyway. And some Germans and Japs helped the replacement rates. Owning one's own personal yacht is bad enough; one finds it hard to find good deckhands and ppl to serve the drinks without spilling them, or drinking it all themselves, but owning a whole damn navy would be nothing but a ghastly expense. *I only heard of one instance where 'float testing' actually happened, and it was totally by accident. * And the divers had to go find it (and it ended up being replaced). Ah, well. *That 'reality' thing getting in the way. For THD measurements I would be inclined to use a 24-bit sound card to generate a perfect sine wave at the desired frequency, then capture the audio on the output with the same sound card and do a discrete fourier transform on the waveform and calculate the THD as a geometric sum of the harmonic amplitudes divided by the fundamental (I think that's the right calculation). *It's been a while since I've done wave analysis. DFT is better than FFT since it lets you have sample counts of "not a power of 2" for a single cycle, so ANY frequency can be used. And we all have fast computers nowadays so a DFT for 50 harmonics would take less time than the audio capture. I still enjoy the irrationality of not having a PC with fragile software in my dirty filthy work shed. I agree about the modern approach, but I am too addicted to fixin good old things which once fixed, tend to stay fixed. I have a 302A analyser and I expect it can sniff along bands for stray F. I hope it works, but won't be surprised if old age has eaten into it. So far only had to replace the input fuse and holder which got smashed by previous owners. Meanwhile the wien bridge oscillator I was using gave up because of yet another failing pot. So in my junk boxes I found an old good quality wafer rotary switch, 2 poles, 12 postions, previously used in a valve tester but luckily not worn or fried, and with firm contacts and the R can be switched for 12 different F per decade of F range between 2Hz and 20Hz and so on up to 200kHz. When I make the 333A into a variable wien bridge oscilator, it should give 5Hz to 600kHz, and the HF is just high enough to cover AM radio IF at 455kHz, and to test the reactive behaviour of OPTs at extreme HF. 200kHz is not quite enough for me. The light dependant resistances used as part of the HP wien bridge for tracking some signal with slightly varying F may be useful for the oscillator NFB path which needs to have varying R somewhere to keep the output level constant at all F. Usually a light globe is used or a special thermistor mounted inside a vacuum tube. I've never seen any really suitable thermistors like the good old ones that were made years ago. If the idea of using a light dependant resistor and lamp to control the oscillator does not work I can always use a solid state device as a variable R controlled by a rectified output Vdc. I have this in my oscillator with TL072. Such a thing helps to lower THD. While the pot for varying F was good the signal bounce was low, but it became worse as the pot slowly became intermttent, and open on one small section of a track. Cleaning and wrapping cardboard to keep out dust didn't help. Plus the contour of the log pots I have is a bit odd, maybe real logarithmic contouring in Taiwanese log pots leaves much to be desired. These cheap pots gave part of the F band all crowded together. I tried another Taiwan replacement but this had such a difference between track R at about midway along the band that the oscillations would not start. A switched set of F determined by carefully calculated R values will be OK for most of my testing work. I have found WB oscillators to generally be very frequency stable and once warmed up the F variation is negligible while testing some amp. One Topward oscillator I had which used a pot to vary a DC level to a digital chip had quite awful distortion, 0.3%, and such high frequency drift it was hopeless to try to use when testing THD and needing to null out the fundemental. Home made oscillator calibration is easy though; I have a modern digital F meter which I built from a cheap kit, goes up to 50MHz. Just rotate the pot or the switch until the F numbers come up and write the marks in ink on the cardboard scale glued to the front panel. When done, a coat of varnish seals it. Lasts for years and will see me out, and can be re-done any time. A 24 pos switch would be luxury........ Patrick Turner. |
#11
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It would have been shortly after the 333A was developed by HP that
opamps were invented and VOILA, all the efforts HP went to became immediately obsolete. I began to think the HP meter circuit was like a horse and buggy compared to a good motot car, and the circuit working seems to rely on all resistors never changing in value, and on many other things like cap leakage, and perhaps bjts and other bits becoming dodgy and almost unidentifiable in the years since 1967. |
#12
Posted to rec.audio.tubes
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HP333A examination and impressions
Patrick Turner wrote:
Usually a light globe is used or a special thermistor mounted inside a vacuum tube. I've never seen any really suitable thermistors like the good old ones that were made years ago. If the idea of using a light dependant resistor and lamp to control the oscillator does not work I can always use a solid state device as a variable R controlled by a rectified output Vdc. I have this in my oscillator with TL072. You could create a wein bridge VCO using an LED current-driven by a signal, lighting up two identical photo-resistors with a few wraps of black electrical tape around the assembly to keep the light out. That should do for your wein bridge if the circuit I have in mind is used, i.e. 2 caps, 2 resistors for an RC bandpass filter and oscillator startup via a non-linear device, often a low voltage incandescent lamp. I've seen this done with 2 triodes and a notch filter also, with positive feedback using the lamp (an old heathkit sine/square generator specifically). There are also MOSFET opto-isolators (analog switches) that should effectively give you the same effect. Some claim proportionally controllable output as I recall, based on light intensity, which is what the photocell thing would also do. Such a thing helps to lower THD. While the pot for varying F was good the signal bounce was low, but it became worse as the pot slowly became intermttent, and open on one small section of a track. noisy pots. Bleah. I suggest good quality contact cleaner sprayed into any openings followed by a rapid succession spinning the knob from one extreme to the other, several dozen times. Works for guitars and guitar amps. Regarding cheap Taiwanese parts, can Digikey ship to your area? They tend to have decent quality stuff, a bit pricey but reliable. You might be able to improve THD of a wein bridge by tweeking the response curve with active filter elements. Anyone consider swapping out the incandescent bulb for a correctly biased tunnel diode? Just a thought. |
#13
Posted to rec.audio.tubes
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HP333A examination and impressions
There is a very good reason for the lamp in a Wein Bridge oscillator.
The _thermal_ time constant of the lamp must be long compared to the period of the lowest frequency of oscillation. I do not have the time to fully discuss this here, but Jim Williams of Linear Technology Corporation has written quite a bit on low distortion oscillators. I suggest both Patrick and Bob download Linear Technology's Application Note 43 on bridge circuits, http://www.linear.com/pc/downloadDoc..., P1213,D4134. Jim presents a number of simple Wein bridge oscillator designs that can produce THD levels from below 0.01% to such a low level (around 3 parts per million) that most commercial harmonic distortion analyzers have a noise floor greater than the distortion being measured. Appendices to this application note include a discussion of Bill Hewlett's oscillator built for his Master's thesis, and "Understanding Distortion Measurements" by Bruce Hofer of Audio Precision, Inc. I think you will find this LTC application note to be very educational yet presented in a way that is easy to understand. If this link does not work for you, you may have to register at the LTC site to download it. I suggest looking through their extensive library of application notes for many good ideas when you have time. 73, Dr. Barry L. Ornitz WA4VZQ |
#14
Posted to rec.audio.tubes
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HP333A examination and impressions
On Dec 31 2010, 10:41*pm, Big Bad Bob BigBadBob-at-mrp3-
wrote: Patrick Turner wrote: Usually a light globe is used or a special thermistor mounted inside a vacuum tube. I've never seen any really suitable thermistors like the good old ones that were made years ago. If the idea of using a light dependant resistor and lamp to control the oscillator does not work I can always use a solid state device as a variable R controlled by a rectified output Vdc. I have this in my oscillator with TL072. You could create a wein bridge VCO using an LED current-driven by a signal, lighting up two identical photo-resistors with a few wraps of black electrical tape around the assembly to keep the light out. *That should do for your wein bridge if the circuit I have in mind is used, i.e. 2 caps, 2 resistors for an RC bandpass filter and oscillator startup via a non-linear device, often a low voltage incandescent lamp. *I've seen this done with 2 triodes and a notch filter also, with positive feedback using the lamp (an old heathkit sine/square generator specifically). *There are also MOSFET opto-isolators (analog switches) that should effectively give you the same effect. *Some claim proportionally controllable output as I recall, based on light intensity, which is what the photocell thing would also do. For a variable F sig gene and for the NFB regulating R I ended up using the on resistance of a j-fet biased with dc derived off the op- amp output with diode detector. For low THD, the adjustments for level became touchy, and bounce was bad if range was changed or F was changed. To avoid bounce and to get quick settling, THD may be allowed to be up to 1% with limiting clamps on the op-amp output. But in an oscillator I am working on now to get 6V at 0.001% or less THD, I have a wien bridge using NE5534 plus fixed R and C but with a double gang radio tuning C with series 250pF padders to get a C variable range of about +/- 30Hz each side of 1kHz. The use of a lamp is then fine. I have one which is 60 ohms cold and 200 ohms at 2V, so the NFB network at 6V output is about 600 ohms and does not overload the opamp. With rails of +/-14.7Vdc, I am getting up to 7Vrms with THD under 0.005%, with best THD of 0.0041% at 6vrms output. The F adjustment with tuning cap does not make signal bounce, and F may be tuned for best null into distortion measuring notch filter. To get oscillator THD down to less than 0.001% at 6Vrms, I am using a couple of cascaded BPFs with R+parallel tuned LC, about 22mH + 1.1uF, with Rw of the coil at 5 ohms for Q about 10. There is -6dB insertion loss but two opamps are used each with NFB to give gain of 2 to overcome insertion loss. Such a thing helps to lower THD. While the pot for varying F was good the signal bounce was low, but it became worse as the pot slowly became intermttent, and open on one small section of a track. noisy pots. *Bleah. *I suggest good quality contact cleaner sprayed into any openings followed by a rapid succession spinning the knob from one extreme to the other, several dozen times. *Works for guitars and guitar amps. Yes, but not for long. Regarding cheap Taiwanese parts, can Digikey ship to your area? *They tend to have decent quality stuff, a bit pricey but reliable. The operation and stability of variable tuning caps is superlative. You might be able to improve THD of a wein bridge by tweeking the response curve with active filter elements. Maybe variable state types with multiple FB paths and 3 opamps will do if the THD produced is low. But I am using opamps with simple R+R series voltage NFB paths and then passive R+LC filters. I can get 40dB attenuation of any 2H in the oscilator signal, and more atten for all other H.. Anyone consider swapping out the incandescent bulb for a correctly biased tunnel diode? *Just a thought. I have never seen this. Patrick Turner. |
#15
Posted to rec.audio.tubes
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HP333A examination and impressions
On Jan 1, 3:07*pm, "Nordic Breeds WA4VZQ"
wrote: There is a very good reason for the lamp in a Wein Bridge oscillator. The _thermal_ time constant of the lamp must be long compared to the period of the lowest frequency of oscillation. *I do not have the time to fully discuss this here, but Jim Williams of Linear Technology Corporation has written quite a bit on low distortion oscillators. *I suggest both Patrick and Bob download Linear Technology's Application Note 43 on bridge circuits,http://www.linear.com/pc/downloadDoc...C1,C1154,C1009,.... Jim presents a number of simple Wein bridge oscillator designs that can produce THD levels from below 0.01% to such a low level (around 3 parts per million) that most commercial harmonic distortion analyzers have a noise floor greater than the distortion being measured. *Appendices to this application note include a discussion of Bill Hewlett's oscillator built for his Master's thesis, and "Understanding Distortion Measurements" by Bruce Hofer of Audio Precision, Inc. *I think you will find this LTC application note to be very educational yet presented in a way that is easy to understand. If this link does not work for you, you may have to register at the LTC site to download it. *I suggest looking through their extensive library of application notes for many good ideas when you have time. * * * * 73, *Dr. Barry L. Ornitz * WA4VZQ Thanks Mr Doctor Barry Ornitz. I would say the pdf there is the *most informative thing on wien bridge oscilators" I have ever seen. BUT, it has ocurred t me that a bridge formed with R + LC on one side and R + lamp on the other side should also work well and with low THD because the Q of the LC is far higher than the Q for the RC wien bridge arrangement. So therefore any harmonic of the oscillator fundemental fed back via the PFB path is much more attenuated than using the wien RC array. The NFB passes all the harmonics back according to the R ratio. So with LC the ß fraction of distortion signal applied to the op-amp diff inputs is effectively much greater with LC. Changing the F with LC is more difficult which is why nobody uses LC for audio F oscillators unless one likes to have switched taps on a range of large air cored coils and switched capacitors which is actually not a bad idea. Q will vary, but its always more than wien RC. But for between 100kHz and 2MHz the normal radio tuning cap and lamp regulation in a bridge scheme might work very well, but I ain't seen any sample of such a thing. Patrick Turner. |
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