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Restoring a TV-7/U Tube Tester



 
 
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  #1  
Old January 14th 14, 12:59 AM posted to rec.audio.tubes
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Default Restoring a TV-7/U Tube Tester

Hello all,

Just picked up a TV-7/U tester today. I've used these before many times, and I've owned a few testers over the years, but never a TV-7 until today. I'm pretty happy with it, but it does have some issues.

I'm going to go over the whole process of restoring it so I might as well do it here.

Here are a couple pics. One is the unit as it looks from the outside, and as you can see somebody decided to make a hole in the top and move the power cable. Also, there's a pic of the inside where a really sloppy set of diodes was put in.

https://www.dropbox.com/s/z90wa6laqkpj46t/TV7_top.jpg

https://www.dropbox.com/s/yac82k8agl...%20%281%29.jpg


So, there will be some work to do here! Already, I have noticed that while the line check seems ok on this tester, every tube I've tested so far has been showing perhaps half of the reading it should be getting. The meter movement seems ok, but something is amiss...

I've worked on several testers before, B&K, a Hickock here and there (TV7s are mostly Hickocks), and some oddball older ones of dubious quality. So I'm a bit green on this one, if anyone sees me screwin' up, please chime in.

First step was to test the 5Y3 and the 83. Well, 83s being mercury vapor recifiers either work or they don't, and the 5Y3 tested fine. So we have a circuit problem. It would be nice if it were those sucky diodes somebody added. I'm going to start digging for schematics and maintenance manuals and I will report back soon.

Cheers,

-forkinthesocket
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  #2  
Old January 16th 14, 01:10 AM posted to rec.audio.tubes
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Default Restoring a TV-7/U Tube Tester

Hi all,

Just a quick update here.

First, thanks much to the kind RATter who sent me such nice scans of the manuals. I will be putting them up on dropbox later.

Well, I'm off to RadioShack to grab some more diodes. First thing I am going to do is a -proper- replacement of those gnarly ones pictured in my last post. They represent a hazard as they are now, so I will fix that, post some photos of how I did it, and we'll go from there!

Cheers,

-forkinthesocket
  #3  
Old January 16th 14, 04:16 AM posted to rec.audio.tubes
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Default Restoring a TV-7/U Tube Tester

Hi again,

So, I was able to get to RadioShack in time, but much to my dismay, the one closest to me was in the process of packing away all their items to be shipped to other stores. Luckily, the other store was only a couple miles away, and I was able to pick up a pair of IN4001.

Much like in the Sherwood project, I soldered these diodes to a terminal strip, and the result can be seen he

https://www.dropbox.com/s/wmtt94daib...iode_Strip.jpg

One terminal has a positive facing diode, the other a negative, and the mounting lug has the other two ends of those diodes.

To install this, I first had to de-solder and clean the three terminals in the TV7 that these diodes connect to. In order to get access to the two terminals on the push-button strip, I had to first unscrew its two mounting screws from the face plate. This gave me enough room to work after pushing some wires aside.

After de-soldering and cleaning the connection on the rotary knob, I stripped back and threaded the original wire through the hole in the knob terminal and through the hole in the diode strip mounting lug. I then clamped it in placed, pulled the connection tight, and soldered it in place.

After preparing the connections on the button strip, I soldered a red wire to the positive connection, a green wire to the negative (I'm out of black at the moment) and left about 3" of wire on each sticking out as I re-attached the buttons to the face plate. I then gently bent the diode strip into place and connected the positive and negative leads.

The connections can be seen he

https://www.dropbox.com/s/iuv84wy7lx...onnections.jpg

The red circle shows where the positive diode will connect, the black circle shows the negative connection, and the blue circle shows where you solder the mounting lug of the diode strip, and the common connections of the two diodes.

After re-mounting the buttons and soldering on the positive and negative wires, the finished diode installation looks like this:

https://www.dropbox.com/s/4youxfuerx..._Installed.jpg

The whole thing fits in there quite nicely and after checking the diodes with a meter to make sure they survived the heat of soldering, I was ready to fire it up.

The next step is now attempting a line calibration, and I will get into that in my next post.

Cheers,

-forkinthesocket

  #4  
Old January 16th 14, 04:21 AM posted to rec.audio.tubes
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Default Restoring a TV-7/U Tube Tester

As a side note to the section below, this picture shows the diode strip installed, but the connections on the push button strip have not yet been de-soldered, cleaned, and had their new wires attached. These are the correct points, though.


> https://www.dropbox.com/s/iuv84wy7lx...onnections.jpg
>
>
>
> The red circle shows where the positive diode will connect, the black circle shows the negative connection, and the blue circle shows where you solder the mounting lug of the diode strip, and the common connections of the two diodes.
>

  #5  
Old January 16th 14, 09:11 AM posted to rec.audio.tubes
Andre Jute[_2_]
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Default Restoring a TV-7/U Tube Tester

On Thursday, January 16, 2014 4:16:33 AM UTC, wrote:
> Hi again,


> So, I was able to get to RadioShack in time, but much to my dismay, the one closest to me was in the process of packing away all their items to be shipped to other stores. Luckily, the other store was only a couple miles away, and I was able to pick up a pair of IN4001.


Yo, Dorkinthesocket, if you're such a newbie that you don't even have a junk box, you really want to start in electronics with something less dangerous than tube voltages. -- Andre Jute
  #6  
Old January 18th 14, 10:31 PM posted to rec.audio.tubes
hugeshows
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Default Restoring a TV-7/U Tube Tester

Hello again,

I've made some more progress on the TV/7. It was very important to pause and really read over the instructions carefully in order to make sure that I was reading the correct procedure for this particular unit.

After reading the calibration steps several times, it became fairly obvious that resistors R123 and R125 are very important, since they form the meter bridge circuit. They are 1K 2W resistors in my unit, and they measure out at 1.084K and 1.175K. That's fairly close, but not as close as I want. I started digging around in my stash, and found a nice box of NOS Mallory 1K 5W precision wirewounds. Out of a box of 5, I figured I'd get a pair that were well tighter than .5% I actually lucked out and found a pair that both measured exactly 1009 ohms. Here's a pic of them sitting on top of the transformer before installation:

https://www.dropbox.com/s/kx8o2sccse..._Resistors.jpg

R125 and R125 are the two 1K resistors on that turret board in this picture..

These had been messed with before and the results weren't very clean, so I decided to completely de-solder those terminals before putting the new resistors in. Here they are installed:

https://www.dropbox.com/s/3sb6ytcmes..._R123_R125.jpg

They fit quite nicely. In case you're wondering, I did check first to make sure they wouldn't bottom out once the tester goes back in its case. A quick measurement shows that they're still matched perfectly, so on to line check and bias adjustment.

The first step now that we have a good meter circuit with new diodes and resistors is to power the tester on for a couple hours and let it burn in. Then we will start adjusting the bias voltage.


That's all for today probably. Cheers!


-forkinthesocket
  #7  
Old January 19th 14, 12:39 AM posted to rec.audio.tubes
hugeshows
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Default Restoring a TV-7/U Tube Tester

Oh Yes, the manuals I keep talking about are he

https://www.dropbox.com/s/k984kab5v9...%2011-5083.PDF
https://www.dropbox.com/s/ykudra7u1r...625-274-12.pdf
https://www.dropbox.com/s/zt0mgdjoay...625-274-35.pdf

Thanks again to the kind RAT member who sent these to me, they are quite helpful and the service manual is actually searchable in Acrobat, which is nice. I've seen lots that aren't.

So, on to bias calibration steps...

To set up this test, you set the meter to HS5-3460. Once again, I want to remind you that this step is for the TV7/U (my tester) and when you look at the above manuals, you will see that there are different measurements and steps not to mention additional pots and resistors in various versions of this unit. So, moving forward with the TV7/U....

Get your DMM set to measure from pins 5 & 6 of the octal socket (uppermost two holes). Warm up the tester for at least an hour in the box, and perform the line adjust procedure until the meter is centered at 60 when line adjust is pressed. Let it sit for an hour, re-adjust it, and proceed to measure voltages according to the table in the manual. On the left, I show what voltage readings you should expect when the BIAS knob is turned to a certain value. You see what you expect versus what I got:

BIAS Spec. Volts Tester Before Tester After Final Tweak
22 3.00 +-.2 2.515 3.093 3.005
50 13.4 +-1.0 10.91 13.60 13.22
75 25.0 +-1.4 21.42 26.26 25.66
100 40.0 +-2.0 32.08 40.1 39.08

As you can see, before adjusting the band on variable resistor R130, the readings were completely off. Incidentally, they were taken before I replaced R123 and R125 in my last post. Now with those replaced, and having adjusted clamp A on R130 a few times, I arrived at what I feel is reasonable. Since most tubes I will be working with are tested with the BIAS knob at lower settings, the readings at 22 and 50 are more important to me than at the higher end of the scale. I realized that part of the difficulty in finding the best adjustment was because the dial was moving slightly on the knob shaft, introducing a sizable error. The final reading was taken after I had cemented that dial in place. It appears that this early knob design is prone to having some wiggle between the numeric ring and the knob itself, and I strongly recommend you fix that issue before attempting to squeeze that last little bit of calibration out of your tester. I added some CA glue where the aluminum meets the brass as seen in this picture. I put a decent amount, but was careful not to let it leak out the other side from over-application. Here's the underside of the knob:

https://www.dropbox.com/s/rvjdscejo8...Knob_Glued.jpg

Not a great shot, but you see where I added glue I hope. After it dried all play in that number dial was completely gone.

So now that we have a good set of bias readings, next step will be to measure and adjust the plate voltage.

Cheers!

-fokrinthesocket



  #8  
Old January 19th 14, 08:05 AM posted to rec.audio.tubes
hugeshows
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Default Restoring a TV-7/U Tube Tester

Hello all,

Well, I have now finished the first and most important calibration procedure. The rest is more related to diodes and shorts, but the actual circuit you use most of the time is now calibrated. It was kind of a pain, and the army manual, while having all the information in there, puts the step that should be first near the last, IMHO. The plate voltage adjustment step really adjusts the meter circuit, and therefore all of the prior calibration steps that required you to be at line set condition are now screwed up because you have to play with the meter to make the plate voltage correct.

See, it turns out that the plate voltage, like the other voltages, are directly related to the the line voltage adjustment. However, unlike the bias and other voltages checked during calibration, there's nothing you adjust to change the plate voltage except the line adjust. In other words, the plate voltage stays a constant and everything else gets adjusted around it. If the meter doesn't show 60 at line check when the plate voltage is at 150, then you adjust the meter circuit, not the plate voltage.

So the army manual totally wastes your time in my opinion, by having you do tests and adjustments that will get wiped out should the meter circuit not be calibrated. That's military intelligence for you.

Knowing what I now know, the things I did in this restore would be completely rearranged in the following way. Let's go over that before I outline the final steps in calibrating this thing and how I did it.

First, I should have checked R123 and R125. If those weren't close to 1K, but more importantly closely matched to one another, I'd replace them.

Second, I'd inspect and/or replace the meter diodes, CR101 if need be.

Now, knowing that the meter circuit is out of whack from new resistors and diodes with a likely higher output voltage than the original, I'd skip straight to the step where we compare plate voltage to the meter at line check.

Now with the new diodes, the meter is reading a high line check when the plate voltage is at 150v. It should read 60, and in my case it's reading 68. Because it is too high, I have to compensate by shunting a resistor across the outputs of the meter diodes. The manual suggests 40k-60k, but this turned out to be too low. Probably because of the voltage from these diodes, I achieved the desired 60 reading with a resistor of 8.2k. Here it is in place:

https://www.dropbox.com/s/jvonmcikyh...iode_shunt.jpg

I just added it right on the output terminals for the diode. I think it looks right at home there.

So, now having gotten the meter down to a proper 60 at line check, with 150 volts coming from the plate, then I'd do the screen grid check. In my case it came out to 130 on the nose, which is exactly what it's supposed to be.. Looky here, no adjustment was necessary now that the meter is calibrated! Why did they put this near the end???

The next test, you press diode and mut. buttons at the same time (buttons 2 & 3) and you should get 56v +- 1.5 I wound up with 68.9 and so had to adjust clamp B on R130. R130 is shown he

https://www.dropbox.com/s/cyy4pik2xa9ucu8/TV7_R130.jpg

Clamp A is the bottom clamp, the one we adjust for the bias calibration. Here we adjust clamp B, the top one, until 56v is obtained. I managed to get it to 56.2 so, I'll call that done.

Now having gotten that right, I had to go back and re-do the first steps where I calibrated the bias knob, adjusting clamp A on R130 a few times until the bias voltages were correct.

After this was all done, it is now calibrated as far as I can tell. Tubes that tested below acceptable before (when I knew they were good) now test quite well, and give readings that I would expect them to give. Obviously, with meter deflection never being very linear, and tolerances being what they are, you're lucky if any two TV7 testers are in 5% of each other when calibrated. So only time will really tell me how I did. But all of the voltages are now correct, the critical resistors are tightly matched now, and the diodes are brand new. It should be smooth sailing from here on out.

At some point I will update this with adjusting the shunt portion of the circuit for diode testing, etc. and using a resistor to verify the shorts tester is working correctly. Then there will eventually be the cosmetic end of things.

That's all for the moment. I'll look at this again soon and try to follow up with anything that I've missed or gotten wrong. Probably will need to do that when I'm less tired.

Cheers,

-forkinthesocket
  #9  
Old January 29th 14, 02:57 AM posted to rec.audio.tubes
hugeshows
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Default Restoring a TV-7/U Tube Tester

Hello All,

Today I received a little package in the mail from Daniel Nelson. If you need to get parts for your TV7 or need an overhaul, he is somewhat of a legend who specializes in this tester. My tester was missing a few things that I wanted, so I contacted him and he gave me a very good deal on some parts.. First off, if you're missing the tube holders like I was, his reproductions are economical and work. I ordered two socket savers, the tube holders, and a socket adapter that my unit was missing. Here's what I got:

https://www.dropbox.com/s/knxqgtha1v...lson_Parts.jpg

I thought that this installing the socket savers and tube holders would only take a couple minutes, but as it turns out I had to do a little fiddling to make it all fit. First, the tube holders. They mount to posts that are threaded through the bracket to which the sockets are mounted. After test fitting the holders, it seemed that my tubes were too tall or the posts not long enough for the holders to fit into place. After a bit of investigating I discovered that if I removed the two screws holding down the resistor board and put it to the side, I had access to the backs of the posts where I could re-adjust perhaps another centimeter of length out of the rods.

https://www.dropbox.com/s/6ehhrzatak...lder_Posts.jpg

As you can see I pretty much took all the play there was. There are nuts on the other side that must be tightened when you adjust these as well. Once I had that done, I was able to fit the new tube holders and here they a

https://www.dropbox.com/s/pa4g1z4vb9...be_Holders.jpg

As you can see, I really did need every last bit of length on those two rods. But once that is done, these work quite nicely. Ten bucks was a good deal since I can't even look at sheet metal without getting a cut.

Now onto the socket savers. Not being a big power triode fan, I typically use octal and noval sockets in my amps. Since those are primarily the tubes I test unless I'm (rarely) working on radios, I bought and octal and noval socket saver. I thought this would be easy as well, but it proved a bit of a pain. First, before I installed them I prepped the old sockets a bit. Working with a dental pick and some de-oxit, I both cleaned and tightened the terminals in the original sockets. After putting in the octal socket saver, I quickly realized that the fit was a bit tight underneath. As it happens, the octal socket is right over the power transformer and leaves little clearance for the mounting hardware.

https://www.dropbox.com/s/1nvmcp0lbo...cket_Saver.jpg

As you can see, it's tight and I had to settle for slightly crooked. It turns out that the only way to even get the white plastic collar and metal washer in place was to loosen the three screws mounting the transformer to the top plate of the tester. Once I was able to move the transformer back a bit, I was JUST able to get the other bits in place. This doesn't have to be perfect, it just need to be tight enough to stop the socket saver from pulling out instead of the tube.

The noval socket provided another challenge - it had some solder or something obstructing the hole through which the center bolt passes. That required some boring out on my part. Once done, the hardware fit fine, but was easiest installed with the power transformer loose.

After that, I put it all back together and am now very happy with the tester. I will get to the shunt portion of the calibration at some point but since it is only used in diodes and rectifiers, I am not terribly worried about it now. Also, something tells me that with the plate voltage and meter bridge correct, the shunt is going to be spot-on unless somebody has messed with it.

Here's a shot of the tester in action, testing some NOS KT-90s I bought back in the glory days of RAT, the late 90's. Notice how well the top plate cleaned up as compared to the first picture. Alcohol for the tough stuff, and windex overall did the trick nicely.

https://www.dropbox.com/s/75zzf7q4ih...ting_KT90s.jpg


Cheers!


-forkinthesocket



  #10  
Old October 13th 14, 03:45 PM posted to rec.audio.tubes
Peter Wieck
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Default Restoring a TV-7/U Tube Tester

On Tuesday, January 28, 2014 9:57:45 PM UTC-5, hugeshows wrote:
> Hello All,
>


A little late - by nearly 10 months - but a bit of advice when dealing with any Hickok-pattern tester:

Lose the 83.

First, in most Hickok testers, it is placed in a sideways position - they are not designed for that. Second, you are correct that they either 'do' or 'don't', but they also are somewhat spotty when they do - after they have aged a bit. Often enough they will show an otherwise good tube as either weak or bad, especially high-draw output tubes such as those KT90s.

http://tubesound.com/2009/01/16/soli...t-for-83-tube/ is a How-To link that will get you a worry-free SS drop-in replacement.

I keep a 539B, and after I replaced the 83 and did the calibration, it is as accurate as any tester gets, a fairly wide window in any case. And the readings are much more consistent overall.

Peter Wieck
Melrose Park, PA

p.s.: Nor would I use junk-box 'pulls' on any piece of equipment that is of any value, more specifically, something as difficult to 'get into' as that tester. Fine for testing and fine for diagnosis - but as a permanent install? Not hardly.
 




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