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#1
Posted to rec.audio.tubes
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How to test audio output transformers
I recently purchased a Fisher 800B receiver and want to test the audio
output transformers for good/bad status and wanted to know if there was an out of circuit (or in circuit) way to do same before I start to retube and spend a good bit of dollars. |
#2
Posted to rec.audio.tubes
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How to test audio output transformers
powerdoc wrote:
I recently purchased a Fisher 800B receiver and want to test the audio output transformers for good/bad status and wanted to know if there was an out of circuit (or in circuit) way to do same before I start to retube and spend a good bit of dollars. See if you can smell any burnt order. But proceed anyway with the ohmmeter. First I'd check with an ohmmeter for continuity of the primary and the secondary. If it's a push-pull amp, each section of the primary should be roughly similar resistance. One side being 10 to 20% higher resistance would not be unexpected. See if both transformers yield very similar resistances. Next step is to test for shorted turns. Odds of a shorted turn vs it being blown open circuit are fairly low, though. Especially if you don't smell a faint burnt odor when sniffing around the transformer where its wires come out (with the power off of course!) you can probably skip the following test, and assume that the transformers are probably good (however, if the transformer is heavily potted, this lack of burnt odor test won't be valid). You'd need an audio test generator that you can measure voltage and current on. Or a variac *AND* isolation transformer (variacs do not provide isolation from the powerline). And an AC milliamp meter and AC voltmeter. With no load on the transformer under test secondary, you want to measure any large amount of current draw when you crank up the variac voltage. Any shorted turns will cause large current draw. At some point when cranking up the variac voltage you may see the current suddenly go up. Back off immediately. What you are seeing if this happens is the core going into saturation. Normal. Both transformers should behave the same. Try loading the secondary with an 8 ohm resistor, you should see the current draw go up. Both transformers should produce the same current draw when doing that. If you pass the above, the transformers are probably as good as the day they were made. Of course if the design was poor, this test won't tell you that, but the above test should reveal any abuse that might have happened. |
#3
Posted to rec.audio.tubes
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How to test audio output transformers
"powerdoc" wrote in message oups.com... ** Groper alert ! I recently purchased a Fisher 800B receiver and want to test the audio output transformers for good/bad status and wanted to know if there was an out of circuit (or in circuit) way to do same before I start to retube and spend a good bit of dollars. ** First connect a sine wave oscillator set to around 400Hz to the 16 ohm secondary and check the primary side gives the same voltage ( on a scope or DMM ) across each half winding and that the two trannys test the same. Each primary half should have about 10 times the voltage imposed across the 16 ohm winding. Also check the DC resistance of each half is the same +/- 10%. Insulation may have deteriorated inside the tranny but this will only show up under high voltage conditions. Driving the 16 ohm secondary with the output of another amp, at say 400Hz and 30 volts rms, will check for insulation break down. Be very careful, doing this generates very high primary voltages. ........ Phil |
#4
Posted to rec.audio.tubes
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How to test audio output transformers
robert casey wrote:
powerdoc wrote: I recently purchased a Fisher 800B receiver and want to test the audio output transformers for good/bad status and wanted to know if there was an out of circuit (or in circuit) way to do same before I start to retube and spend a good bit of dollars. See if you can smell any burnt order. But proceed anyway with the ohmmeter. "Burnt ordure"? I know some kit is sh*t, but really |
#5
Posted to rec.audio.tubes
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How to test audio output transformers
powerdoc wrote: I recently purchased a Fisher 800B receiver and want to test the audio output transformers for good/bad status and wanted to know if there was an out of circuit (or in circuit) way to do same before I start to retube and spend a good bit of dollars. The other guys have all offered good advice. The burnt smell is not a reliable indicator of a stuffed OPT and many such fused or problem OPTs have no smell. With the amp turned off and cold the primary halves should both measure near equal dc resistance between anode connections and CT for the B+, and be the same as in another amp with equal circuitry. If the digital meter won't read the dcr due to a lot of inductance of the primary, place a shorting link across the secondary. One may use a signal gene with at least 20Hz to 200kHz range connected to one primary anode connection. Mine produces 6.4Vrms at about 4k7 output source resistance and the response signal at the secondary should be a small voltage of about 1/10 of the signal across the 1/2 primary. An oscilliscope should be used to monitor the secondary signals across the AF range. The F response should be -3dB at say 10Hz and 30kHz, but if the LF -3point is say above 1kHz, then you have a shorted turn in the primary. Also betwen 20Hz and 50kHz the same voltage and bandwidth with opposite phases should appear at both anode connections. Insulation breakdown can be seen using Phil's method, and yes, it is potentially dangerous because if you have a sec to pri turn ratio of 1 : 20, then with 30Vrms applied across the secondary, there will be 600Vrms from anode to anode. But under real useage conditions there will be say 350V dc PLUS the ac signal voltage of say up to 1,000pk ac V under fault conditions, so primary to secondary voltage could reach +/- 1,500pk V easily at each anode connection with respect to 0V. The source resistance for the signal input to the OPT sec under test should be less than 4 ohms, so another power amp with a 3.3 ohm buffer resistance can be used so that as the input voltage signal is raised then if there is a an arc from the primary to ground or groundeds secondary the short won't damage the other amp supplying the test signal. If there is an intermittent interruption of the signal input to the OPT under test as the signal is raised then it indicates a short due to insulation break down. An alternative way to test P-S insulation is to simply apply +2,000V dc to the CT. with anode and CT connections disconnected from the amp. This requires a HV supply above able to make the 2,000V, and 6 x 220k 1 watt resistances in series between the HV supply and the CT so that if there is a short the current flow is limited to a harmless amount. Careful HV measurement techniquiqes are required since a shock from 2,000V is quite deadly. The dc V test need not be carried on for longer than a minute, lest the presence of the HV cause a corona to form and ionisation and an arc over a longer time. New transformers should all sustain dc HV for a minute, 4,000V being used typically. But usually such shorts from primary to something earthy can be fatal to an OPT since an arc started by a high ac voltage will sometimes then be sustained by an arc of dc current of B+ supply to 0V. A fuse might blow but many amps don't have a fuse to protect against such an arc. I have repaired one amp with such problems and I could hear and see where the arc would form, and I was able to scrape out resulting burnt area carefully and soak the OPT in wax. Its been in use for the last 6 years without any more trouble, and probably it was moisture in the ancient 1950s OPT which caused the problem to start. And this was in a Geloso 15W PA amp B+ = only +320V for a pair of 6BQ5. But usually arcs due to failing insulation ruin an OPT, and don't occur where they can be seen. If an OPT is possibly damp, ( a Katrina victim? ) then the only way to dry it out is to heat it up to about 110C and if possible place it in a vacuum chamber while still hot and suck the air off with the water vapour. Patrick Turner. |
#6
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How to test audio output transformers
Thanks to all replies; will get on it this w/e. |
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