Strange behavior in a vintage Sony TA-3120A amplifier - diyAudio
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Old 10th September 2013, 12:49 PM   #1
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Default Strange behavior in a vintage Sony TA-3120A amplifier

I recently picked up a vintage Sony TA-3120A amplifier, 1970-era. This thing is stock, so quite due for some electrolytic cap and resistor replacement. Nonetheless, I decided to plug her in and give it a short listen before digging into it.

The output is capacitor coupled, single voltage power supply - your standard bog early solid-state circuitry. (The service manual for the older 3120 is available online, but can't find the specific one for the 3120A which is slightly different).
SONY TA-3120 SM Service Manual free download, schematics, eeprom, repair info for electronics

Now here is the strange part - the sound comes and goes at a rate of approximately 2 seconds. I pulled out the DMM to check some voltages. According to the manual, voltage at the main power supply cap should be 88. I get a reading of 82... which is quickly dragged down to 65 (audio cuts off)... and then cycles back up to 82 (audio goes back on).

Audio going in and out happens across both channels, indicating a big current draw. In one channel, I have a pair (out of four per channel) output transistors that get really, really hot compared to the others.

I've just started looking at it (and I'm more of a tube guy, so my solid-state-fu is a little on the low side) but any pointers would be greatly appreciated.
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Old 10th September 2013, 01:09 PM   #2
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What happens if you disconnect the speakers?
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Old 10th September 2013, 01:11 PM   #3
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Check the bias pots!
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Old 11th September 2013, 04:23 PM   #4
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My guess is one channel is breaking into oscillation - possibly because of defective electrolytics.
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Old 11th September 2013, 04:39 PM   #5
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I have seen such early amps do such strange slow voltage pumping when the caps are virtually just a bit of paper and foil rattling inside the can! Replace the caps first, then ponder what might be the point of knowing why a crippled amp behaves so badly or how hot the transistors can get before they snuff out.
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Old 11th September 2013, 06:51 PM   #6
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thanks all - will go ahead and replace the caps. Never seen this kind of issue with a vintage amp before, and I've worked on a number of them - but admittedly mostly tube.
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Old 11th September 2013, 07:29 PM   #7
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this could be a lot of things forcing one channel into overload and triggering the protection circuit.

Before committing to replacing caps, just "tack" a couple of similar values across the power rails. If this cures the problem THEN replace the caps!
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Old 18th September 2013, 12:40 PM   #8
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I ended up replacing all the small PCB mounted capacitors and even the speaker output caps. In the end - those circuit traces are fragile! - I got the amplifier working. The original PS cap is still in place and will eventually be replaced when I locate one of the appropriate diameter.

Just a note for anyone else tackling a TA-3120A, it appears to be different than the 3120. The latter "A" model doesn't appear to have a relay or safety circuit. Some of the small capacitor values are different too or not even on the PCB - like the 10uF input cap.

With a bevy of Elna Silmics and a pair of Elna LAO caps on the output, the end result sounded much better than I expected. Very nicely constructed unit but as mentioned above, work with a low temp soldering iron and be careful when running new component leads through since the (copper?) traces like to lift off the board.
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