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Advice re: Vintage Sony Amplifier drifting out of spec?

ArthurTut

ArthurTut

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I have a Sony TA-3120F amplifier that was completely recapped by a professional two years ago.

Last night, Saturday, 8/8/26, I noticed something weird with one of my speakers. The left was not playing as loud as the right one. Then, I turned the left up and I heard distortion in the left.

I spent about two hours taking all the wires off and putting it all back together and testing along the way. And then both speakers seemed to be working fine. I thought maybe it was just a loose wire.

However, on Sunday morning, 8/9, the problem came back or persisted. I spent another 2 hours testing and swapping components. I discovered my Sony TA-3120F amplifier is out of spec somehow.

The left channel has -164 mV to -176 mV of DC on it. (should be 0 millivolts).
The right channel has -225 mV to -235 mV of DC on it. (should also be 0 millivolts).
(Not good. How bad this is, idk.)

The TA-3120F amplifier is almost always played very softly, so the problem went unnoticed for an undetermined time. But, on Saturday night (8/8/26) when I turned it up, I noticed the right channel was louder. Tired to use balance on my TA-2000 preamplifier and the left/right volume control on TA-3120F amplifier to match L/R sound. But turning left channel up to match the right caused the noticeable distortion in the left.

When the problem returned or persisted on Sunday morning, I isolated problem to the TA-3120F amplifier because I have two sets of speakers on it (so, tests show the problem is not the main or remote speakers) and two amps on this preamp (so it’s not the TA-2000 preamplifier). I removed the TA-3120F and installed a different amplifier (a TA-3200F) in its place. System function restored. It must be this TA-3120F.

After I pulled the TA-3120F amplifier out of the system, I tested it with a multimeter. See the test results above.

Please post any comments you think might help me. Thank you.
 
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This thread has useful information, especially if you plan to refurbish.

bobdod

Thread 'Sony TA-3120F Japanese Market Power Amp Refurbishment'

I am starting this thread to document, and hopefully get some help, on refurbishing a little gem I picked up a few years ago. The model in question is a Sony TA-3120F power amplifier. This unit was not sold in North America as far as I know, and perhaps not even outside Japan. From research I have done, I believe this amp was available in Japan between 1969 and 1971. It is a predecessor to the widely acclaimed TA-3200F, and has an almost identical exterior package, but retains some of the (IMO) nicer build quality of the original late 1960s original "ES" line. This example is all original...
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Update. Problem solved. One resistor had drifted out of spec. It has been replaced.
 

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Also, the bias and DC offset have been adjusted, so the idle current and output voltage are now properly calibrated; close to 0 millivolts of DC.
 

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I love this unit. I have used it for several years, and I don't plan on parting with it. I would really like to find a service manual, even if it's only in Japanese.
 

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curtisw99 said:
Sheesh! All that DC from one resistor ...
Click to expand...
No. Probably not.
In fact, the problem does not appear to be resolved.
I hadn't started using it again, but I warmed it back up to test it again today.
I tested it again and some DC is back on the outputs.
The left channel has -90 mV of DC on it.
The right channel has -85 mV of DC on it.
I will try to adjust these again and see if I can get these closer to 0.
 
I adjusted the timmer pots again until both channels are as close to zero as I can get them. However, the heatsink on the amplifier got very warm in this process. I tested the amplifier with speakers afterwards, and it sounds fine. However, the temperature concerned me, so I turned it off. I recall it running warm before, but I don't recall it getting this warm before. I wonder if this is an issue.
 
I didn’t measure it. It just felt hot. I could touch it but not comfortably keep my finger on the heatsink for too long.
 
It would help to find out the temperature rating of the transistors attached to the heatsink. It's possible the temperature could be well within their max temp rating.
 
eiraved said:
It would help to find out the temperature rating of the transistors attached to the heatsink. It's possible the temperature could be well within their max temp rating.
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Eiraved. Thank you. I will have to do some research into this question before revisiting this amplifier. I don’t have this information at this time.
 
I decided to back those trimmer potentiometers off to about where I started this afternoon. I let the amplifier warm up for about 20 minutes and I ended up with about -40 mV DC on each channel. Then I let it play for about 15 minutes. It sounded fine and it did not get significantly hot at all. (The heatsink for the right channel does get a little bit warmer than the one for the left channel; could be some imbalance there?). I disconnected it again for now though. I will consider additional tests later to make sure it’s going to stay working. I wish I had the service manual. Even using the one for the TA-1120F is a problem because it’s in Japanese and not English. I spent a couple of weeks in Tokyo in 2018, but I never learned to read Japanese.
 
ArthurTut said:
I decided to back those trimmer potentiometers off to about where I started this afternoon. I let the amplifier warm up for about 20 minutes and I ended up with about -40 mV DC on each channel. Then I let it play for about 15 minutes. It sounded fine and it did not get significantly hot at all. (The heatsink for the right channel does get a little bit warmer than the one for the left channel; could be some imbalance there?). I disconnected it again for now though.
Click to expand...
I'd consider that a win, unless OCD takes over because of the slight temperature difference.
 
Thanks, Eiraved. I am concerned that the bias adjustment is not correct. I am considering revisiting that issue. Not sure yet though.
 
ArthurTut said:
(The heatsink for the right channel does get a little bit warmer than the one for the left channel; could be some imbalance there?).
Click to expand...
Don't guess, get a infrared thermometer and verify the temperature difference. There very well may be a slight difference, 5° or possibly less. It's not unusual.
 
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