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Old 20th January 2003, 03:02 AM   #1
PedroPO is offline PedroPO  Portugal
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Default Help on sony Amplifier

Hi guys!

I'm searching for help with a repair that I'm trying to make on a old Sony amp from the 80's.

It is Sony TA-212a

It had a major hum in one channel, even with volume at minimum.

I fixed that by resoldering the power transistors and the volume control - I dunno what was wrong, but the hum stoped.

Now it has a minor but annoying hum that is stronger on one channel.

I tried to see the references on the transistors, but I guess they are coded, since I performed a search on google and I cannot find them, so I will not mess with that unless one of you might help me in some way.

Is it possible that the hum is "not a problem"? its a old amp!!

What else may be wrong? The PSU is common for both channels,
no burned stuff.

it seems to be two power devices /ch, since I detect a simetry on the boards and they are attached to a heatsink that burns like hell! ( its a small heatsink).

Could a cap or resistor be damaged and not visible from the outside?

Thanks,
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Old 20th January 2003, 05:25 AM   #2
jleaman is offline jleaman  Belgium
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I'd say it is a Bad cap. Also look for bad solder joints.


P.s Sony amps are the worst to work on. Well the newer ones are any way.

Jason.
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Old 20th January 2003, 05:28 AM   #3
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Old electrolytic in power supply?
Check the ripple while jiggling the caps.
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Old 20th January 2003, 07:41 AM   #4
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Default Sony

Did you say the heatsinks "burn as hell"? If it really gets too hot (and remember, if you barely can touch it without burning your fingers it still may not be above 50 degr Celsius), it could it be that the quiescent current is way too high. Such a constant high load current often causes hum as well. Are both heatsinks at the same temp?

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Old 20th January 2003, 11:48 AM   #5
PedroPO is offline PedroPO  Portugal
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thanks guys!


When you say bad caps, do you mean electrolytic, ceramic or both?

In the PSU or the main board?

The PSU has two big elna 6800uF caps that are glued to the board, and they look just fine.

The PSU like I said is common to both channels. Is it possible that the Hum (low freq only) is bigger on one channel that the other?

For the bad solder joints, I actually resoldered the hole board just in case, but didn't help

janneman:

the sinks are TOO HOT. trust me. I built a SOZ and he was hot too, but had more 8kg of heatsinks heheheh ( this sony has a little alu plate with 150g for BOTH channels!!

I guess none of the sony designers work at Passlabs now

If I cant fix it I guess I put in the beautifull sony case a Zen amp.

I'll post a picture of the case and the boards later.
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Old 20th January 2003, 11:56 AM   #6
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Quote:
Originally posted by PedroPO

The PSU has two big elna 6800uF caps that are glued to the board, and they look just fine.

the sinks are TOO HOT. trust me. I built a SOZ and he was hot too, but had more 8kg of heatsinks heheheh ( this sony has a little alu plate with 150g for BOTH channels!!

Sounds like a pretty craptacular amplifier to me.. similar to my techincs one, except that is only 30watts, and doesn't get hot at ALL!! But the caps are teh same value.. lol.. perhaps these could do with an upgrade if you get rid of the noise.. lol
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Old 20th January 2003, 12:06 PM   #7
PedroPO is offline PedroPO  Portugal
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Yes its a crappy amp, but its old and deserves some respect just for that.

I'm trying to fix it for the challenge also.

I really dont know much of electronics, just ohm's law as I usually say!

A simple test , as NP said is to replace the PSU caps for new ones.

what about the diodes? could they be a problem also?
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Old 20th January 2003, 02:58 PM   #8
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Quote:
Originally posted by PedroPO
thanks guys!
[snip]the sinks are TOO HOT. trust me. I built a SOZ and he was hot too, but had more 8kg of heatsinks heheheh ( this sony has a little alu plate with 150g for BOTH channels!![snip].
Well, in that case, do check the quiescent current. Too much of that will heavily load the supply which can definitely cause hum without anything being broken, really.

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Old 20th January 2003, 03:02 PM   #9
PedroPO is offline PedroPO  Portugal
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how can I do that? I dont know what the transistors are(they are coded)!

I measure the current and what is the maximum value?
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Old 20th January 2003, 03:11 PM   #10
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Pedro,

Work your way back from the speaker cable, through possibly a speaker switch and/or relay to the output circuitry. I bet you get to the junction of two largish resistors, which would be the current sense resistors. You should measure the voltage across each (no signal) and then check or measure their value, then calculate the current through them. Unless it is a class A amp, that current should be something like 20 to 50 mA, certainly less than 100mA.

If it is too high, the fun starts. You will have to find the trimpots that adjust the quiescent current. There may be two trimpots per channel, one for the quiescent current and one for the output offset. But first see if you can mneasure Iq. What type Sony is it? Right, TA-212A. Don't know that one, but as one of my (former) girl friends once said, if you've seen one, you've seen them all (just joking of course).


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