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,
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,
Member
Joined 2002
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.
P.s Sony amps are the worst to work on. Well the newer ones are any way.
Jason.
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?
Jan Didden
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?
Jan Didden
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.
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.
PedroPO said:
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
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?
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?
PedroPO said:
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.
Jan Didden
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?
I measure the current and what is the maximum value?
sony
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).
Jan Didden
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).
Jan Didden
Is I can see the power resistors (0.33ohm), and I can measure the current.
the top value is 100mA right?
There are actually 4 trimpots on the amp and 4 output devices!( i allready messed with them, but the hum didnt go away. Maybe I put them on a wrong position afterwards,( I marked the position, but...)
I guess you have seen them all afterall!
The amp is a integrated amplifier in a silver brushed alu front pannel with two VU meters, a great volume knob.
I guess that its a class AB amp with feedback and tone controls. I don't know the output power, but I would guess around 40W/ch.
do you think like NP that the problem might be the PSU caps?
thanks again for the help
the top value is 100mA right?
There are actually 4 trimpots on the amp and 4 output devices!( i allready messed with them, but the hum didnt go away. Maybe I put them on a wrong position afterwards,( I marked the position, but...)
I guess you have seen them all afterall!
The amp is a integrated amplifier in a silver brushed alu front pannel with two VU meters, a great volume knob.
I guess that its a class AB amp with feedback and tone controls. I don't know the output power, but I would guess around 40W/ch.
do you think like NP that the problem might be the PSU caps?
thanks again for the help
Pedro,
I'm not sure we are communicarting. I did say in two posts that it might be the quiescent current because you said the sinks were very hot. That's why I suggested to check the standing current. That isn't the same as a broken PS cap, isn't it? It CAN be that Nelson is right and that it is a PS cap, but the very hot sinks IMO point to another problem. Let me know what current you measure? (How are you going to measure it?).
Jan Didden
I'm not sure we are communicarting. I did say in two posts that it might be the quiescent current because you said the sinks were very hot. That's why I suggested to check the standing current. That isn't the same as a broken PS cap, isn't it? It CAN be that Nelson is right and that it is a PS cap, but the very hot sinks IMO point to another problem. Let me know what current you measure? (How are you going to measure it?).
Jan Didden
Dear Jan Didden,
dont get me wrong!
I deeply apreciate your suggestion and I'm going to do it.
I am going to measure it with a clamp meter or if the current is to low for it i'll just measure the tension in the resistors terminals and then I=V/R where R=0.33ohms (as you mentioned).
I believe that hot sinks are a bad principle also.
after you info, I was thinking of exchanging the sink for a finned, bigger one, more, one for each device (to avoid insulation), but now I'll try your (more scientific) aproach.
thanks again,
dont get me wrong!
I deeply apreciate your suggestion and I'm going to do it.
I am going to measure it with a clamp meter or if the current is to low for it i'll just measure the tension in the resistors terminals and then I=V/R where R=0.33ohms (as you mentioned).
I believe that hot sinks are a bad principle also.
after you info, I was thinking of exchanging the sink for a finned, bigger one, more, one for each device (to avoid insulation), but now I'll try your (more scientific) aproach.
thanks again,
Hi Pedro,
I only know the Sony TA-F210/220 but those´ve got a integrated preamplifier (uPC1298V).
Those IC´s (one for each channel) get a separate supply voltage that goes through a regulator (pass transistor) before.
Maybe your hum comes from that specific power supply.
If you have this IC in your amp it´s maybe similar to the one I had and so I could send you the schematic.
Just give me a ring.
Regards
Jens
I only know the Sony TA-F210/220 but those´ve got a integrated preamplifier (uPC1298V).
Those IC´s (one for each channel) get a separate supply voltage that goes through a regulator (pass transistor) before.
Maybe your hum comes from that specific power supply.
If you have this IC in your amp it´s maybe similar to the one I had and so I could send you the schematic.
Just give me a ring.
Regards
Jens
A sony amplifier completely discrete?
That´s quite rare.
Mostly I´ve seen these uPC1298 or even LM3886 for the whole job.
But still the preamp could have separate power supply.
Regards
Jens
That´s quite rare.
Mostly I´ve seen these uPC1298 or even LM3886 for the whole job.
But still the preamp could have separate power supply.
Regards
Jens
sony
Hi Pedro,
The extra heatsinks would be a good idea, but are really only a temporary fix; they definitely should not be too hot to touch. And we still have that hum, I presume. I wouldn't bother with the clamp meter, too much risk to break anything, and the current as you already mentioned is probably too low to measure. Your method with the calculation is spot-on.
Jan Didden
Hi Pedro,
The extra heatsinks would be a good idea, but are really only a temporary fix; they definitely should not be too hot to touch. And we still have that hum, I presume. I wouldn't bother with the clamp meter, too much risk to break anything, and the current as you already mentioned is probably too low to measure. Your method with the calculation is spot-on.
Jan Didden
Caps
I would definitely check the big caps in the PSU. Also look for other caps all over the circuit that don't look "fresh" anymore, specially those that are near heat sinks (the get hot, don't they?) Well, the caps also get hot and they don't like it. When you see the plastic around the cap has shrunk or the cap is a little swollen at the top, replace it.
Also when you thake out a cap and looking at the print you see some wet acid, replace it.This and bad solder joints are the most common problems in old gear.
Hugo
I would definitely check the big caps in the PSU. Also look for other caps all over the circuit that don't look "fresh" anymore, specially those that are near heat sinks (the get hot, don't they?) Well, the caps also get hot and they don't like it. When you see the plastic around the cap has shrunk or the cap is a little swollen at the top, replace it.
Hugo
Pedro,
I have a Sony TA- 212 on a shelf here in my office.
This amp is rated at 40W/40W, and although is old, was not too bad in it's day, but not fantastic.
No time to open mine just now, but the 4 trimpots are for DC offset and bias current.
These sorts of amplifiers usually ran around 30mA or so bias current.
.030A x 0.33 ohms = 10mV across each emitter resistor.
Clip on, or soldered leads that make perfect contact with the resistor leads is required.
The trimpots can go noisey and cause overbias condition.
Another cause is dry solder joints.
My usual permanent fix is to just blanket resolder the output and power supply stages, adjust bias an DC offset, and then you will have a reliable amplifier.
Eric.
I have a Sony TA- 212 on a shelf here in my office.
This amp is rated at 40W/40W, and although is old, was not too bad in it's day, but not fantastic.
No time to open mine just now, but the 4 trimpots are for DC offset and bias current.
These sorts of amplifiers usually ran around 30mA or so bias current.
.030A x 0.33 ohms = 10mV across each emitter resistor.
Clip on, or soldered leads that make perfect contact with the resistor leads is required.
The trimpots can go noisey and cause overbias condition.
Another cause is dry solder joints.
My usual permanent fix is to just blanket resolder the output and power supply stages, adjust bias an DC offset, and then you will have a reliable amplifier.
Eric.
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