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Help with Modular Component Series 3222 Receiver

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DannyBad

New Member
I have an MCS 3222. When I scope it, it distorts at the bottom of the wave much sooner than the top. Both channels are equal now after I re-capped it but something must still be wrong because they both still distort at the bottom of the waveform much earlier than the top. Here are the other clues:
1) When I turn the balance to the left the right channel goes away and the left stays the same. When I turn the balance all the way to the right, the left goes away but the right wave starts moving quickly.
2) DC offset is 46mV on the left channel and 28mV on the right channel.
3) Same problem happens on Aux inputs and tape inputs.
4) Same problem happens regardless of which frequency I send through it.
5) I re-capped everything except the tuner section which I plan on leaving alone.

Can anyone offer any advice or thoughts?

Thanks in advance.
 
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Don't find a manual online, but what you are seeing may be normal. I assume the distortion you are seeing is when the output is near maximum. You are never going to get a perfectly balance amplifier, one side or the other of the output waveform is going to start clipping first. In that amplifier, it just may be that the negative side clips first by design.

As for the balance, it is not unusual for the balance pot to be a dual lot/anti-log pot. When you turn it to the left, the left side increases only slightly while the right drops significantly. By design.
 
DannyBad said:
I have an MCS 3222. When I scope it, it distorts at the bottom of the wave much sooner than the top. Both channels are equal now after I re-capped it but something must still be wrong because they both still distort at the bottom of the waveform much earlier than the top. Here are the other clues:
1) When I turn the balance to the left the right channel goes away and the left stays the same. When I turn the balance all the way to the right, the left goes away but the right wave starts moving quickly.
2) DC offset is 46mV on the left channel and 28mV on the right channel.
3) Same problem happens on Aux inputs and tape inputs.
4) Same problem happens regardless of which frequency I send through it.
5) I re-capped everything except the tuner section which I plan on leaving alone.

Can anyone offer any advice or thoughts?

Thanks in advance.
Click to expand...
Clean all your controls (POTs), especially the balance control using FaderLube. Then if it still seems to act up, we'll continue.
 
I cleaned all of the pots. When I started the left and right channel were different but now they are the same. The balance pot seems to be fine unless I turn it all of the way to the right. My main concern is the distortion at the bottom of the wave. It seems to be a bit extreme and distorts on the bottom much sooner than the top.
 
DannyBad said:
I cleaned all of the pots. When I started the left and right channel were different but now they are the same. The balance pot seems to be fine unless I turn it all of the way to the right. My main concern is the distortion at the bottom of the wave. It seems to be a bit extreme and distorts on the bottom much sooner than the top.
Click to expand...
It is indicative of DC balancing being off to have the wave square off on one side before the other. A possibility is one bad, or going noisy diode in the rectifier bridge that feeds the negative rail. Or possibly a weak ground. These kinds of things aren't always very easy to find, but sometimes you end up with a forehead slap after stumbling upon the cause. Given the age, it is very possible there is some silicon beginning to fail somewhere. The trick is finding it. Some freeze spray might help.
 
quaddriver said:
clean any switchy things too...like tape mon.

can you snap a hi-res photo of the amp section and send it?
Click to expand...
Absolutely. And sometimes pots need 2 or 3 flushings to actually behave. The balance pot improving in a good sign and may need another dose.
 
DannyBad said:
I have an MCS 3222. When I scope it, it distorts at the bottom of the wave much sooner than the top. Both channels are equal now after I re-capped it but something must still be wrong because they both still distort at the bottom of the waveform much earlier than the top. Here are the other clues:
1) When I turn the balance to the left the right channel goes away and the left stays the same. When I turn the balance all the way to the right, the left goes away but the right wave starts moving quickly.
2) DC offset is 46mV on the left channel and 28mV on the right channel.
3) Same problem happens on Aux inputs and tape inputs.
4) Same problem happens regardless of which frequency I send through it.
5) I re-capped everything except the tuner section which I plan on leaving alone.

Can anyone offer any advice or thoughts?

Thanks in advance.
Click to expand...
Did you thoroughly examine all solder joints? As old as it is, I would be hitting each and every one just to eliminate them as being a possible cause. Also, since the TO-3 output transistors have the wires soldered directly to the transistor leads, and the collector wire on a solder tab, check those too. A loose TO-3 collector screw may not be getting a solid connection.
 
Skywatcher said:
Did you thoroughly examine all solder joints? As old as it is, I would be hitting each and every one just to eliminate them as being a possible cause. Also, since the TO-3 output transistors have the wires soldered directly to the transistor leads, and the collector wire on a solder tab, check those too. A loose TO-3 collector screw may not be getting a solid connection.
Click to expand...
I have not gone through every solder joint. I will check on your suggestions.
 
Skywatcher said:
It is indicative of DC balancing being off to have the wave square off on one side before the other. A possibility is one bad, or going noisy diode in the rectifier bridge that feeds the negative rail. Or possibly a weak ground. These kinds of things aren't always very easy to find, but sometimes you end up with a forehead slap after stumbling upon the cause. Given the age, it is very possible there is some silicon beginning to fail somewhere. The trick is finding it. Some freeze spray might help.
Click to expand...
Freeze spray? What's that?
 
ok, based on the pics, lets start simple - rebuild the power supply. change EVERY cap in there, especially the tall gray burned looking ones.

change the 4 diodes that form the bridge...I dont know what they say on them but 1N5405 or equivalent should cover everything up to 3 amps, else find 6A10 or equivalent to cover up to 6 amps. this is a 60hz circuit so ultra fast recovery not required.

then test again, but looking at the amp, the low hanging fruit in there seems to be the input pairs which I suspect are around Q501/502 and Q503/504 if my eyesight works on the pic (it blew up poorly)

IF they are 2SAxxx or Axxx on them match up 4 KSA992F (TA/BU) to within 1% on the trusty hfe meter (they will test out generally between 370-410 on my meter) 1%=4

that alone should clean up most of that 46/26mv dc offset. Not worried about VAS, drivers or outputs at this time.

A DMM should tell you right now, today, if the +/- rails are within the same ballpark.

Report back...
 
quaddriver said:
ok, based on the pics, lets start simple - rebuild the power supply. change EVERY cap in there, especially the tall gray burned looking ones.

change the 4 diodes that form the bridge...I dont know what they say on them but 1N5405 or equivalent should cover everything up to 3 amps, else find 6A10 or equivalent to cover up to 6 amps. this is a 60hz circuit so ultra fast recovery not required.

then test again, but looking at the amp, the low hanging fruit in there seems to be the input pairs which I suspect are around Q501/502 and Q503/504 if my eyesight works on the pic (it blew up poorly)

IF they are 2SAxxx or Axxx on them match up 4 KSA992F (TA/BU) to within 1% on the trusty hfe meter (they will test out generally between 370-410 on my meter) 1%=4

that alone should clean up most of that 46/26mv dc offset. Not worried about VAS, drivers or outputs at this time.

A DMM should tell you right now, today, if the +/- rails are within the same ballpark.

Report back...
Click to expand...
Thank you for the details on this. I have already re-capped the entire receiver except for the tuner section. The re-cap made everything better but after capacitors, my knowledge is limited. Your explanation is really good and is leading me into understanding the diodes and transistors. I will get back to this as soon as I have some time. I really appreciate your insight and clear explanation.
 
Skywatcher said:
I use canned air with the can upside down. It sprays liquid that will freeze things. Just a spritz of liquid is all it takes to quickly drop the temperature of a component to watch for changes on the scope.
Click to expand...
And when one changes quickly, that tells you that it is the component with something wrong? That makes sense. It would show an overheating component and an overheating component is a possibility for the problem.
 
DannyBad said:
And when one changes quickly, that tells you that it is the component with something wrong? That makes sense. It would show an overheating component and an overheating component is a possibility for the problem.
Click to expand...
It doesn't necessarily mean a component is running hot. But rather can show it is struggling to operate properly.
 
quaddriver said:
ok, based on the pics, lets start simple - rebuild the power supply. change EVERY cap in there, especially the tall gray burned looking ones.

change the 4 diodes that form the bridge...I dont know what they say on them but 1N5405 or equivalent should cover everything up to 3 amps, else find 6A10 or equivalent to cover up to 6 amps. this is a 60hz circuit so ultra fast recovery not required.

then test again, but looking at the amp, the low hanging fruit in there seems to be the input pairs which I suspect are around Q501/502 and Q503/504 if my eyesight works on the pic (it blew up poorly)

IF they are 2SAxxx or Axxx on them match up 4 KSA992F (TA/BU) to within 1% on the trusty hfe meter (they will test out generally between 370-410 on my meter) 1%=4

that alone should clean up most of that 46/26mv dc offset. Not worried about VAS, drivers or outputs at this time.

A DMM should tell you right now, today, if the +/- rails are within the same ballpark.

Report back...
Click to expand...
The transistors in this power section say BC559B. Can I replace them with what you're suggesting?
 
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