Help! Bizzare Yamaha C4 Shut down issue

Markus111

Big Horns & Many Tubes
Subscriber
I recently acquired a C4, and have been going through it slowly recapping it and replacing the VD1212 with two 4148 diodes in series. Everything works well and sounds great, EXCEPT when the tone bypass switch is depressed. When I depress the bypass switch, the sound changes radically (very bass heavy), and then about 20 seconds later the preamp shuts down. All the lights go out, and no sound. If I turn it off, and reset the tone bypass switch, all is well. This is a real head-scratcher for me. Any help would be appreciated!

Mark
 
Just to verify, does it occur when the tone control is bypassed (the green light on) or when t.c. is in the signal path (green light off)?
 
:scratch2:

:lurk:

I'll bet the +/- 30 vdc regulated supply is dropping out. Ask me why?
 
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This occurs when the green light is on (bypassed). The sad part is that I can't remember if it worked while it was bypassed before. I know it worked ok with the tone controls engaged before I started working on it, but I'm not sure that I ran it through its paces with them disengaged.

I did find one 2.2 uF cap that was completely smoked on the flat amp board. I don't know the reference number, but it was sitting right next to a heatsink. I had assumed it was damaged by heat.

When the amp goes kaput, the 30.5v drops a bit, down to 29 or 30 volts.

So far, I have only recapped the power board and the flat amp board - I haven't got as far as the function board yet.
 
This occurs when the green light is on (bypassed). The sad part is that I can't remember if it worked while it was bypassed before. I know it worked ok with the tone controls engaged before I started working on it, but I'm not sure that I ran it through its paces with them disengaged.

I did find one 2.2 uF cap that was completely smoked on the flat amp board. I don't know the reference number, but it was sitting right next to a heatsink. I had assumed it was damaged by heat.

When the amp goes kaput, the 30.5v drops a bit, down to 29 or 30 volts.

So far, I have only recapped the power board and the flat amp board - I haven't got as far as the function board yet.

Thats different than this one. When it acts up, I lose the regulated supplys completely.This one is so intermittant .Might act up once in a while.
 
Yours may just have a problem with the -12 volt supply feeding the muting relays and muting circuit as well as the indicator lamps.

Only caveat is. I don't see the tone switch tie in.Other than the lamp.Might check for solder bridge on that switches solder connections.
 
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Thanks for the comments so far - it has given me some things to look at. Now the plot thickens...... I spent some time yesterday working on it, and it appears that all is well with the bypass switch EXCEPT - when the headphones are plugged in! If the headphones are in and the bypass is activated (light on), the sound through the headphones can be described as lots of bass with distortion. I'm guessing that there is something wrong on that board........
 
Thanks for the comments so far - it has given me some things to look at. Now the plot thickens...... I spent some time yesterday working on it, and it appears that all is well with the bypass switch EXCEPT - when the headphones are plugged in! If the headphones are in and the bypass is activated (light on), the sound through the headphones can be described as lots of bass with distortion. I'm guessing that there is something wrong on that board........

The headphone amplifier has 4 - 2SC458 transistors that I usually replace, as well as 10 electrolytic caps .First try to deoxit the HP volume level control,for your distortion issue.
 
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Some definite progress here. The shutdown issue magically disappeared. Ok - it probably wasn't magic. I've now recapped all of the boards except the headphone board - I'll start that tonight. One thing that I did end up replacing that I didn't see on any of the other threads were the 820 ohm resistors that couple power into several of the circuits. They showed a lot of heat wear, and all of the solder joints on these resistors looked pretty weak. I found a couple of solder joints where the solder basically crumbled away.....

Again, thanks for all of your help. I'll let you know how the headphone circuit goes....
 
Look close at all those solder joints. I have 4 C-4's and every one has had bad solder joints. :thmbsp:
 
Some definite progress here. The shutdown issue magically disappeared. Ok - it probably wasn't magic. I've now recapped all of the boards except the headphone board - I'll start that tonight. One thing that I did end up replacing that I didn't see on any of the other threads were the 820 ohm resistors that couple power into several of the circuits. They showed a lot of heat wear, and all of the solder joints on these resistors looked pretty weak. I found a couple of solder joints where the solder basically crumbled away.....

Again, thanks for all of your help. I'll let you know how the headphone circuit goes....

Those 820's are 4 sets of 4ea -820s in series-parrallel-- basically the 4 together represent one 1 watt 820 ohm for dropping the rail voltages from 30 volt to 6 volt for the MC amplifier rails.
 
This has turned out to be one of my most complex projects yet. In a nutshell, it still has a problem, just a different one now. I'll keep working on it, and updating as I go along.

This is the summary below:

1.) started off with a strange problem, where the headphone would become very bassy and distorted when the tone-bypass was pushed in (green light on).

2.) Recapped the whole unit, and the headphone problem was still there.

3.) Adjusted the 30.5 volt and dc-offset voltage. I noticed that the dc-offset voltage would swing wildly if I pushed down too hard on the adjustment pot. I determined that pushing on the circuit board anywhere near the pot would cause the problem. I resoldered the components near the pot, and the problem went away!

4.) I thought I would try resoldering components on the flat amp board - that bypass switch was still puzzling me. After resoldering every component on the flat amp board, I have a new problem. Now when the bypass switch is out (light off), there is a low level hum from the output. When the switch is in (light on), everything sounds good. However, I can no longer adjust the 30.5 volts. Measuring on the tie points, it is 38 volts when the tone bypass is out (light off), and 30 volts when the tone bypass is in. The adjustment pot has no effect in either position. So I'm guessing i got too much heat on something while soldering. I'll start measuring voltages again tonight.

Mark
 
.) I thought I would try resoldering components on the flat amp board - that bypass switch was still puzzling me. After resoldering every component on the flat amp board, I have a new problem. Now when the bypass switch is out (light off), there is a low level hum from the output. When the switch is in (light on), everything sounds good. However, I can no longer adjust the 30.5 volts. Measuring on the tie points, it is 38 volts when the tone bypass is out (light off), and 30 volts when the tone bypass is in. The adjustment pot has no effect in either position. So I'm guessing i got too much heat on something while soldering. I'll start measuring voltages again tonight.
RechecK your soldering. Most likely you have a solder bridge across one or more connections.
 
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