Another Yamaha C-4 Audio Muting Circuit Thread - Maybe

[craigsc]

I recently picked up a nice Yamaha C-4 at a local auction house. Good cosmetic condition on the inside and out, but it has a problem of not initially playing for several hours after I turn it on. From what I've researched I seem to have the symptoms of bad diodes and caps in the muting circuit. I've read a few posts here with similar symptoms. Before I order the parts and make the swaps I wanted to get a little confirmation from the experts or those that have had these symptoms before.

1. When I initially turn on I get no sound on any source. If I crank it all the way up with source connected to aux for example, I can faintly hear the input on both channels through the headphones.

2. After many hours unit works fine and I get sound from all the channels.

3. I've checked the power supply voltage (30.5 initially but adjusted to 30.0) and the DC offset (around 500 mv on both channels and adjusted to less than 100).

4. I've reflowed the solder throughout the power supply and flat amplifier boards just to clear up any problems (no change).

Should my next step be to replace the caps and diodes in the muting circuit?

Looks like,

D 306, 317, 318, 304
C 312, 313, and 331

Anything else?

Thanks,
Craig

 

[avionic]

Looks like,

D 306, 317, 318, 304
C 312, 313, and 331

Anything else?

D303 as well.

 

[avionic]

My bets on the caps..

 

[craigsc]

Thanks Avionic, will report back on the results.

 

[Jazzmuso]

Avionic needs A PLACE IN THE YAMAHA HALL OF FAME - such encyclopaedic knowledge of the components of so many models. He reminds me of a good professor who really knows his specialisation or field. I feel privileged to have even this remote association with him. Thanks Avionic

 

[craigsc]

Well replaced the caps and diodes in the muting circuit and am getting the same results. Something I didn't report before, because I hadn't really checked and assumed that the lamps were all out is that I don't get any lamps lit. So symptom is the same, very faint sound through both channels, louder sound right when you start up but then it dies down, and no lamps lit. Seems to me like a power supply issue, although I have 30 volts on the nose and its easily adjustable. Any other ideas?

 

[avionic]

Measure your voltages around TR623 B,C&E. It's the relay driver transistor. It has a small heatsink. When the C-4 acts up. ....
Are the lamps actually burnt out ?

Ohm them out , if in doubt.

 

[avionic]

Next time it acts up..Also might try. Tapping on the 3 reed relays with the plastic end of a screwdriver. The reed relays are silver colored metal boxes. Two are on the right hand side of the rear circuit board. The other one is right next to the rear of the volume control.

 

[craigsc]

I've tried tapping the relays with no effect. Measured voltage at the b,c,e of TR 621 and get -.7 at all 3 with the unit on. The bulbs were not actually burned out. This morning I did find what appeared to be a couple of bad diodes at d313 and d314 and replaced with double 1N4148s. This didn't seem to help, in fact now I have voltage at 41 v at test point 7,8 and I can 't adjust. Still have the same symptom of louder sound when I first turn on and then it fades to very quiet. Seems similar to a thread back in Jan where you helped willg restore a C-4. Not crystal clear what fixed his problem, but he did replace transistors throughout and all the diodes, include d311 which I haven't replaced. What's the correct replacement for d311?

 

[avionic]

Measured voltage at the b,c,e of TR 621 and get -.7 at all 3 with the unit on

Black test probe connected to chassis ground and probing transistor with the positive test lead ?

 

[avionic]

What's the correct replacement for d311?

D311 is a VD1212 varistor. -- Two 1N4148 diodes in series usually does the trick.

 

[avionic]

This morning I did find what appeared to be a couple of bad diodes at d313 and d314 and replaced with double 1N4148s.

D313 and 314 ..Should have been replaced with one 1N4148 in each position.

 

[avionic]

Measured voltage at the b,c,e of TR 621 and get -.7 at all 3 with the unit on.

Did you check the fuses under the transformer cover panel ?

 

[craigsc]

Don't know how I got confused on the diodes. Replaced D313 and D314 with one 1N4148 and D311 with a Double. Measured the jumper voltages:

+45: 43.1
-45: -42.7
-12: -21.8
+30: 40.9
-30: 1.56

and

T626 is -41.3, -42.6, and -40.7

consistent with being the 40.9 on the 30 volt rail I guess. Checked the fuses and they were all good.

 

[craigsc]

To confirm, what type of diodes should I have been using in:

D 306, 317, 318, 304?

 

[avionic]

+30: 40.9
-30: 1.56

That doesn't look good.

 

[avionic]

D306 -- 1S1555 --- 1N4148
D317 -- WZ130 ------- 13 volt zener@500mw
D318 -- VD1212 -- 1N4148 X 2 in series
D304 -- HZ6C --- 6 volt zener @ 500mw

 

[avionic]

-12: -21.8

Thats the correct voltage that should be at that point.( -22 vdc approx.)

 

[craigsc]

I used BZX79 Zener that I had on hand instead of the 13 and 6 you have listed. Would that cause the +30, -30 voltages to be off?

 

[avionic]

Would that cause the +30, -30 voltages to be off?

Regulated +/- 30 PS isn't working.