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Yamaha C-4 pre-amp help

O

Osito22es

New Member
Hello all,

I acquired a yamaha C-4, M-4, and T-1 tuner combo at an estate sale. I'm very excited about it.

The Yamaha C-4 pre-amp has no sound out but the headphone channel works fine.

I understand that the likley culprit is that TR621 is not closing the RY301 relay.

I have read many of the forums and done a lot of work on the muting circuit with no resolution.
Have replaced C313 (the common culprit), C312, all of the VD1212 double diodes in this circuit with two 1N4148 diodes in series
I replaced TR622 with a 2SC18115GR NPN TO-92.
I have resoldered many joints.

There had been work done on the board before, obviously, since C313 had been replaced with a 47 microfarad rather than 33 microfarad capacitor and TR622 had been replaced with a PNP transistor rather than NPN with emitter, collector, base pinout.

I still hear no relay click on power on.

The voltages I have for the rails include
+45 / +41
-45 / -40
-12 / -20 (I know this should be around -20)
D / -21
E / 0
+30 / +30
-30 / -30
0L / Black Lead

The voltages around the key transistors include (from left to right pins looking at the back (soldering side) of the board and relative to the black lead placed on 0L.

TR624
-12.7, -19.4, -12.1

TR623
-11.5, -19.4, -12.1

TR622
-5.6, -6.2, -6.2

TR621
-11.3, -11.2, -10.6

I noted the following voltages around C312 (-21 (negative) -> 0 (positive))
and C313 (0 (positive) -> -5.6 (negative)

I diode tested TR624 and TR623, and as far as I can tell, they are behaving as expected. I do get a little confused which is PNP vs NPN and EBC vs BCE. I only took them out to test them and put them back in the same orientation.

I'm stuck and cannot seem to find a solution.
I would be grateful for any help!
 
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Osito22es said:
Yes I did this. To be honest
My test of whether pre-out is working is whether I see levels on the M-4. I don’t have speakers hooked up but I don’t see any levels!
Click to expand...
might not be the preamp but the meters are not working on the amp.

Hook up your multimeter to the amps speaker outs and set it to measure AC , can you see the voltage change as you raise the volume with a signal into the preamp/amp combo?
 
Nashou66 said:
might not be the preamp but the meters are not working on the amp.

Hook up your multimeter to the amps speaker outs and set it to measure AC , can you see the voltage change as you raise the volume with a signal into the preamp/amp combo?
Click to expand...
Thank you and avionics for your help. Unfortunately voltages are 0 on the m-4 speaker out with attenuation nobs at 0 or at 10. I made sure the speaker buttons were on and the pre out buttons were engaged. I do feel like I hear a faint chirp on the c-4 on power on and power off but no signal that I can detect with the M-4. I don’t have another power amplifier to test it on. I am still able to hear audio through the headphone channel but curiously it now has a hum and the volume responds to the main volume nob now. I don’t think it is supposed to function that way. I’m worried I might have made some mistake. I didn’t think the tr621 voltages were what they were supposed to be but your responses make me think they look ok. Thanks again for your guidance!
 
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