Yamaha C4 help needed

dirtyblues

Member
Hi

I've had a Yamaha C4 preamp in the basement for a few years now. Nice unit. Used to be in my father's system. Then one day it wouldn't turn on.

At the time I thought maybe a fuse had blown but it wasn't that and I forgot about it.

So yesterday I decided it was a shame to let that thing gathering dust. I opened it and tested:

Continuity on the fuses --- ok
The power cable --- ok, tested with a lightbulb
The transformer --- checked the resistances on the primary and secondary, seems ok
Checked the power switch --- ok

I also unsoldered the lead wires on the power board to examine it. I see what I would call leaky capacitors. (see pics)

Note that I know very little about electronics, I have a multimeter and the service manual, I should be able to test a resistance or a voltage between two point of the circuit but that's about it.

Because it stopped working suddenly my hunch is that it's a power supply thing. Is there a thing I can test on the power board?

thanks in advance
 

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Need to check some voltages.. + / - 30 volt regulated DC PS and the -12 volt regulated DC power supply..
 
Reinstall that board..The regulated power supply is not on that board.
 
You have the manual..do the 30 volt power supply adjustment on the alignment page. That will verify the 30 volt supply.
 
hi avionics

thanks for the prompt answer. ok. so is the regulated ps on the flat amp board?

if so, it is a bit difficult to examine without unsoldering from the function board. or is it better to look at it from the bottom?
 
hi avionics

thanks for the prompt answer. ok. so is the regulated ps on the flat amp board?

if so, it is a bit difficult to examine without unsoldering from the function board. or is it better to look at it from the bottom?
You don't need to unsolder any boards--the function board and tone board unplugs.The remaining boards are accessable without removal.
 

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thanks

i see the tp7 and tp8, from the bottom, will be able to do it. but first i need to resolder that power board...
 
Chances are real good that all three lamps are burnt out.

just'n'case 14.5v@80ma
 
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Check the solder connections on the two pass transistors mounted on the two large heatsinks.Reflow the solder recommended.. See the fine red lines in photo to locate the 6 solder pads. If your heatsinks don't have the 4 screws - find some ! The heatsinks are already threaded.Some units have the screws-some don't. If they don't, the transistors can loosen up and crack the solder connections. I usually replace the heatsink compound between the transistors and heatsinks. Do not install with mica insulators , just thermal compound! The white stuff is fine.
 

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I resoldered the two transistors but I still measure around .45V ...

my heatsinks didn't have screws in them, nor were they threaded, they look like they used to be glued to the board, but now it's more like dirt. I put everything back in place with small screws that fit in the holes and son threadlock to hold them in place.

Any other ideas?
 
Oh yeah..stand by..Measure the voltages on ..The E,B,C of TR626-629. With the neg test lead clipped to chassis ground. Probe voltages with the Pos test lead.
 
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just to be sure: the 629 is one on the two i have resoldered, right? and 626 is the one right next to the other one, they are marked 29 and 26 on the flat amp board 1.

also, I measured using diode/transistor metering on my meter, with the preamp unplugged. and chassis ground is just the probe touching the chassis.

so if all above correct:

629 -- b=.60v c=0 e=0
626 -- e=0 c=.58v b=0

hope i got the pins right


petehall347, it is the light, I have tested continuity across each fuse.
 
you might well be better off checking the voltages with power on rather than the way you are testing ..
 
just to be sure: the 629 is one on the two i have resoldered, right? and 626 is the one right next to the other one, they are marked 29 and 26 on the flat amp board 1.

also, I measured using diode/transistor metering on my meter, with the preamp unplugged. and chassis ground is just the probe touching the chassis.

so if all above correct:

629 -- b=.60v c=0 e=0
626 -- e=0 c=.58v b=0

hope i got the pins right


petehall347, it is the light, I have tested continuity across each fuse.

As pete has already stated these voltages need to be checked with power applied. Using DC volts function on the meter.
just to be sure: the 629 is one on the two i have resoldered, right? and 626 is the one right next to the other one, they are marked 29 and 26 on the flat amp board 1.
Yes..
 
Did you check TP7 and TP8 (30 volt PS) with power applied? I hope...
 
When you reinstalled that board that you removed did you resolder all the little jumper wires to the flatamp board. Especially the ones labeled + 45 and -45 and -12 ?
 

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