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Yamaha C-4 Preamp Recap

OK. Removed two transistors ( TR632 and 633 ). Powered up and still same result.

I did a search 'TR632' and found a number of threads with this issue. Like this one:

Yamaha C4 help needed

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...
audiokarma.org

Am I taking these measurements with TR632 and 633 removed? Is this a good place to start?:
"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." "voltages need to be checked with power applied. Using DC volts function on the meter."
 
Jake O Bob said:
Am I taking these measurements with TR632 and 633 removed?
Click to expand...
Reinstall them. Make sure you get them installed correctly in the correct locations.

Disregard that thread.

Follow my directions no more ,no less.
Do you know how to use a schematic ?
 
1704945190077.png

This is the actual +/- 30vdc regulated power supply circuit. Located on the " Flat amplifier" circuit board.

+/- 45vdc is and unregulated supply for the headphone amplifier.


Note : Typo in the schematic. The ( base of TR626 and/or emitter of TR628 ) should be around -30 vdc not -42.1vdc..
 
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Using TP-8 for your meter ground connection. Measure the DC voltage at TP-7(emitter of TR627) and the emitter of TR626. These two transistors are mounted on the two largest heatsinks.

1704947015832.png
 
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Measure the emitter voltages of TR626 and 627. Looking for + & - 30vdc..
 
Attached photos to make sure I'm doing this correctly. All of these measurements were taken with multimeter grounded to TP8 and preamp powered on.

TP7 33.1v
TR626 Emitter 1.4v
TR627 Emitter 1.9v
 

Attachments

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  • TR626 Emitter.JPG
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  • TR627 Emitter.JPG
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Missing your -30 volt rail.. Remove TR632 transistor and remeasure..

TR626 Emitter 1.4v

--------------------------------------------------------------------------------------------


If you have + 33 volts at TP-7. Then you have +33 volts at emitter of TR627..
 
BTW there is and adjustment to get it back down " in spec " to +/- 30vdc.. Which you can do. After we figure out what is killing the negative rail. Adjustment won't fix that.

Page 8 , step 1 in the service manual.
 
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Jake O Bob said:
TR626 Emitter 1.4v
TR627 Emitter 1.9v
Click to expand...
Your not measuring on the correct transistors !! TR626 and TR627 are the two TO-220 transistors. Mounted to the two largest aluminum heatsinks on the left side of the front, lower, main circuit board.
(2SB566 and 2SD476)

TO-220 transistor

1705067816767.png
1. base
2.collector
3.emitter
 
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avionic said:
Missing your -30 volt rail.. Remove TR632 transistor and remeasure..



--------------------------------------------------------------------------------------------


If you have + 33 volts at TP-7. Then you have +33 volts at emitter of TR627..
Click to expand...
TR632 and TR633( crowbar ), TO-92 transistors are located on the right hand side of those same two large aluminum heatsinks..

TO-92 case style
1705069519130.png
1705079000727.png
 
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Oh, yea I didn’t realize there were other transistors labeled the same on different boards. This is what i was seeing and working from before. Thanks for clarifying and for being patient with me here. I’ll get back to this tomorrow.
 

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Jake O Bob said:
Oh, yea I didn’t realize there were other transistors labeled the same on different boards. This is what i was seeing and working from before. Thanks for clarifying and for being patient with me here. I’ll get back to this tomorrow.
Click to expand...
Hmm . I can see where that would be a little confusing..I hate it, when that happens ! I hadn't noticed that error/typo/mistake before.. Probably because I've never had any issues in that part of the pre-amp.:idea: I have at least twenty or more of these C-4 control amplifiers under the belt so far.:biggrin:
 
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Adjusted the power voltage to 30v now

I think I’m in the correct spot (next to the heat sinks). Latest measurements:

TR626 Emitter .71v
TR627 Emitter 30v
 

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Jake O Bob said:
Adjusted the power voltage to 30v now

I think I’m in the correct spot (next to the heat sinks). Latest measurements:

TR626 Emitter .71v
Click to expand...

That is correct...
avionic said:
Missing your -30 volt rail.. Remove TR632 transistor and then re-measure..

TR626 Emitter voltage.

--------------------------------------------------------------------------------------------
Click to expand...


Have you removed the power supply TR632 yet ?
 
Removed TR632 now

Remeasured TR626 Emitter at -1.1v
I see my meter flashes -29v for a brief second then goes to -1.1v and stays there
 

Attachments

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Find TR628 and measure and record the emitter,base and collector voltages. TR628 is right in front of TR626.
 
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