Another Yamaha c-4 problem

maxcombor

New Member
Hello to all ,first of all sorry about my bad english .
I buy this preamplifier in mint state but inside is haos
Here's what I've done so far
Full Recap with Elna ,Panasonic And Nichicon KZ and some Nic
All small 1uF with wima MKT
all vd1212 change to double 1n4448
all potentiometers and switch`s cleaned in Ultrasonic Bath
Variable resistors changed and tuned to +-20mV

First problem is the ends diodes d315 and d316 i measure
Left 1.45v and 0.9v
Right 0.7v and 0.2v
and normal is 2.4v and 2.1v

I already tried
Swap output transistors from first chanell to second one, Polystirene cap,
Change d315 diode etc.
Measured value is the same

Thanks
 

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Welcome to AK.....


Reverify the correct installation of your VD1212 substitutes.Should be two 1N4148 or equiv. wired in series.
 
Was this pre-amp in working order before you started ?
 
All is the same , with old vd1212 and two in series 1n4148.The preamp is working naw and before, but measured value is different between my preamp and shematic . I tried to cut off the entrance to the r305 and input fet ,all is the same .
 

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Last edited:
Is the C-4 working now ? I don't have a working C-4 at present to compare voltage measurements.Just a couple cannabilized parts units.
Keep in mind.Yamaha like everything else has been known for errors in the service manuals.
 
Are those 1n4148 soldered together wrong? I didn't thing I soldered mine that way. Look at his picture.
 
All is the same

The voltages posted on the sch are the ideal values if everything were perfect. In the real world the tolerances can stake up differently. If you look at your measured values you see there basically the same differential as what's on the sch so my guess is all is well ... :thmbsp:
 
I know that a lot of times instructions are not true, but this big difference between left and right chanell. 1N4148 are soldered ok
 
Avionic Yamaha is working. But is little bit granular in high and middle tones ,on left chanell is more ,also As long as it's switched on sound is worse and worse. Measured voltage on beginning is +30.4 -30.0 after one hour or more difference is more than volt and offset is worse and worse again.
 
Is offset more than 200mv ? It has a real wide tolerance of +/- 200mv per the SM.
 
What voltages are you measuring at the base and collector of TR607 ?
 
The voltage is measured on emitters tr627 and emitter tr626 it is output voltage from power board .Another thing is vr123 and vr124 After 10-20minutes of working offset is ok and drifting from +-20mV to maybe 60mV ,but if I leave longer, (about 2 hours) the offset is going about 600-700mV .If I adjust trimmers again to measure good offset ,same situation after few minutes offset is going mad.
 
I'm sorry if I bothered anyone or if something was wrong from me.
I just asked for help.
Sorry .

:lurk: This just means I'm subscribing to your thread and also interested in what the information or outcome may be. I am waiting patiently like you, and am having a little popcorn to while away the time. It's meant as a friendly comment.

I don't have a lot of specific experience but have questions as well. Before you recapped, was the preamp holding offset properly? If it was, then you may have a bad cap or solder joint in the new work. I would assume all caps properly oriented polarity wise?

Did you replace any trimmers, sometimes they start acting up so you may consider an appropriate 10 turn trimmer to use instead of the original.

Do your input differential transistors have a means of being thermally linked or coupled so that as they warm up, they track each other?

Otherwise, as I understand it, if your offset is drifting a lot, the differential transistors may have become leaky. You may have to find and match some new replacement transistors in order to get offset down and stable.

Also, you may wish to double check any passive or active parts that set the current for that part of the circuit. Sometimes a current source will become wonky, especially old resistors or current source diodes/zeners.

Hope that gives you some direction. However, Avionic is the master at this with Yamaha C-4's.
 
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