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Yamaha C-4 | Proper Transistor Replacements

Cergei

Cergei

New Member
This unit had some issues such as randomly after some time of being working began to distort, and gradually being powered down.
I changed all those VD1212 by pairs of 1N4148.

After recapping this unit I bought 2nd hand in Argentina in 1997, I checked some of the transistors and I am quite sure some technician has done some repairs and put the replacement “he could find” —I do not think that the representative firm for Yamaha in Argentina had the proper parts stock, and I know in that country electronics components are not available in a variety as they are available in North America—, and found that 2SB566 had a not quite suitable substitute (one 2SB546), and I replaced by a TIP32.

The unit works, and apparently that issue has gone.
However…

I found the unit has several 2SA942 instead of the original 2SA941, and some of the 2SK68A “were replaced” by something as A999.

The maximum Base to Collector and Collector Emitter voltages of the original 2SA941 are 120V, while those voltages on the 2SA942 are 90V. I know I can replace properly the 2SA941 by the KSA992 (same voltages120V, and higher power dissipation 0.5W).

Could anybody give some thoughts and advice in particular about how to replace the missing 2SK68A ?

Thanks in advance.
 
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I realized I got confused. I read the schematics wrongly. The are no missing 2SK68A. Those A999 are in place of the 2SA673A. Should I find a better replacement ?
 
No... 2SA999 . Is a good cross reference.
 
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Cergei said:
original 2SA941 are 120V, while those voltages on the 2SA942 are 90V.
Click to expand...
Technically for this app.. 2SA942 will work just fine. The working voltages won't exceed - 30v in the C-4.
 
Nothing wrong I am aware about the 2SA999.
In regards to the 2SA942 I remember I measured the voltage of TR628 emitter /TR626 base and did not get the -42V that the schematics shows, instead was about -30V, so I thought it could be the fact of not having the original 2SA941.
Or my assumption is wrong? Thanks
C-4-Output.jpeg
 
Cergei said:
Nothing wrong I am aware about the 2SA999.
I measured the voltage of TR628 emitter /TR626 base and did not get the -42V that the schematics shows, instead was about -30V,
View attachment 1915563
Click to expand...
-40 is still a long ways from -90..

What voltage are you seeing at the collector of TR626?

I'm thinking there is a discrepancy in the schematics posted voltages. -29/30vdc makes more sense.

You can replace the A942's with the A992's but probably won't change anything.

Pull R384 100Ω and check its resistance. Also pull and check D314.
 
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avionic said:
-40 is still a long ways from -90.
Click to expand...
Agreed !

R384 is fine (98.8Ω) and D314 it's 0.593 in diode check

Collector of TR626 is -42V and its base /TR628 emitter is -30.3V (So perhaps something wrong in the schematics?)

Base or TR628 is -30.8V matching what the schematics shows.
Thanks
 
avionic said:
I'm thinking there is a discrepancy in the schematics posted voltages. -29/30vdc makes more sense.
Click to expand...
Look at the base of TR627/emitterTR629 in the schematic.. Should be the polar opposite.
 
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