Yamaha J-25 Guitar amp

[sawmichael]

A friend of mine Yamaha J-25 Vintage amp bias trim pot burnt and doesn't know the value.
Help also with value and current setting .And also Japanese vintage NISSHIN ONPA (Maxon) GT-10 guitar amp output power amp Ic lost and not known, Could TDA 2003 be a replacement?thks

[Jazz P Bass]

I do not think that there is schematic available for this particular amp.

That said, the bias pot on most Yamaha amps like this one is 500 ohms Linear.

I have attached the JX 40 power amp & adjust proceedures for the bias (Note: for reference)

If your output section is similar, the bias test points are on TR9's emitter resistor.

JX 40 Pwr Amp.pdf
JX 40 adjust.pdf

[sawmichael]

thks,Jazz P Bass.....

[Mick Bailey]

With your GT-10, does the power amp IC mount onto a circuit board - if so, how many pins?

The TDA2003 could be made to work, but there could be compatibility issues that prevent a straight swap. Without a schematic it's down to trying to work it out from scratch. A start would be to identify the ground, power supply and signal input/output connections.

Do you have any test equipment?

[J M Fahey]

Agree, TDA2003 possible only on a very old amp, needs 12/15V single supply; TDA2006/30/40/50 are split supply and incompatible with 2003 .
They might even have used some Japanese IC , uPC sometging or reallyt anything else.
Post an IC closeup picture, is any printed code visible?
Even if partial.

Of course state what kind of supply (single/double) and voltages do you read, as suggested by Mick Bailey .

Edit: here's JX30, we're getting closer
JX30BYAMAHAServiceManual-EN.pdf

[sawmichael]

Finally,TDA2030 worked in old GT-10,thks J M, so much for JX30 schematic. What a Classic manual.....

[juturnadee182]

A friend of mine Yamaha J-25 Vintage amp bias trim pot burnt and doesn't know the value.
Help also with value and current setting .And also Japanese vintage NISSHIN ONPA (Maxon) GT-10 guitar amp output power amp Ic lost and not known, Could TDA 2003 be a replacement?thks

YAMAHA J-25 has a trim pot of 470 OHMS.. i just repair 1 this week..
you can tweek the trim between 2 and 3.. that is were its originally set.

there is a reason why the trim pot is burned.. you have to check the transistor in the power amp stage, there's a shorted 1 or 2..

[sawmichael]

Thanks alot juturnadee182.
My amp has to be gathered alot of parts including 470 Ohms Pot and Output ICs.
Thanks for the information and Hope I can repair upon Your datas.
Bye.

[juturnadee182]

Thanks alot juturnadee182.
My amp has to be gathered alot of parts including 470 Ohms Pot and Output ICs.
Thanks for the information and Hope I can repair upon Your datas.
Bye.

sawmichael how's your friends j-25 amp now?

[sawmichael]

It was already damaged.My friend put it outside in the rain and the next time I went to see it's finished.