distortion in amp Please help

[adlaic]

I think I might have found the problem. On the PSU it has 41v ac in that is all working ok and the rail voltage is 132.9 + and - on both channels it has 15v ac + and - and that test ok. Plus it has 12v dc + and - when I test the 12dc - it test -12v but when I test the 12v + it is +21.2v must be somthing wrong with the PSU.

 

[Netlist]

Nice find. Keep us posted.

/Hugo

 

[adlaic]

I fixed the 12v side of the amp and it is not as distorted, but it is still distorted. Could a bad cap cause it to distort? I tested all of the transistors on the board that I took apart and they all tested good.

 

[Elso Kwak]

Now figure out which parts could be damaged from the over voltage.....

 

[adlaic]

Does anyone know where I could find some Toshiba a1937 and c4793 transistors, and when I test them should I test them like I did for the 2sa1943 and 2sc5200?

 

[adlaic]

does anyone know how I would go about testing the bias of the amp?

 

[bobo1on1]

Try measuring the voltage over the bases of the driving transistors to check the bias voltage or place a multimeter in series with the output transistors and power supply to measure the idle current.

 

[adlaic]

I hooked up my volt meter to my speaker out and it read around 500mv. so i turned the pots till it was down to about 10mv and the sound cleared up after about 5min it blew a fuse.

 

[clem_o]

I hooked up my volt meter to my speaker out and it read around 500mv. so i turned the pots till it was down to about 10mv and the sound cleared up after about 5min it blew a fuse.

I certainly hope you took note of the original pot settings... turn them back to where they were, try putting in a new fuse and keep your fingers crossed. If the new fuse blows the instant you power up the amp, you destroyed your output transistors the last time.

Dont go around turning potentiometers, not unless you know exactly what each pot is supposed to do (and how to set each properly). From the looks of it, you managed to over-bias the amplifier into thermal runaway.

See if you can locate a schematic or service manual first.

 

[adlaic]

If I can't get a manual how would I know what to set the bias to? There are only two pots on each channel. are they both for the bias or one for the bias and one for something else?

 

[infinia]

The other pot is most likley output voltage offset adjust
(the offset adj should be located closer to signal input.)

(the bias adj pot should be located near center of pca or closer to the heatsink)

 

[adlaic]

how would you adj. the output voltage offset?

When the amp blew the fuse it also blew a big hole in one of the output transistors.

 

[infinia]

Turn the volume control to min and adjust the Voffset pot very slowly while monitoring Vdc the + speaker out for lowest reading usually around 10-30mV or lower if lucky.
Unfortunately you have to repair the scrapnel first.