help troubleshooting fender ultimate chorus

[guitar james]

got a ultimate chorus 2- 12's.one side crackles and distorts as notes dissapates.does the same with another 12 speaker, is it capacitor??? or output transistor??the other side works fine.i am trying to fix this so my friend can do a show tommorrow.any advice would be great

[Enzo]

Disconnect the "good" speaker and tape the wires so they don't touch anything. Now we can listen to only the "bad" channel. Try turning the effects off and on - any effect on the noise? In particular the chorus effect, since it uses only one channel, the right channel. (I think it is the right)

There is a stereo efects loop jack, it is TRS - tip is left channel ring is right. Send this output to some other amp for a listen. Is the distortion present there? If so it comes from the preamp, if not, then the power amp has the issue. In that case we could input a test signal at the stereo loop return jack - also TRS - to test the power amp only.

I would assume the signa coming out the mono effects loop was clean, but is it?

And find U7 over in teh corner behind those stereo jacks. is there any DC offset on pins 1 or 7?

[guitar james]

so many thanks in your advice.i checked what you said and it is in the power amp section for sure.i got to the u7 part.how do i check dc on these pins.where do i go from there? i checked for broken solder joints and all apears to be ok.nothing looks burnt.thanks g james

[Enzo]

U7 provides the drive for the stereo loop SEND. If the problem exists in the power amp from a signal put into the stereo RETURN, then U7 would not be involved.

To check voltages anyway, ground your meter to chassis and probe each pin. Pins 4 and 8 are power pins and should show -15v and +15v respectively. Pins 1 and 7 are outputs and should show essentially zero DC.

The power supply is common to both amp channels, so filter cap trouble would affect both equally.


On to the amps.


I don't know which channel is the bad one here - left or right. Each power amp has a 1436 IC right near th heatsink. PIns 7 and 4 are its power pins, and it runs on +/-26v, so look for more or less +26VDC on pin 7 and -26VDC on pin 4. Then look for zero DC - or no more than a few millivolts - on pin 6, the output pin for the IC.

After that, it is a matter of getting in there with a scope and looking at what is going on.