peavey musician mark III 400 GH 1978... loud constant buzz

[g1]

Also, the pdf has 2 versions of the power amp module. The 400GH version (pg.5) shows a 4741, not TL074.
The 4741 is probably not available anymore, but TL074 will be fine as a sub.

[Tone Meister]

I just piddled with one of these in the last several days, but my problems were much different than yours: http://music-electronics-forum.com/t35563/ In my thread, Enzo had instructed me to pull the connectors from the preamp to the power amp for testing purposes, so yes I'd say that you could do the same here to see what your power amp is doing.

Part of my problem was CR18. On the schematic I worked from, the U1 power amp IC was a 4741. Here are the schematics from which I worked. See if there are any differences in these and the ones you have.

[wheatley]

well i havent plugged the pre amp up to the power amp yet. i replaced U1 from the power amp with U21 from the pre amp. this substitution took care of the DC ont he output, the pre amp is still missing its ic U21, would it work run the preamp without U21?

[52 Bill]

can i run the preamp without on the that ic to test the amp while i wait for the part. i feel like ive tested almost every component that i can on this thing, mostly out of ignorance. is there anything else i should look out for since the ic was bad?

The preamp uses the TL074 for the graphic eq. If you have removed one, then the graphic will not work, so turn it off with the front panel switch and you should be able to test all of the other preamp functions.

Consider the testing as a learning experience. At one time or another all of us have spent time testing parts that were perfectly good. It makes you learn to focus on the symptoms of the problem and to look for the cause. The next one you fix will be easier.

[g1]

Ok, I think I get it. The 4741 in the power amp was bad, you replaced it with a TL074 from the preamp. If you ordered TL074, it will work fine in either position.