help need on bx1200

[selmax]

i am new to this i have read all the post on the bx1200 and i have done all the checks and i am still getting a hum. i am getting 11.5v on all the outputs and inputs on IC 7,8 and 10. i am getting 11.5 v, 13.2v and 12.6 on T5 also getting 9.8v, 13.2 and 12.6 on T8 also i am getting 11.5v on the active and passive input jacks also getting 11.5v on the direct out jack. Power rails 40v+, 40v- and 15.5v+ and 15.5v-. i am wondering which IC or transistor i need to change or could it be something else.

[drewl]

If you're seeing voltage on the input jacks, then you are either measuring something wrong, or you lost a ground connection.

[Enzo]

Notice that all those things have one common element, IC7, a TL074. Replace it.

Well first verify power to it. It is fine that you have both +/-15v power supplied, but make sure it is actually getting to the two power pins of this IC. I expect it is, but check anyway. But i am betting you just have a plain old dead IC.

Having several of them dead though is real suspicious, again, check to see that both 15v rails are actually at the IC pins. If the outputs are all at some plus voltage, look for the negative to be missing or real low.

You do need to have a proper ground for measuring as drewl mentions. The input jacks are isolate by caps, and should not show DC. However, the ring contact on those jacks does sit at some negative voltage when a plug is inserted. That has to do with the muting through IC9.

[selmax]

remove the preamp board and found a burnt out ground line between two pots. thanks drewl for the lost ground idea. enzo IC7 is ok i also found a few bad caps c14,c17 and c22. it's working with no hum thanks.

[Enzo]

Col.

The loss of that portion of the grounding would upset the DC voltages as well.