Trace Elliot GP12SMX Bass Preamp Hum Problem

A buddy of mine has a Trace Elliot GP12SMX bass preamp with a bad 60 Hz hum problem. It's on all outputs regardless of whether or not there is an input. It goes away only when I switch in the ground lift switch. I've already checked the power supply voltages for proper levels and checked for AC ripple. I swapped out the 12AX7 tube (but I'm not a tube guy) just to eliminate that as suspect. The hum is still there in standby mode but not as loud. I'm assuming I've got a cap leaking to ground somewhere but I don't know where to start. Oh yeah, I do have a schematic I can post if the file size isn't too large. Can anyone offer any advice on where to look as I'm at a loss. I'm an electronics technician but I usually work on test equipment, not guitar amps.

Thanks,
DD
 
If it goes away when you lift the ground, it means that you have a ground loop between the other things you are using. Try to connect everything (pre, power amp, FXs...) to the same wall outlet...

Don't lift the ground: in a pre you need the safety ground, because it is connected to the bass which is connected to your body. Maybe you can lift the ground at the power amp: but it is a very inadvisable solution, don't do it. Resolve the ground loop instead.
 
Look at solder connections. On solid state instrument things it is the most common faliure. Check from the input jack right to the power supply. Many times I repair amps that hummmmmm, they're just a broken solder joint on the main filter capacitors (in combo amps they vibrate a lot - if not glued to the pcb the solder joint tends to break). Maybe you used to put it on the bass cab... just open it up and start checking at the joints and connections.

Tip: an extreme difficult to find fault is a faliure of contacts in those enclosed pc-mount input jacks. Actually in an amp I repaired there was the whole jack's tip in it (it broke off - really!).

Hope this helps!
 
I've gotten most of the hum out but not all. The main culprit was the center pin on the 3 pin connector which connects the transformer to the main board. When I pulled the connector off, the center (ground) pin on the board came off with the connector. One other pin was loose too so I resoldered all three. I still have the problem with the XLR DI outs not working. I do know the output of one side of the power supply (-15 VDC I think) has a sine wave riding on it. I can see it with an oscope when I probe the metal top of the main filter cap. I'll probably just replace all the filter caps. Parts are always the cheapest part of a repair.

Mortalwombat - I'll check my other computer for the link to the schematic. The file is too big to attach here (670K). In the meantime, check http://gibson.com I think that's where I found the schematic but I'll have to look. If I can't find the link I can send it to you from work Monday (I'm on dial up here). Send me your email address by private message on this BBS if you want me to email the schematic. Don't worry, it won't go anywhere else. You can check my rep at http://www.homerecording.com/bbs (DigitalDon) or on eBay (deafmonkeystudio).

DD