Trace Elliot GP7SM 130

[pnut5150]

Got a problem with this bass amp..very
weak sound through the main input, bass plugged in the send or return of the effects loop is loud like it should be. I do not have a schematic for this one. It does have the j50/k135 I assume MOSFETs in it..any ideas?

[Jazz P Bass]

Have you tried running a jumper cord From the Send To the Return?

[pnut5150]

Sure did, no change

[The Dude]

Got a problem with this bass amp..very
weak sound through the main input, bass plugged in the send or return of the effects loop is loud like it should be. I do not have a schematic for this one. It does have the j50/k135 I assume MOSFETs in it..any ideas?

Need to clear up a few things. By "main input" do you mean the guitar/bass input? When you say, "bass plugged in the send or return of the effects loop is loud like it should be", what do you mean? You mean the amp has output when the bass is plugged into the effects return, or do you mean the preamp has output when you use the effects send to drive another amp?

I take your statements to mean that the amp section works when you plug the bass into the effects return, but your description is a bit vague and unclear as to exactly what you did. If this is the case, the MOSFET type is irrelevant because the power amp is working and you have a preamp problem. Again, if I'm interpolating your description correctly, my next check would be solder on the input jack.

[pnut5150]

Weak output when bass is ran through the input, bass by itself plugged into fx loop send or return and power amp works great. I listed the MOSFET type due to several different variations of this amp being listed online while looking for a schematic. I will double check input jack, pretty sure I already have, but will look again. Thanks

[The Dude]

Can't find and don't have a schematic, but if you already checked the jack and the power amp works, next I would look at the +-15v op amp supplies. Maybe one is down. (making some assumptions with no schematic)

[pnut5150]

Is that the 4 IC's on the preamp board? Do you have any idea of voltages I should have on them?

[The Dude]

Typically -15 on pin 4 and +15 on pin 8. If one of these were missing, your output would be very low.

[oc disorder]

Found this schematic for a series 6 100watt. It has 2 Mosfets for output 2SK135 and 2SJ50.

http://hpbimg.someinfos.de/diy/allge...t-GP7-100w.pdf

The GP7 preamp comes with lots of versions !
e.g.
http://hpbimg.someinfos.de/diy/allge...ot-GP7-MK2.pdf

Here's a later version for 130 watts remove TR7 and TR9

[The Dude]

THX OC.

Ok, now that we have a manual, it looks like the op amps use 30VDC and ground instead of +-15VDC. Still something to check. If the supply is ok, do you have a scope?

[pnut5150]

Thanks for the schematic and replies!! I'll check those IC voltages in a lil while and post them, and I do have a scope

[pnut5150]

Here we go:
IC 1:
Pin 1: 17.02v
Pin 2: 17.02v
Pin 3: 16.91v
Pin 4: 33.96v
Pin 5: 15.41v
Pin 6: 1.609v
Pin 7: 33.40v
Pin 8: 17.02v
Pin 9: 17.02v
Pin 10:17.02v
Pin 11:069.0mv
Pin 12:17.02v
Pin 13:17.02v
Pin 14:17.02v

IC 2:
Pin 1: 17.02v
Pin 2: 17.02v
Pin 3: 17.02v
Pin 4: 33.90v
Pin 5: 16.90v
Pin 6: 17.02v
Pin 7: 16.92v
Pin 8: 17.02v
Pin 9: 17.02v
Pin 10: 16.94v
Pin 11: 64.7mv
Pin 12: 17.02v
Pin 13: 17.02v
Pin 14: 17.02v

IC 3:
Pin 1: 17.2v
Pin 2: 17.2v
Pin 3: 16.94v
Pin 4: 33.91v
Pin 5: 17.02v
Pin 6: 17.02v
Pin 7: 17.02v
Pin 8: 17.02v
Pin 9: 17.02v
Pin 10: 17.02v
Pin 11: 64.9mv
Pin 12: 16.85v
Pin 13: 16.97v
Pin 14: 16.97v

[The Dude]

So you have the preamp with TL074's. According to the schematic pin 7 of IC1 is fed directly back to pin 6, so we have a problem there. The voltage should be the same at both points. Check for connection problems there and/or burnt/broken traces. The op amp may have shorted and burnt up a trace, or it could be a simple connection problem.

[pnut5150]

Ok, pin 6 foil pad on IC 1 was bad, I had some new TLO74's so replaced all 3. I crossed pin 6 with pin 7..is that ok? I now have 16v on the two joined together now.

[The Dude]

You probably didn't need to replace all 3. Voltages looked good on the other 2. Voltage on IC1 looks correct now so give it a try. Does the amp work? If not, that pad (pins 6&7 of IC1) also goes to C17. Make sure it still does. If you had something burn up or break, it could have also broken its connection to C17.