Peavey 5150 Combo Preamp Signal

[Enzo]

Ah, we are a team here.

[g1]

Like the 'high flyers' Gagne and Brunzell?
Or (hopefully) more like the Iron Sheik and Baron Von Raschke!

[Enzo]

Terrance and Phillip?

Siegfried and Lunch?

[g1]

Ha! It took me a bit to get the lunch part. Magician du jour.

[bobloblaws]

So I ordered some P chan JFETs to replace the ones I took out of my other amp. Digikey lists the J174 as obsolete so I ordered and installed what looks like the next best thing which is the J175. They seem to be doing what they are supposed to be doing in that I can trace the signal at R28 and beyond, but the amp's output is weak, similar to what was going on in the other 5150 after I fried the JFETs, and the signal is noisy and distorted at another test point that I'm familiar with from. So what I am asking is, do you think my current problem is a result of using the J175 in place of the J174 or is it more likely there is something else going on with this amp that just happened to crop up (maybe I kaiboshed something removing and then replacing the JFETs)? Some of the specs of the J175 and J174 are different but I don't know enough about transistors to determine from the datasheets whether they should be interchangeable or not for this application. The following page compares specs.

InterFET Corp. Page Selector - J174 datasheet

[Enzo]

Did the amp work OK WITHOUT the JFETs? Hopefully?

By clipping them out, we removed their function. But we didn't - if I recall - determine if they were at fault themselves, or if the circuit was not turning them off properly. SO go back now and check for gate voltage.

[bobloblaws]

Did the amp work OK WITHOUT the JFETs? Hopefully?

By clipping them out, we removed their function. But we didn't - if I recall - determine if they were at fault themselves, or if the circuit was not turning them off properly. SO go back now and check for gate voltage.

Actually, we did determine the JFETs were at fault. When I replaced them with the pair out of my OTHER amp the problem was corrected (and I confirmed with simple resistance check that they were blown). Now I'm trying to restore the amp I took the working parts out of. But since chances are I will have dissasemble again at some point I will try clipping them out.

Edit: I verified there was gate voltage, 21.5VDC. I clipped out the J175's and there is still the same problem with a very weak output. So something else has happened, I'll need to begin troubleshooting.

[g1]

Jumper FX send to return for starters.

[bobloblaws]

Found the problem, one of the jumpers from the top vertical board to the horizontal board developed a bad solder joint. Probably not unusual with these when you end up bending everything flat to work on the board(s) and then square it all up again before putting back in the amp. So I'm guessing the J175's will actually work in place of the J174's when I put them back in.

Edit: And yes, the J175s work fine. Also, if anyone comes across this thread in the future that needs to remove the main circuit board contraption, here's a tip. The secret is to remove the small FX send/return board as well as the one for the speaker outputs first. That wasn't obvious to me at first and I struggled to remove that main board the first time.