Boogie Lonestar low output

[LarBal]

I'm super frustrated with this amp, that came in for a pot replacement.

Apparently it took a fall and the master for channel 2 broke off. Easy enough. I installed it and turned it on, and channel 1 worked fine but I got zero output from channel 2.
Essentially, both channels are the same, with different tonestacks, and channel 2 has the option to add in another stage.
Channel 1 sounded great though, and owner reported that the amp had been working fine since the fall, he just couldn't adjust the volume on chan 2.
Anyways, after about 10minutes of trying to figure out what was going on,
I noticed that the wiper lug on the pot I installed was barely touching the back of the pot above it, and shorting the signal for channel 2 to ground.
I bent it back again and channel 2 came to life.

I proceeded to play it for 30 seconds, then switched the drive function ON, on channel 2, adding in a second stage after the shared 1st stage.
I cranked the drive control to max out the gain, and within 10 seconds all volume on all channels went to conversation levels. Great.

Oh well gotta be a tube or a plate resistor right?

Well, no funny business with any of the voltages dropped across any plate resistors, but I started changing some out anyways.
I should say first I changed out every single tube in the amp, and so far I've changed:

1st stage 100k Rp, both PI Rp, both couplers .1 off the PI.
I've got good voltages on the 6L6, and they're pulling decent enough current(original mesa tubes so cold, 23mA or so)
If I turn the loop on, and plug into the fx return, I get some signal out of the speaker, but just about the same amount as through the input, with every single volume maxed, and that's hardly loud at all, slightly louder than conversation.
The tone of the amp still sounds good, and the switching is functioning.

Oh yea I swapped in another 100W OT , to no change.
It;s not the speaker, I've tried different cables, guitars, output jacks

The only clue I've got is that if I tap on pin 7 of the PI tube, I get a loud sound out o the speaker, and if I tap on pin 2 I get nothing.

with the loop off, I still get the same quiet signal, so I think the loop is fine.

I can eliminate both tonestacks, and the drive gain stage as any source of problems, as they are all "working" but all have the same low output.

THEN owner sends me a video of the amp "crackling" from saturday , at idle, and said it seemed "Like the amp was clearer before". So this is likely an issue that's been mounting, and it decided to crap the bed while I had it

Any ideas on what to look for next is appreciated.

boogie_lonestar.pdf

[The Dude]

Got a scope? I'd insert a test tone and start tracing signal through the amp and see where gain drops. If no scope, use a DVM on ACV.

[LarBal]

Ok thank you, that helped. I'm losing all the signal at the master level wiper, after v2. It's strong at the input lug of the pot, but all but gone at the wiper.
Also, when each channel is selcted, the resistance to ground from the wiper(when dimed) of that channel changes from 1M to 99ohms.

So I'm figuring its the diodes or J175M transistor?

Could I have fried that when the wiper lug was accidentally shorting to the chassis when I installed that master level channel 2 pot?I don't see how, as that would be the equivalent of just turning the pot to zero,
but it seems very suspect, as that happened about 30 seconds after I removed the lug from shorting to the chassis. Could be a coincidence I suppose

[Jon Snell]

In a 20 year old amplifier, I would suspect the pot as there should be a signal on the wiper relative to its position.
The FETs don't fail, they are killed by severe signal conditions.
The J175s are cheap but not on my list of suspects until the pot issue is sorted out.
If you do find the J175 is at fault, (a continuity test Source to Drain will tell you) don't be tempted to cut the old one off with long leads and stick a replacement on the leftover leads. That will cause problems in the future!
Touching pin 2 of the PI should provide no signal. That is how the PI works.

[LarBal]

Thanks Jon. I should have been more clear, in that the same loss of signal is happing on both channels, and at both master level wipers(when each channel has been selected with signal applied)
.
I was just reading a lot of threads out there about how the J175 is part of the muting matrix, for relay pops, and can send the amp into "partial mute" if it dies.

Now that I think about it, the failure mode of the amp happened only when I maxed the gain at every point in the amp. Would the protection diodes for the j175 start to clip at some point, and cause "severe signal conditions"?

I'm going to get back on the amp this morning.

[LarBal]

I clipped out the J175M and the amp returned to its proper volume level. I don't have another on hand, but customer has a gig this weekend, and doesn't even switch channels, so.....
The pop, after the first one, is very slight