Laney LC15 problem

[Prairie Dawg]

I've got a Laney LC15 with reverb here that's acting hysterical. Here's what happens. If you start it up from cold you may get a few crackles and then it goes into auto blat mode, 60 cycle hum, real loud. If it doesn't do that to start with, some power chords at high volume will do the same thing. The chord sort of collapses and it goes right into auto blat.You can also get it to do it with a thump on the cabinet.

I also took the jack from my guitar cord and sort of raped the inlet, which made it go into auto blat.

So I pulled it apart, took the board out, didn't see anything remarkable, resoldered some dodgy looking connections, and stuck it back together. Same problem only worse.

I've checked the tubes and also substituted them. Is there something obvious I should be looking for or am I on my own here?

Thanks

[Fragger]

If you`re serious about what you did to the inlet, you`re on your own.

[Prairie Dawg]

Perhaps I should have said 'inserted vigorously to see what would happen' ain't that always the way?

[Prairie Dawg]

Just as a follow up I have the chassis out on the bench with it connected to the speaker. On firing it up and tapping the chassis with a pencil it appears that the problem can be replicated only right around the gain pot. Does anyone have an Alpha dual 16mm PC mount pot with a 100kA section and a 1megA section that I can buy? The only place I've found that actually has them is the Tube Amp Doctor in Germany and I'm not about to spend the kind of dough they want for this.

[pecorporation]

LC15-110(2008).pdf

LC15_LC15R.pdf

[pecorporation]

Did you touch up the solder on that pot before?

[Prairie Dawg]

Did you touch up the solder on that pot before?

Turns out the gain control was not at the heart of the matter. The flaw is located right around the phase inverter tube and socket area. By tapping the board with the eraser on a pencil you could get it to go into hysterics but at this point it quit working completely, zero output.

This board is not made with any eye toward repair of any kind. I'm in the process of putting together some special probes (needles soldered to alligator clips) to see if I can isolate the problem further.

Thanks for the service manuals by the way. Just for speculative purposes are new boards available and what do they cost?

Thanks

[Prairie Dawg]

Three brand new, TAD 12AX7s all had exactly the same problem. They'd all go crazy if the chassis was tapped on with a pencil.

[52 Bill]

So is it working with the 4th new tube?

[Prairie Dawg]

It's a five tube amp.

I removed all three TAD preamp tubes-the EL84s seem to be OK, but the fault was definitely with 2 of the 3 preamp tubes, and it was the same fault with both of them.

Because of that, substituting one tube at a time-which is what I did when I got it-did not disclose the problem. Two were bad for sure and I'm not reinstalling the third one.

Tapping the chassis with a pencil would induce a heavy 60HZ buzz that could not be modulated and looked for all the world like a problem with the circuit board or some component on it. Interestingly, tapping the envelope of the tubes themselves would not produce the same problem.

Needless to say I've just lost a lot of time I cannot charge the customer for. Part of it was my own fault, I suppose, for assuming that all these nice new tubes were ok-and you know how assume goes.

Well. Something learned for sure. Never say "never", perhaps?