1998 JCM2000

[LarBal]

Ok since the amp is stable, but getting no signal from going in/out of either FX jack, is it reasonable to start looking at IC2 as a problem?
Amp has all new tubes, voltages at all tube sockets are good. I haven't started looking at low voltages, I'm looking for some guidance there.

I edited together 3 schematics to get what I *think* is my amp. Certainly the topology would be correct at least

[DrGonz78]

I would test the low voltage power supply to ensure function. I would ohm out all computer cables they used to connect all those circuit boards. Ground connections have a star location right by the FX board. I have found those to need tensioning at times.

[g1]

You had suspected the mute circuit, did you verify it is ok? Any particular reason you want to move to the FX jacks?
A mute is at V3 grid (TR3). Do you have signal before and after V3?

[LarBal]

Welp, looks like today is the day I learn how to use a scope. I've been using my voltmeter and iphone as sig generator, but I just inherited a lot of equipment, including a Velleman HPS5 "Personal Scope" from the 90's.
I'm getting to know it now, but am confused by the statement in the manual about coupling. It's kinda hard to tell if there is an internal capacitor to block DC while reading AC signal, or if I need to physically place one between my probes and the test point(when DC is present).

Depending on (part of) the signal to be measured, the input can be connected to the signal through a direct link or by using a decoupling capacitor. Select DC for measuring DC voltage. Press the “AC/DC” key to select either AC or DC input coupling (see indications on the screen).

Manual : https://cdn.velleman.eu/downloads/0/...ual-hps5_h.pdf

[Helmholtz]

As the scope provides AC coupling, it contains a coupling/DC blocking cap engaged in AC mode.
In case of doubt, measure a 9V battery and switch between AC and DC input.

[LarBal]

You had suspected the mute circuit, did you verify it is ok? Any particular reason you want to move to the FX jacks?
A mute is at V3 grid (TR3). Do you have signal before and after V3?

Ok well I've made a bit of progress thanks to the scope
I have signal at V3 grid pin7, and at the cathode of the following stage.

I have Signal at R38(between treble and volume), but not at R40 (right underneath "MUTE") . So my confusion is what the mute circuit actually does, and what could be wrong with it.
The relays are just channel switching, if I'm reading that correctly. I do have -15V and +15V (though its 16V) at IC2 also...

[The Dude]

Do you have signal on the wipers of VR2 and VR4 (making sure the controls are not all the way down in ground position)?

[LarBal]

Do you have signal on the wipers of VR2 and VR4 (making sure the controls are not all the way down in ground position)?

I couldn't get to them unforunately, the traces seem to go right to the relays out of reach. I did the have the volumes up though when I tested at R40

[LarBal]

Turns out I was losing signal at the cathode of the cathode follower, and it was reading short to ground.
Last week, the owner told me he had taken it to a "guy" that had it for months and couldn't fix it. I asked him to please report back on what he had done, and my customer said the guy said he did nothing.
The owner also let me know before he took it to him, and before it went silent, that it was working fine for years.
That turned out to be BS because a couple of the molex connectors were in the wrong place. The scope made it pretty easy to sniff that out!
Only reason I tell that story is that I had that quote I see here from Enzo swimming around in my head all week as I worked on other amps...about not finding excuses not to check something. He was right!