|guitardad||8/2/2018 3:02 PM|
|Crate GT80 No sound in tube mode|
This one has a boatload of FET's for switching and I think I may have that problem.
Non tube modes work fine. All PS's for tubes and there, filaments, plate, and signal.
The Tube LED lights up when you select that mode, but there is no sound. If I use a sig injector, some sound leaks, not much.
The schematic is not that great. Having a hard time seeing where the PS feed is to the main switching FETS.
The board components are not marked either, so that looks like fun too.
Signal feeds to the FETs Q4 and 24 but is not getting to the first tube (seems to be muting). No measurable shorts on the FETS unless I am overlooking leakage when under load.
Old amp that was brought in for donation, probably not worth fixing, but I'd like to donate it locally to kids that can't afford gear.
These were not that bad of an amp in their hayday. I had another one with a blown PT not long ago, but customer said to dispose of it.
I bet this PT would have fixed the other one that was in better condition.
Any ideas? TIA
|The Dude||8/2/2018 3:15 PM|
|What about IC2? Do you have signal on it's input (it might be hard to see). What are the DC voltages on the IC's pins?|
|guitardad||8/2/2018 7:43 PM|
Seems simple enough, but I can't read the source of signal or voltage from the gate of Q4.
I inspected the board for cracks, bad solder etc. Has to be the fets or whatever is driving them. The TP voltages 20-27 measure correctly though. At least I thought so. It's hard to trace the TP's with no component markings on that board.
Schematic has no component layout.
|Enzo||8/2/2018 8:06 PM|
|You usually can't see signal at op amp inputs due to their virtual ground. There is signal at pin 7 or there is not. You have two channels, I tend to doubt ALL your JFETs are bad. One channel or the other should work.|
If the amp is silent, isolate the problem. Plug your signal into the LINE IN (FX return) Hear it now?
Your power amp is pretty basic but has a 12AX7 triode input stage and a pair of 12AU7 triodes to drive. Use you meter as a signal. Don't ground the black probe but use the red one as a signal. It will act like an antenna and inject hum into the circuit. Touch pin 7 of the 12AX7, hear hum?
Go back to the preamp and touch pin 2 of that tube, hear hum?
You can inject real signals if you prefer, I just use my probe or even a screwdriver for a quick go/no-go test.
|guitardad||8/2/2018 8:20 PM|
Hardest part of this is having no markings on the board for components. Takes a hell of a lot more time to sort out what is connected to what. That's why I started with the obvious things like the IC In and Out, Gain pot, Input of Ic2.. yeah nada there, so I still think the switching is out of whack.
|Enzo||8/2/2018 9:28 PM|
|Call Loud Technologies and ask for the schematic file. It will include the layouts, especially if you specifically mention them.|
Did you inject a signal right into pin 6 of the IC?
JFETs are ON until turned off by a gate voltage. Are they getting gate voltage when you want them OFF?
|guitardad||8/2/2018 10:11 PM|
I can get noise via Pin 6 and touching the pin.
I think there is some print missing on the left where all the switching is derived. Only print I could find though. Loud said they didn't have one, it's too old.
|Enzo||8/2/2018 10:27 PM|
|I am sorry they told you that. They have all the drawings. Your guy just didn;t want to look for them. They got all the documentation along with the hard assets when they bought the line.|
Specifically, do you have the GT80 or the GT80DSP? I have both in my paper files, but lord knows when I can get at them, they are buried in a storage unit.
I think the missing switching is mostly just the jacks. The lines from the jacks control those transistors.
So look at the gate legs of some FETs and see if they are being controlled or not. If I can get at those binders, I will pull my Crate files.
|guitardad||8/4/2018 5:57 AM|