Crate GX140C schematic needed

[Tone Meister]

I thought I'd try getting a schematic here before enduring the torture of calling Loud. Have a GX140C, and both power amps work. I can trace signal all the way to C34 and R31 Level pot, then lose it.

Supposedly a "known problem" with these amps, but as of yet, I don't know. I plan to start a thread on the guitar amps section. Thanks in advance.

-Stan

[Enzo]

Known problem? According to whom? Just what are you trying to fix? These amps are made just like a zillion other Crate amps of the era, and are no more nor less likely to fail in some particular way.


There are two power amps, both working?

If you have no throughput, did you check the INSERT jack on the preamp?? Might be called an FX loop jack. And there are also FX loop jacks on the power amps. The return is a stereo or TRS jack, check that. That preamp insert jack can be used as an input to see if the amp works past that point, and an output to see if signal is getting that far.

Turn up the reverb some and rock the amp to crash the springs. Does that come through?

[Tone Meister]

Sorry Enzo, I forgot to watch my words (again). Anyway, here is what the first post should have said ...

I have a GX140C, and both power amps work when injecting a 1K signal to the FX loop line in. I have also patched a jumper cable from each of the the FX sends to FX return and get no throughput. I realize the return is stereo and uses a TRS connection, and I also realize the loop has both mono and stereo sends.

I can inject a signal to the front panel INSERT jack and get throughput to BOTH amplifiers. When the signal is injected into the main INPUT, I can trace the injected signal as far as the way to C34 and R31 Level pot, then lose it.

-Stan

[Enzo]

I don't see R31. COntrols all have a part number with a P, as in Pot. So P1, P2, etc. C34 can only affect channel B.

Here:

[DrGonz78]

When you say R31 Level pot do you mean P7 volume pot? I see where R31 is on the schematic and we should just look at TP1 for that reading. Is TP6 where the signal is lost or just after C34? Just keep going right here on this thread by tracing all the test points on the preamp. What sort of reading are we getting on channel A and then go to channel B.

[Tone Meister]

Now that I have the schematic I can attempt tracing it in a systematic fashion. My R31 comment should have stated "by the level pot."

To answer your questions, I started by injecting a signal at various points (INPUT, INSERT, and FX RETURN). When connected to INPUT, I get no throughput to INSERT, FX SEND, or SPEAKER OUTPUTS whether channel A or B is selected.

Here are the readings at each test point under each of the three conditions:

[Enzo]

I asked about reverb spring crash, did you do that test?

It works after the INSERT.

You have no signal anywhere. After the first op amp, the signal splits into two paths, and neither works, so I;d be tying a new TL074 in IC1 myself. I would first verify that both power rails were present at the IC pins.

[Tone Meister]

No reverb crash. Both power rails present at pins 4 and 11 respectively on IC1 TLO74.

[Enzo]

It works after the insert, but you get no reverb crash. That points me to a very narrow space.

Hmmm, the schematic looks like there could be a spring reverb Or a digital FX unit, I assume you have a spring reverb?

The reverb return mixes through IC4 pin 6. That IC makes TP8. Assuming the reverb works, then we have signal into IC4-6, and none at the insert jack.

See Q25 there? That is a mute. Follow from its gate back to the input jack, there a sleeve cutout contact turns on the mute - or off when the jack is full. See if that is working. Just below Q25 are a couple resistors to -15v, thart voltage needs to get to the gate of Q25 to turn it off. Does plugging in and opout of the main input jack turn that gate on and off?

Now the transistor itself, measure resistance source to drain while it runs, and the input jack is occupied. Just measure R38, its probably easier. If you get a low resistance, the JFET is stuck on or shorted. You can remove it and see if the sound returns.

[Tone Meister]

It is a spring reverb, the DSP you're seeing is a stereo chorus processor. R38 (10K) measures ~42 ohms.

R41 (100K) measures 180 ohms, R42 (22 ohms) measures 151 ohms and looks toasty, and R43 (10K) measures within tolerance at ~9.9K.

Also, R11 (1Meg) measures ~64 ohms. R11 is in parallel with R7 (1.5k) and R9 (680 ohms), so should I expect to read 1 Meg there or the paralleled value of ~468 ohms?

[Enzo]

Um...


R38 measures 42 ohms, so there's your problem. And that is a Q25 issue so do the things I described there.

WAIT, did you take those readings with the amp ON and running? That is when it counts.


R41-43. Where are they? Oh, reverb. Resistors don;t go down, so 180 ohm R41 has a parallel something. The ONLY place R41 has anaything parallel is inside that op amp next to it, IC5. Pull it and recheck R41. Sounds like the 22 ohm is toast, so replace it, and I'd imagine a new IC5. But those are reverb, still get rid of IC 5 for now.

R11 would only affect one channel. R11 is not in parallel with R7 or R9, nor are those two parallel to each other. R11 measures low because it has the JFET across it. Again, did you take that reading with the power off? It will always read that way with power off. That is part of channel switching too, so it will read low or not depending on how the channel selector is set.

[Tone Meister]

@#$%^&*()*&^%$#@!!!!

OK, amp on this time ...

R38 = ~565K
R41 = ~182 ohms
R42 = ~151 ohms
R43 = ~9.9K
R11 = ~1 Meg

Enzo, your patience is legendary, and the depth of your electronics IQ exceeds that by a very wide margin.

[Enzo]

OK, so no cheap trick with the mutes...

Then just track it down. I'd put a strong signal into the input and scope along. But equally valid is signal tracing. Google "signal tracer". it is essentially just another amp with a probe at the input, and we use it to listen to various points in the amp we are fixing. All you need is a cap to block any DC.

We start early in the amp and follow the signal path along, listening in as we go, until we get to the point the signal stops progressing.


The other side of that coin is signal injection. We start near the rear and inject a signal. That is what we are doing when we plug a signal into the FX return or the insert return. But with a little probe or just a wire, we can inject signal most anywhere along the path. Again, if we use a signal geenerator we block any circuit DC with a cap. We start at that insert jack, then at R53 then C51, then TP8, then pin 6 of IC4.

You don;t really need an official signal. We are looking for the amp to pass signal at all, so I just use hum from my environment. If I don't ground my meter, then its probes act like hum antennas. I would just touch a probe to IC4 pin 6. A working circuit would make hum then.

[Tone Meister]

Actually, I started first by signal tracing. I have a decent tone generator with an amplitude adjustment and a handmade probe (600V cap) and cheap DC powered amplifier that I use as a signal tracer. I also have a Tek 475A scope but am still learning my way around on that, so I tend to be more confident with the signal tracer. Funny thing is, an experienced tech would have found the issue in 10 minutes. But I do love dabbling so here we are.

I will start back in the morning and go through the schematic and see where it takes me. Good night John Boy ...

[Tone Meister]

OK, I'm set up to both signal trace and inject signal from the function generator. I have tested the ability to "inject" the signal into the circuit and know it works.

Amp ON, speaker connected. I used a barrel connector to complete the reverb circuit for testing purposes. Going backwards and injecting a60 mV, 1K sine wave at each of the test points, here is what I have:

TP8 - no throughput
TP7 - no throughput
TP6 - no throughput
TP5 - no throughput
TP4 - no throughput
TP3 - no throughput
TP2 - no throughput
TP1 - no throughput

I injected the same signal at each leg of every IC, and only the ICs that produced a response are listed, and only the pins that produced a response are listed; the other ICs and pins are omitted.

CLEAN CHANNEL, all levels at "5"

• IC8, pins 6 & 7 - can produce chorus effect on pin 7 that can be controlled by P14
• IC7, pins 5 & 6 - can hear the 1K tone faintly
• IC9, pins 2 & 3 - produces healthy level of hum (schematic indicates it's not used)
• IC6, pins 5 & 6 - produces healthy level of hum, pin 9 lower level of hum

OVERDRIVE CHANNEL, GAIN 1 selected, all levels at "5"

• IC8, pins 6 & 7 - can produce chorus effect on pin 7 that can be controlled by P14
• IC7, pins 5 & 6 - can hear the 1K tone faintly
• IC9, pins 2 & 3 - produces healthy level of hum (schematic indicates it's not used)
• IC6, pins 5 & 6 - produces healthy level of hum, pin 9 lower level of hum

OVERDRIVE CHANNEL, GAIN 2 selected, all levels at "5"

• IC8, pins 6 & 7 - can produce chorus effect on pin 7 that can be controlled by P14
• IC7, pins 3, 5 & 6 - can hear the 1K tone faintly
• IC9, pins 2 & 3 - produces healthy level of hum (schematic indicates it's not used)
• IC6, pins 3 & 13 - can hear the 1k tone faintly; pins 5, 6 & 9 - produces healthy level of hum

Nothing when injecting signal at J12, J13, J14, or J15