Crate V32 palomino and GTD120 Problems

[bnwitt]

I just received a Crate V32 Palomino and Crate GTD120 amplifier in for service. I've got an intermittent output loss in the Palomino after warm up and a low distored output out of all three channels in the GTD120 though the power amp works fine with an input in via the insert jack.

I can probably find the Palomino problem with some chopsticking but the
GTD120 issue is one for the tracer. The problem is I emailed the Loud Tech guys for schematics and got back an auto response announcing a 3 week delay in email response. Does any one have a schematic for these amps? If so, I'd appreciate a copy as I'm on a deadline with the GTD120.

Thanks,
Barry

[bnwitt]

Ok, so I managed to get in touch wih Loud Technologies today and they sent the schems. Thanks to their tech department who are really nice folks. I also heard that they are taking all repairs in house because they want them fixed right. Interesting.

Back to the amps. I started working on the GTD120 which has distorted output from the preamp section but clear output from the insert jack. I set up the signal generator and scope as listed on the schem and so far I discovered good clean signal at IC1-A pin 1 but a clipped signal at IC1-B pin 7 with setup C. Also I could see two slightly out of phase very jumpy sine waves on the scope side by side at IC3:B pin 5 So, at this point I'm thinking IC1 and or JFET Q1 may be the culprits. Any thoughts out there?

[Mars Amp Repair]

I don't see an IC-3 on your schematic & IC-1 as well as the only 2 IC's in this schematic appear to be in the reverb section. If that is the case, you would be barking up the wrong tree for distortion in the main signal line. Is the thumbnail you've attached the one you are referring to? glen

[bnwitt]

I don't see an IC-3 on your schematic & IC-1 as well as the only 2 IC's in this schematic appear to be in the reverb section.

That's because I'm a bonehead and put up the Palomino schematic instead of the GTD120 schematic. Sorry, I replaced it with the correct one. Thanks for looking Glen.
Barry

[Mars Amp Repair]

HMM,
It seems any kind of open in the tone stack for channel 1 could create an overdriven issue to IC3 p5. You can't always see much of a signal at the input of most closed loop opamps because of their + - cancelling input properties, so it's best to back up a resistor or so & look at the signal there.

If the Middle control were open, that could cause an overdrive condition to the input of IC3. I'd be certain all those tone pots have a reaction or just measure them.

Also the volume pot is really controlling the gain of IC3 in the FB loop...it could be open on one leg, too.

Is the signal also bad from the crunch channel? How is the signal at IC-1B pin7? Is it clipped there, too?

Also, you can get a very good indication if an opamp in closed loop operation (most audio opamps in guitar amps are connected that way) by merely being certain that the DC voltage on the input pins are the same as the output pins. If the opamp has + & - supplies to pin 4 & 7 then most often the voltages of input & output pins are all ZERO (5 = 6 = 7 and 1 = 2 = 3)

glen

[bnwitt]

More time today

Glen,
thanks for the info. I am going to have some more time with the amp and scope today so I'll do some more analysis. I didn't think about it at the time, but the reading on pin 7 of IC1 would be weird unless I was in the B switching mode and Q1 were conducting. I was in the C mode when I did the test. The distorted low output is present in both channels but good output can be obtained by injecting a signal into the insert jack. I'll check the signal to IC3:B between C56 and P10 and at all the official test points in all modes.

I'm still trying to figure out the switching system. It looks like the solo/overdrive and clean/overdrive just short signals to ground.

[bnwitt]

Ok, here is where I am with the GTD120 IC1:A and B have good signal which matches the "Condition" table in the C setting for both the waveform and the P-P voltages. IC3:B also has good signal. IC3:A has no signal but I don't expect it to in the C conditions setting. IC2A and B have almost non-existent distorted signals which I could barely see on the lowest range of my scope (5 millivolts). IC4:A and B have heavily distorted signals also with low P-P voltages.

So now I'm thinking I have a switching problem at Q21. I need to check it out next. I have a question however. If you look at the schematic, there are several JFET switches and the gate of each has a note regarding it's signal source. There is a switching logic table on page two of the schematic which shows state for each signal node in each test condition. What stumps me is some of the JFET bases have notes beside the A,B and C listed in the table. There is an A-B, a B2, and a C2 shown at points. Also, I see the sources on page two for the A, C and A-B signal nodes in the switch section but I don't see a source for B. Another oddity on the schem is the JFET shown on page two above the Solo/Overdrive switch. Where is that connected in the amp? I know it is connected to ground, the A signal source but where is the other "Clean" note on the schematic. What am I missing here?

[bnwitt]

Ok, I believe I'm starting to understand the circuit a little. In the clean channel setting C:

switching node A is at 0 volts
node B is a +10 volts and
node C is at 0 or +10 volts

That being said, in the clean channel mode, the signal would go from IC1:A, thru the tone stack, in and out of IC3:B, past the volume control, thru Q21, IC4:B and A, then thru the insert jack's R-T contacts and on to the power amp.

Since I have good strong clean signal all the way through IC3:B I have to assume that between that point and the insert jack R terminal I have a defective component. So I will next check if I have good signal at the junction of C49 and Q21's source. If so, I'm going to replace IC4. If not Q21. Does this sound right to anyone out there?

[bnwitt]

Well, it turns out that IC4, Q21, Q25 and Q29 were all bad. The amp is up and running.

[digtrip]

Do you still have the V32 schematic?

[bnwitt]

Palomino Schem

Here it is digtrip