Crate gx-1200h blows fuse when loaded

[Spike1956]

I am working on a Crate GX-1200H from 2007 schematic 07-249-22. The TIP142 has a hole blasted in it and the fuse was blown. I replaced all the output transistors and checked around for anything else that was taken out but couldn't find anything. I powered it up on the limiter without a load and checked the signal with a sine wave and an oscilloscope and all looked good. I loaded it with about 6 ohms of power resistors took out the limiter and powered it up with the variac. It blew the fuse. I took the load off and powered back up with the limiter and no problems. I put the load back on and kept the limiter on and it is showing a short and the speaker hums with no sine wave in the speaker. All of the schematics I have don't corelate to the screen on the board making things harder to trace. I checked the emitter resistors 0.47 ohms and they are good I also checked a couple of transistors 2n5088 and 2n 5087 and they are good. Anyone have some suggestions where to check next? I am using a schematic for a GLX 1200H marked as PWA-Guitar Amp (posted on this site) which has the same layout as well as the FXT 120. It is not the same as the schematics I see on the GX1200H that uses the 0.33 emitter resistors. Any advise would be appreciated.

[The Dude]

Did you check the amp for DC at the output before hooking up a load? If so, what did you measure? Also please post the schematic or schematic link you are using, so we have some idea of what you are working on.

[Spike1956]

I did check for DC. I got 0 but if I hook up the speaker I get 7V. I have attached the schematic I am using. Thanks for taking a look!

[g1]

Did you adjust the bias to the spec. before you ran with load?
What is different about the schematic compared to your board, component names? Give some examples and maybe we can find a match.

[Spike1956]

I did set the bias at 7.0 mV the differences on the schematic and the board are

SCHEMATIC BOARD
D5 D29
Q5 Q12
D12 D17
Q11 Q13
Q15 Q15
Q23 Q16
EMITTER RESISTORS
R23 R61
R85 R74
R33 R64
R83 R72
OTHERS
C21 C25
C39 C17
Q9 Q8
Q18 Q11

Thanks. I looked all over for a schematic that matched. Hope you have better luck.

[Spike1956]

OK, so I traced the schematic back from the output through to the long tailed pair of Q14/Q20 and changed the part references on the schematic to match the board. I run into problems trying to trace the front end since the schematic doesn't match the circuit I have but it is basically a 3 channel setup with TL072's driving the signal which looks normal to me on my scope. So I guess my big question is is there any way I can separate the front end from the back ens to eliminate the front end as being the problem and concentrate on the power amp section. I don't understand why the amp looks good unloaded and fails when loaded up. I can't check the voltages. I have checked all the parts near the output stage and everything tests ok. I have an ESR Meter, Scope. Semiconductor analyzer, VTVM and most of the parts on hand that go in this amp. Anybody have a guess at what my next step should be. I can run it on a 60 watt light bulb limiter and nothing gets hot. Should I go up to a 100 watt bulb (0.83 amps) or higher? The fuse is 3 Amp slo-blow so theoretically I can go as high as a 300 watt bulb to get me to 2.5 Amps. I know enough to be dangerous.

[g1]

Please post your corrected schematic.
Plugging your signal into the FX return jack will take you straight into the power amp and eliminate the preamp.
If it is good with no load, but fails loaded then that is an issue with delivering current to the load.
I would not recommend going to a higher wattage bulb. Components can be damaged before the fuse blows.

[x-pro]

I did check for DC. I got 0 but if I hook up the speaker I get 7V.

what polarity?

[Spike1956]

g1, First, thanks for your comments. Duh, why didn't I think of the effects insert! I can't post the corrected schematic unless I scan it but why would you need it? You can reference the one I posted and I can do the translation to the board unless of course I am missing something. x-pro, I think it was positive but I will check again. I will be working on it again today. I also will be taking voltage measurements from the driver circuits no input signal vs input signal to see which transistors are turned on or off. Maybe a clue there. Thanks again everyone!

[Spike1956]

The DC is +7.5 on the TIP142 emitters and +6.6 on the TIP 147 emitters. DC at the speaker is +6.5 This is with signal and with no signal applied at the insert jack.

[Jazz P Bass]

Q9 & Q18 are limiters.
Q9 may be at fault.
You can safely remove them for initial testing.

[Spike1956]

I found a schematic that matches the board. It is the one on page 2.

[Spike1956]

I pulled Q10 and Q11 and no change and they tested good. Q10 and Q11 are on the schematic I just sent. I assume they are the ones you asked me to pull.

[Jon Snell]

Don't allow yourself to get confused as if you do, you will never repair it. Check power supply voltages first!
Test points. Test points are useful.
Q19 and Q20 are the front end, control amplifiers.
If the base of Q19 rises from 0volts the output will swing in a positive direction and visa versa. Q20 samples the resultant output voltage and adjusts the Emitter of the long tailed pairs which in turn compensates for any offset. Positive on Q20s Base means positive on Q19s Emitter which means an effective Negative on Q19s Base and a positive on Q14 so Q15 switches off whilst Q13 switches on (until reaching the drive offset voltage) and then will sit there until something adjusts it further, like a signal or a leaky capacitor.
Get what is happening?

I would check for leaking Electrolytic capacitors. C62, C65 and especially C61 and C29.
If you lift C26 you will remove any DC offcet from the main amplifier, if that is the issue.

Drink plenty of Coffee or Tea, walk away to do something else from time to time and when you have fixed it, try a glass of your favourite brew to celebrate.
Have fun and remember, you are the superior being, working on a simple amplifier. Make it fun.

[x-pro]

Leave Q10 Q11 alone. They don't work when the power signal is low, much less when there is no signal on the input.
I agree with Jon Snell about checking C61.
By the way, indicate the voltages on Q13 Q15 bases with respect to TP13 (minus of the voltmeter to that point).