Crate GT1200H circuit draw at speaker jack

[BStringThumper]

I have a problem I haven't seen before, I'm hoping someone could please help/shed some light on it. I have a Crate GT1200H head. With all cables connected there is a hum through the speaker but no musical notes when strumming the guitar. I plugged the amplifier into a light bulb current limiter and tried it again. The bulb lit brightly, something is drawing more current than it should. It's not blowing the fuse when plugged in directly. I started checking the input and speaker jacks. When I pulled the speaker cable out of the jack the current draw stopped, I plugged the cable back into the speaker jack and the amp works. All channels work, lights, it works. I shut the amp off and looked at the jack. It looks fine, but I swapped it with one of the other less used jacks just to make sure. Resoldered a couple of joints for good measure. I turn the amp back on with all cables connected and the hum and current limiter light is on again. I unplug the speaker cable and plug it right back in and the amp works great, no light, no hum. The speaker jack is the switching type. I've noticed when I pull the cable out slowly that as soon as the tip clears the inner jack contact and it closes that contact there is a pop through the speaker and the light goes out. Any thoughts would be greatly appreciated. Sorry for the long post. Best wishes to Enzo.

[g1]

Have you tried a different speaker cable?

[BStringThumper]

Yep, tried the cable on something else, it works. Everything works after I pull the speaker cable out and put it back in, with the amp powered up. Strange. It's like a capacitor is holding a charge or shorted until the speaker jack contact is closed and then the amp works. Thanks.

[Jazz P Bass]

The fact that the amp is pulling enough current to steadily illuminate the limiter, with the speaker connected, tells me that the output section is faulty.
My guess is that if you where to measure the output, you would see Vdc on the speaker.
I would not plug the amp into a speaker until you get this resolved.
As to what makes the output section pop & then correct itself, I am not sure.
I would resolder the output section.
At test, the first place to look would be the 5532 IC's output pins. Pin 1 & pin 7.
This IC is used as a driver for the output transistors.
If it is failing "on" that would explain the current draw & the hum.
It also may be the stage in front of it that is telling it to turn on.
I have attached the GX1200 schematic. It may be close.

[BStringThumper]

Thanks for the input. I thought the output transistors were shorted until it started working when I unplugged and plugged the speaker cable back in. The schematic you sent is different than the GT1200H, thanks though. This one has 2n5087, 2n5088, MPS-A06, MJE-340, transistors. Then it has TIP142 & TIP147 output transistors. I just found that it has a JFET J175 prior to the power amp. Since the JFET has to have power to function, could the momentary closing of the contact on speaker jack be enough to jump power back through the circuit to activate it??????????

[Jazz P Bass]

I am thinking that the speaker plug/ unplug may be sending a spike back through the feedback loop.
At any rate, the amp needs service.

[BStringThumper]

Do you have any suggestion on what components I should remove for testing in the feedback circuit, please? I've heard of Negative Feedback Loop on tube amps, but not solid state. Thank for the help.

[Jazz P Bass]

That is not what I said.
My thoughts are that the feedback path is sending whatever plugging & unplugging is doing, back into the power amp.
I can only say again that the amp needs to be serviced by a pro.

[52 Bill]

Have you eliminated the possibility that the problem is mechanical in nature and plugging and unplugging things is only causing the problem to come and go? Give the amp a good Enzo whack with your fist. Does this make anything happen?