Best place to start with channel stuck in Protect?

[soulray]

Just thought I'd ask for a few pointers from the more experienced people, as to a sort of checklist, to repair an amp that has 1 Channel stuck in "Protect".

What are the "usual suspects" so to speak, when this happens? The amp in question is a InterM 2000 Power amp, and it looks absolutely spotless on the inside: no signs of any components burnt out, or of any overheating. In short, no obvious signs of damage to the power module at all!

I have a users manual with a circuit diagram, but not a service manual proper sadly.
Any ideas as to where to look/what to do next will be greatly appreciated good people.

[burbeck]

hi
start by testing the output transistors, then the driver transistors and the all the semiconductors around the output stage including diodes

[nigelwright7557]

A good and and bad transistor can look exactly the same.
Transistors dont always explode if a fault occurrs.

[kct]

soulray,

you may also want to check the PS supply to the front end first. Otherwise you might have no other option than to test all output stage and driver transistors as well as the diodes separately. Verify that in deed the output stage is stuck at some DC and that the protection circuit is in deed working correctly first before you go through this trouble.

InterM M1500 & 2000 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics



Cheers

Kay

[soulray]

Many thanks to all of you for your help & suggestions, and to Kct, thanks too for the link to the proper Service manual. That will be invaluable I'm sure, just as soon as I can track down some actual faults!

Kct, I wasn't 100% sure what you meant by "PS supply", but in case it meant Power Supply, I have swapped over the power modules & reconnected, and the channel in protect has swapped too: So now channel 2 is stuck in Protect. Progress of a sort maybe!
I suppose I now know that the fault is definitely with the power module pcb and not the caps or anything else?

Anyway, some pics to brighten things a little:-





I still can't see any damage anywhere on the "faulty" module. There was a dry joint on the other one, (the 1 not in protect ironically, which I resoldered), but nothing I can identify on the problem one. Hopefully, as you can see, the board & amp in general look pristine. And the large power transistors measure identically on both modules, at least in terms of resistance. That is how I'm supposed to check them electrically with my multimeter isn't it?

To check the output stage for DC current, do I just check the speaker outputs? And how to check the Protect circuit is running as beyond me I'm afraid.

In my youthful innocence, I had always imagined damaged components would be a bit easier to spot. How wrong I was!

[tomchr]

If you have the schematic, you should be able to figure out which conditions will cause the channel to go into protection mode. Check these conditions one by one to figure out which one caused the fault. Fix the problem.

Easier said than done, but that would be my methodology. At the very least check if the supply voltages are within specified limits and if there's DC on the output of the amp (measure prior to the speaker relay).

~Tom

[soulray]

Sorry Tom, maybe I misled you, but there are no "which conditions will cause the channel to go into protection mode", it is permanently in Protect from power on. It fails to come out of Protect after 5 or 6secs as it should, like the other channel does.
My guess, is that there is a permanent fault or short on the power module. I just can't identify it.

[sregor]

My 2 cents - start with the protection circuit (inline IC at far right of circle. TA7137. Pin 2 is the "input pin" Check voltages on R175 and R176 (22k). The output is on one side of R175 and should be at zero volts. If it's not, have to trouble shoot the amplifier. If zero, and you get a negative voltage on R176, then the thermal sensor th101 is open (all the way on the other end of the heat sink by Q105 and mounted on the heat sink. If all these voltages are less than half a vote, the IC chip itself is probably bad.
If DC on the output, the most likely problem is the input IC 101 which acts as a gainstage and DC servo for the amp. Signal before IC101 is capacitor coupled, so can't cause DC offset. Also could be any transistor or component although I would expect to see burnt or open resistor for many of them. Hope this helps.
Be careful checking voltages. A set of mini-grabber type clip leads would be helpful, as well as taping probe leads to minimize possibility of making a bad contact.

[soulray]

Sregor, your 2 cents were right on the money! On the good side, R175 & R176 tested at 0.0V, while on the side in Protect, they were both 136.7V! Not good...

Also tested all 12 of the large Sanken 2SA1295 transistors, even though they look perfect, and each one tests at identical resistance value to the corresponding 1 on the good power module. So that's some good news I guess. So it's not the power transistors, or the protect circuit by the look of things. What to check next now?

[sregor]

Obvious suspect is IC101 (NJM 072D) Probably equivalent to a TL072, but that's just a guess on my part. I would do diode tests on semiconductors and check values of low value resistors. Since no smoke or burnt components, suggest something is either open or bad opamp. YMMV good luck and be careful. Lots of power in there.

Biggest problem with trouble shooting here is that everything is in a feedback loop - so a bad component almost any where makes will cause the output to go to the rails. Good luck.