When I checked Q1 and Q12 I used continuity first and then played with the resistance function to see if I could get it to read anything for me.

In resistance mode I got a lot of random measurements as I recall. In continuity mode none of the legs beeped with each other in any combination.

I'm going to diode test the rest of the op amps on the board and those diodes you mentioned next... Maybe i'll get lucky. If not I might call Carvin and tell them whats going on.

Sorry for the confusion. I work in commercial/industrial electrical so 'rang out' is a general term for doing a continuity test.

There were no shorts (no beeping) between any of the legs of either Q1 or Q12. As I recall the lowest resistance reading I got between legs was something like 3.5 megohms, with all of the other combinations being significantly more.

On a diode test I should get continuity in one direction and high resistance in the other, right?

Well I pulled Q1 and Q12 off the board and tried to turn it on, but it wouldn't do anything. I checked them with my DMM but I don't have a diode tester, only resistance and continuity (and of course VDC, VAC, etc.). They rang out fine with no shorts between the legs.

I put them back on the board. I guess i'll check all the other onboard transistors and such for shorts between the legs? I still need to fire it up to make sure i'm back where I started before I took Q1 and Q12 off the board...

How do I diode check Q1 and Q12? By taking them off the board and running it without them? What should I do if it works without them?

Just to clarify; you're talking about Q1 and Q12 by E1-E6 in the schematic, not the Q1 and Q12 diodes by line out in the schematic right?

I was given a Carvin SX-200 head for free awhile ago by a friend. Apparently it had 'blown up' while he was using it and wouldn't turn on anymore.

Fast forward to two weeks ago. I take the amp apart and diagnose the problem; the VIPer100A (high speed switcher) and K817P opto coupler isolator were damaged, as well as a few capacitors and a resistor. I drove to the Carvin factory near my house and picked up the parts I needed and brought them home and installed them. Now the amplifier turns on but once it powers up, the Power LED indicator flashes on and off and the signal cuts in and out as if the amp is turning itself on and off. I can also hear the small board-mount transformer kick on and off as well. As far as I can tell I get a full signal and everything works, but the amp is automatically turning itself on and off! The on/off cycle is fast too, turning the amp on and off about twice per second. I dont see any other damaged parts on the circuit board and have triple checked all my connections. I installed sockets and plugged the new parts in to avoid possible damage during soldering.

Here is the SX-200 schematic. http://www.carvinmuseum.com/pdf/amps/sxpowerac.pdf
The parts I replaced are the 'Viper' (VIPer100A), OP2 (K817P opto coupler isolator), C31, C40, C41 and R40.

Any help would be very much appreciated. I am really close to bringing this poor amp back to life...