Peavey XR684X Mixer amp with 1200 SC power amp module

[cablercdd1]

I have a 1200 SC power amp module in a Peavey XR684X Mixer That's kicking my butt. It came to me with one channel not working and parts missing. I ordered the parts from Peavey and powered it through a lightbulb. The lightbulb glowed bright so I checked everything I could and couldn't find anything. When I unplug the transformer plug to the low voltage the lightbulb goes out. It appears that the B channel is being over driven so I removed the Q206 driver transistor and the amp stopped being overdriven. That's as far as I have been able to take it. The voltages when it's intact are all low because the power supply can't keep up with the current draw. Any advice on a direction would be appreciated. I've attached the schematics

Paul

1200SC_MODULE_03.pdf

[Mark Black]

With Q206 checked & reinstalled try pulling or replacing U200. Looks like it is direct-coupled to Q206 base so a "stuck-to-the-rail" fault in that chip could cause large DC offset in the amp.

[cablercdd1]

Thanks for the suggestion, I'll let you know how is turns out.
Paul

[cablercdd1]

Ok, I took U200, Q3, and Q4, current limiting bulb still on. Took Q206, and Q208, bulb still on. BTW, I took parts out one at a time and then tested. Took Q215 out and my light bulb went out. Replace Q215, bulb went back on. VR 203 and VR202 have proper voltage when Q215 is out. Checked Q217 and Q221 and they check good. Voltages on ZD202 and 203 are + and - 14 volts but drop a little as the output transistors warm up. Don't know what to do from here.
Paul

[Jazz P Bass]

Have you checked the output transistors.

It sounds like you have a short.

[Mark Black]

Probably not the most cerebral way to go about things, but at this point I would take advantage of having a duplicate known-working channel with similar part reference markings close by and measure forward/reverse on all the semiconductor junctions comparing one channel to the other using "diode check" on my meter looking for a significant difference. This with power off and all components installed of course.

You could also try swapping Q115 and Q215 to rule out a failure not detectable with a meter (see if problem follows swap).

Another thought - might be a good idea to pull SCR100/200 so you can measure for DC offset at the outputs and to reduce current draw under fault.

[cablercdd1]

I pulled all the .22 ohm resistors and measured the transistors with a diode check and they check good. It acts like a short but I think the output transistors are just being driven beyond their capacity.

[cablercdd1]

Taking comparative measurements isn't something I've done yet but at this point I'll try anything.

[Jazz P Bass]

I would not, at this point, swap any parts from the working channel.

Why? Because it's working.

[babasial]

1200 sc module

dear sir please contect me i have same problem with 1200 sc module in peavey xr800f plus
shaheensound555@gmail.com