Peavey XR1200 Repairs In Progress - Help Requested

[GuitSprint]

Hello Folks,

I am part way through diagnosing issues with a Peavey XR1200 powered mixer that I recently acquired.

When received, it was blowing fuses as soon as it was powered on. Cursory troubleshooting found the 'A' side amplifier to be the culprit. The 'B' side operates properly.

Further investigation found a bad output transistor (SJ6357 = MJ15003). I will be ordering a replacement, and may do them all since the price per unit looks to be affordable.

I then temporarily borrowed a known good SJ6357 from the 'B' side to test the 'A' . No fuses blowing, but there remains 15v DC across the speaker terminals. I removed all 8 transistors for the 'A' and the 15v remains. What components are likely to be suspect? What further troubleshooting should be done?

Additional info:
Power Board is a CS400
Driver Board is a 111778
Output transistors are SJ6357, drivers are SJ6344.

Thanks in advance for your help!

Jim

[Enzo]

Not as good idea to run it with no outputs on a side, especially if it is making DC. That leans even harder on the drivers and predrivers. And of course, no speaker load until the DC is gone.


Since ther is DC on your output, we can probably assume the triac across the output is not shorted, but on any blown PV amp, one should always check for that. Triac right across the output.

it is however OK to run with missing outputs, as long as each side has at least one. I often remove the bad output, then fire it back up to see if the rest of the amp is OK.

Yes, I use a variac.

So, fuses hold, DC on output bus.

First, any time a transistor shorts, always check the resistors around it. In this case, check all of the output transistors. This thing is quasi-complementary, so all of them are NPN as are the drivers. Then all of the 10W 0.33 ohm ballast resistors, and don;t forget the extra one in the middle. Check the 5.6ohm base resistors and the 100 ohm driver base. Opens in any of them will get you.

Now we could do a ton of extra testing, but the simplest thing is to swap the driver cards and see if the "bad" one works in the good channel. If not we can fix it, but if it works, then all your troubles are on the output board. And ther are not many places left.


And did you check the two drivers, those 6344?


I usually find the best prices on the MJ15xxx series at Allied.

[GuitSprint]

Further info provided. A few follow up questions.

Thank you for the informative reply.

I have tested and found the offending output transistors. Two bad SJ6357's; parts will be ordered. Both SJ6344's are good.

Guaranteed: a speaker will not be connected until the DC issue is solved.

I seem to be having trouble locating the triac. Isn't it supposed to be on the power board near to where the speaker wiring connects to it? I have found nothing numbered to SAC187/TO220 on that board. Is it possible that my XR1200 does not have it?

Upon further dissassembly, I did find an obviously fried resistor on the power board (traces to pin 5 on header from the driver board). Of course the color code is burned away, but the other 2 like resistors look to be 93ohm (=100ohm, yes?).

All of the 10W 0.33ohm and 5.6ohm resistors are visually ok. Should I lift one side of all of them to test, or am I "done" since I found a fried component?

I also proved that the driver board is ok by swapping it into the channel that was "good".

Thanks for the lead on the parts supplier.

What a terrific resource this forum is!

Jim

[Enzo]

Looking at resistors tells us nothing. Put your ohm meter across each. You don;t need to unsolder it, you can tell if it is open or not.

The triac is RIGHT AT the speaker posts. Not on the power amp board.

[GuitSprint]

Greetings.

I have completed testing of all the resistors on the power board, and it appears I have one bad one. I think it is one of the 100 ohm driver resistors, but it is burned beyond recognition. It is connected to pin 5 on header from the driver board, and is physically parallel to two other 100 ohm resistors that are also both connected to pins on the driver board header (pins 2 and 4 looking down on driver board from component side). This is one of the driver resistors you mentioned, correct? Can I use a 100 ohm 1 watt 5% part?

On to locating the triac.....there is nothing at the speaker posts (1/4" jacks)....just the wiring coming from the power board. I did perform a resistance check across the jack and there was no short. Is it possible this unit has no such protection?

Thank you,

Jim

[Enzo]

Entirely possible. This is a CS400 power module used in your mixer, so I am going by the CS400 schematic.


100 ohms is 100 ohms, you can use any size that fits, well not smaller.