Peavey CS800X - DDT light, low distorted output

[iH8usrnames]

I use this amp to power two 12" subs in my home audio system.
Overkill? Yes.
Was it cheap? Yes.

It had been working without issue for months. Issue showed up this week.

I have swapped speakers and line ins, issue remains on channel B.
I have worked the Bridge/Stereo and DDT switches multiple times, issue remains.

Basically, the DDT light comes on at the drop of a hat. For example, this is what I went through yesterday:

1. I power on with nothing plugged in (speakers or source) nothing aside from the power lights are lit.
2. Connect anything to the input while powered on, just connecting a 1/4" jack that is not connected to a device at other end, ddt light comes on and stays on until I power cycle the amp.
~ But sometimes I can connect a wire and it does not engage DDT; maybe 10-15% of the time it does not engage. ~
3. If I connect wire and DDT does not engage, I will connect other end of wire to my powered off function generator, DDT light comes on. Will stay on until I power cycle the amp.
4. Powered off function generator connected to the input of the amp, I power on amp - no DDT light. If I unplug the wire from the powered off function generator, the DDT light comes on and stays on until I power cycle the amp.
~ Sometimes I can unplug and DDT light comes on for a few seconds then shuts off. If I then plug that wire back in, DDT light comes on and stays on until I power cycle the amp.~

I have not swapped the driver boards from A to B but imagine the issue will follow.

B+/B- are fine.
26v supply measure 24.33v

[Jazz P Bass]

Have you tried turning the DDT off on the rear panel?
What year is this one?

[Enzo]

The X says it is the wind tunnel version.

OK, just a hunch... Power off at the amp, disconnect any speakers and cables. Now measure resistance between black and red speaker posts on the output of that channel. Is it very low? How does it compare to the good channel output posts?

[iH8usrnames]

Have you tried turning the DDT off on the rear panel?
What year is this one?

Yes. tried that.
Also swapped the drive boards between channels, problem stayed on channel B.
Also swapped the potentiometers between channels, problem stayed on channel B.

[iH8usrnames]

The X says it is the wind tunnel version.

OK, just a hunch... Power off at the amp, disconnect any speakers and cables. Now measure resistance between black and red speaker posts on the output of that channel. Is it very low? How does it compare to the good channel output posts?

I reassembled and tried powering on - immediately let the smoke out.
Pulled it back apart.
Thought I found a faulty diode on output board but it tested fine when removed from circuit.
Tonight I pulled all the output transistors and found two were bad, both on channel B.
- 70473180/9515
- 70473180/9518

As it sits, it only has the four transistors at the center of the output board installed.
I was told the logical step is to power-up without output transistors and check voltages; not exactly sure it means more than just checking the B+/B- and 26v supplies.
I have an electrical engineer friend help me in the process but its not exactly easy; she is in Belgium and I'm outside Chicago.

If you have any wisdom to impart I will certainly listen - you made your living on these things, it seems.

[The Dude]

........OK, just a hunch... Power off at the amp, disconnect any speakers and cables. Now measure resistance between black and red speaker posts on the output of that channel. Is it very low? How does it compare to the good channel output posts?

^^^^^^THAT! I'm guessing Enzo wants to know if the triac in the crowbar circuit is good.

[g1]

I reassembled and tried powering on - immediately let the smoke out.

So you turned it on without doing the check that Enzo suggested in the post that you quoted? Or did both channels measure the same?

[iH8usrnames]

So you turned it on without doing the check that Enzo suggested in the post that you quoted? Or did both channels measure the same?

I checked the outputs on the output board while still apart and got nothing - if I recall correctly. I then reassembled and powered on - not remembering he specifically said not too.

I will do the checks he requested in the morning.

I am kicking myself over this one.

[iH8usrnames]

Fully assembled (all the wires connected), no power, no inputs/output connected, , all output transistors have been removed when these measurements were made.
Speaker terminals:
Channel A = .5 ohm
Channel B = overload

I did swap board positions between channels and do not recall if I returned to original positions. I believe I did but cannot say with great certainty.

[The Dude]

That likely indicates that the Channel A triac (SAC187) on the output is shorted.

[Jazz P Bass]

"Channel B = overload"
This is a common misinterpretation of the meaning of "O L".
Over Limit or Open Line is technically more informative.
This, from a Toyota Tech Brief.