Peavey CS800x - DDT light, low output, distorted

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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
 
As the CS800x is a PA amp instead of a guitar amp, this belongs in PA forum, not instruments forum.
You need a DVM and some alligator clip leads. The voltages in this amp can stop your heart if they flow across it. Use alligator clip lead on negative probe of DVM to speaker ground, to take voltage measurements. Use only one hand with power on. Wear no jewelry on hands or neck, 1 v through a ring at 20 A can burn your flesh to charcoal. Touch no metal until check @ below 1 v even power off. If rail caps discharged once, they will probably do it again & you can speed up. Use safety glasses, solder splashes especially desoldering, and transistors sometimes explode blowing shrapnel to the ceiling.
There are 3 electrolytic caps in the DDT circuit. Measure ESR of them, or replace them. C104, C147, C132.
Peavey operators in the 90's had a habit of making beautiful solder joints that didn't actually connect anything. In your probing around of stupid voltages, if you find a place that makes it get better when you push in it, you found it. Reheat & resolder.
I suggest you plug the 1/4 phone cable in the input, which makes it bad, and look at voltages. Pin 3 of U106? (input op amp) crammed up against rail by Q110, DDT is stupidly active with no input. I'm not entirely sure whether it goes to + rail or - rail, but you should be able to figure it out by probing & looking at the LED. U124 could have gotten oxidized pins in the socket, with power off remove & replace to scrape any oxide off. U124 usually only blows when the output transistor fails sending rail voltage rampaging out the base line through the whole system. This meltdown even can take out the 50 v .1 and .01 ceramic caps peavey used. If unit was half repaired after a meltdown, you have a chance to show how an amateur can exceed performance of a pro repairman by spending more time. Time doesn't cost us diy people. Sign of a meltdown, the output transistors don't have 8 digit part numbers on them. Just industry numbers like MJ15025/24. Or from a cheapskate, OT can be 2n5630/6030. but the cheap ones won't put out 400 W very long.
Happy hunting.
 
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Apologies. I placed it here because in my googling I found another reference to the Peavey CS800 in the instruments area.

Thank you for all the information.
I've had a development since this post - I reassembled, powered on, and the smoke came out.

I pulled all the output transistors and found two bad ones on the problematic channel. Tomorrow I will reassemble again and check voltages - I am getting guidance from an electrical engineer friend. This should not indicate I am not taking your advice to heart - I certainly will check all the areas you mentioned.
 
Well, no. Too late if the OT are out alreay. You can measure the OT's directly now they are out on the bench. Diode scale 0000 b-e b-c both ways it's shorted. Diode scale 350 - 480 mv one way probably damaged. Iceo test @ 12 v or higher before reusing. 550-750 mv probably good, but if it blows with rail voltage, I was wrong.
Iceo test is 12 v power supply series 47k resistor series ma or ua scale of DVM. Case is plus on npn, - on pnp. Base is open. ua=12/47000 transistor breaks down @ 12 v, garbage. ua is 1 to 5 transistor is okay @ 12 v. DVM test at 2 v, not very useful on 160v or 300 v transistors. My 400 mv MJ15024/5 that were blowing the tops to the ceiling on current limited rail voltage failed the 12 v Iceo test. Peavey 7047xxxx 7048xxxx house number OT's. Learned this Iceo test from a guy in a 3rd world country that checks all his new transistors at 150 or 250 v to make sure they weren't substituted for fakes at the border. 150 v will kill you, I had a 12 v battery charger on the bench and it worked - proved that all but one OT were trash.
 
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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.
 
Yes it still matters, I am trying to determine if the crowbar triac across the output has shorted. This often happens when outputs fail. You can replace the bad outputs, but the crowbar remains shorted. It is not too late with the transistors removed. it takes ten seconds, PLEASE do the test.
 
Yes it still matters, I am trying to determine if the crowbar triac across the output has shorted. This often happens when outputs fail. You can replace the bad outputs, but the crowbar remains shorted. It is not too late with the transistors removed. it takes ten seconds, PLEASE do the test.

I performed the test and posted prior to your response above.

One channel had .5 ohm resistance, the other was overload.
 
Okay, OP PM'd me, and my instructions for a simple little DDT problem did not apply to output transistor failure. Furthermore, once the cover is off, it is as easy to check the crowbar triac directly with meter instead of messing with the speaker terminals.
I messaged back following, unless Enzo wants to override my practices.
The safety instructions apply at all times.
The first post was about your little problem of the DDT light coming on when you plugged a cable in. Not usually a problem caused by burned output transistors. Obsolete now that the smoke came out.
Pro shops usually 1. check DC voltage on output 2. if fail replace all the output transistors. crowbar triac comes out at this time 3. Check DC again while on light bulb box. Crowbar triac doesn't go back in until amp works perfectly at full power on resistor bank for an hour or two.
Enzo's post might save a little time if no fault , you don't have to take the cover off. OP took the cover off & took outputs out based on his friend's advice. At that time he can check crowbar triac directly he doesn't have to use the speaker jack. BTW when the triac goes the SBS can go too.
To nigel7557 post, if output transistors are bad, drivers can be bad. In my PV-1.3k, not only drivers, VI limiter transistors & all resistors were bad, couple capaacitors in the VI limiter were bad, the DDT op amp & jfet were bad on one channel, the input op amps were bad on one channel, also the light driver op amps. Most of the OT emitter resistors were bad. Temp sense diodes were bad, also predriver transistors. Up around the op amps some of the 50 v .1 u disk caps were bad. So if OT's are bad there is a lot of checking to do, before you worry about the DDT light coming on because you plugged in a cable.
Note light bulb series the AC cord is good advice in all power on tests after the outputs fail. My light bulb socket is in a grounded steel box with a circuit breaker, so if the wire falls off, it blows the breaker, not set fire to my coffee table or shock me silly.
One further point: if one output transistor is bad on one channel, you replace them all on that channel. The Iceo test I described can determine which ones might survive another use, but you save the survivors for future projects, not put them back in the amp with a few new transistors. Output transistors have to be matched by month of production, ideally same batch, and the 10 you use taken from a rail of 25 by matching the Vcb. Or buy new output transistors pre-matched from Peavey, which is not as cheap as newark/digikey/mouser. What is in the amp is MJ15024/25. I replaced them all with MJ21193/94 which are cheaper but require checking the idle bias current after installation to make sure it is right. then changes to resistors in the idle bias circuit, which has no adjustments.
 
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The crowbar is directly across the speaker terminals. You can certainly test the part directly on its leads but since it connects directly to the speaker terminals, it is so easy to test for it there without having to take anything apart.

The whole point of testing the crowbar is that it is not the outputs, they are a separate issue. Output transistors short the speaker line to a power rail, not to ground. So it is a separate test.
 
well he did say he's got half an ohm on one channel.
is Peavey still using a triac for the crowbar protection?
and as i recall there on the little sub PCB's right at the outputs.


iH8surnames do you have the schematics?



don't or try not to swap board positions until things are sorted.
 
CS800 revisions A, B, C have crowbar speaker protection. Suffix X & S have a speaker disconnect relay. A, B, C are worth rebuilding IMHO because of extreme over design of heat sink.
I don't think OP is here anymore. Putting a hundred or more parts in a severe output transistor blowup is not an economic solution, it is a conservation effort. Save the planet, reuse, recycle. Most people will dump it & buy a Class D piece of trash.
 
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