Cs-800c ddt led on and hums

[jswickhe]

Today, my Old School Peavey CS - 800C power amp has failed... but I can't let go!

I've enjoyed it's use at my wedding, and daily at low volume ever since ... but not today.

When switched on today, a buzz / hum is emitted from the subwoofers attached on both channel A and B while both DDT leds glow solid... with or without audio input plugged in.

With or without DDT defeated, increasing or decreasing the gain of channel A or B has no effect on the volume or nature of the buzz, and I cannot hear any trace of the input signal amid the hum buzz.

Unlike Channel A, Channel B DDT led will shut off when I defeat the DDT, and this behavior follows when swapping the A & B output / driver boards. In either configuration though, hum / buzz persists with absolutely no trace of the input signal.

Absence of both these boards seem to create a short when switched on, but the amp is plugged into a GFI outlet thus it throws outlet's breaker.

Unplugging the speakers alleviates the buzzing sound, LOL, but all other symptoms remain regardless of whether speakers are plugged in.

The fan and speed control switch work on high and low speeds, and the neon light in the power switch glows when powered on.

The amp has always historically operated well and within thermal specification, including the moment I turned it off 12 hours ago.

On internal inspection, I removed a fair amount of dust and debris, but didn't smell nor identify any burnt components.

Plugged in, but switched off, the resistance measured across the output posts are
Channel A: 3.91k
Channel B: 4.75k

Thanks in advance!

[Enzo]

Is this set for bridge or stereo?

Please use as stereo during tests.

Is this buzz/hum really LOUD? And if you can see the speaker cones, do they move one direction and stay there? That would mean DC on the output, and it will damage the speakers, so stop doing that to them. Alternatively, disconnect all speaker, and just measure for DC voltage across the output posts. There should be none.


Look up "light bulb limiter", and make one. Very simple. It will reduce stress to the amp during service.

You may have one bad channel or maybe both, but we need to isolate all problems and fix them, so I suggest not only stereo mode, but also let us pick a channel, fix it, THEN move to the other. Please don't try to work on both channels at once. I work with the amp facing away from me, so I am looking at the rear. There is enough slack in the wires that I can unscrew the two power amp assemblies and set them down on either side of the amp chassis. I like to stick a magazine or something between the module and chassis so nothing can touch the chassis and short out.


Let us check power supply first. You have the two large can caps. There should be about 80v of DC across each. And very low ripple.

Each power module has a four wire molex plug from the chassis. Pulling that connector off disables that channel. You do not need to unplug any of the driver card connectors on the disabled channel. SO pick a channel, and disable the other one for now.

I use these terms, so just to avoid confusion:
Power module or just module - the large heat sink thing with 12 large transistors
Driver card - the smaller board that plugs onto the underside of the module by the row of 8 pins.
Volume board - the small circuit board with the volume control
Output board - the small board with the actual output terminals and jacks. The crowbar triacs are there.
Input board - the board with all the input jacks on the rear panel

If you need to use different terms, explain then so we don't talk at cross purposes.

Sounds like your triacs are not shorted on your output boards, good.

I don't usually operate the modules without a driver, but they should power up OK that way. The end power transistor on each row is the driver for the rest, and each has a little 100 ohm resistor pulling its base down in the absence of a driver card. If the module pops breakers without a driver card, I'd be checking those 100 ohmers and also just going down the rows of transistors checking for shorts.

[jswickhe]

The DC Voltage measurement on the output terminals of both channels reads 0 Volts, however, in the 200 mV range, it reflects some marginal activity.

On the two large capacitors, one fluctuates from 80 - 82 Volts, the other 53 - 54 Volts.
The capacitor that reads 53 Volts is to the rear of the chassis, parallel to the input board.

I haven't proceeded to the power module or driver card examination since there's an inconsistency with the voltage measurement across the capacitors.

Though it's regular use is to power subwoofers, I normally operate this amp in stereo mode.

The subwoofers reside in folded horn enclosures, not readily available for visible inspection.

I consider the hum buzz to be moderately LOUD, it's about as loud as my causal listening volume if I were to play a recording of such a noise; A volume just loud enough that you can still talk over a telephone without having to raise your voice or turn the music down.

The nature of the buzz is more like a hum, doesn't sound like a smooth sine wave though. This may be partially because the speakers producing the sound are subwoofers, and in a folded horn enclosure. I've tried to avoid subjecting them to further abuse, and abstained from listening to the noise through full range speakers to avoid potential damage -- though it would reveal a more accurate production of the noise.

[Jazz P Bass]

When you are reading the main voltages in dc mode it is best if you also flip your meter to read volts ac.
That supply that reads 54 volts dc is either loaded down or the ripple (Vac) is so high that it is confusing your meter.
Go back to the 54 volt supply & measure the Vac ripple.
If it's high. it is likely bad or the soldering broke loose.

[Enzo]

That 50v on the cap is way low, and it sounds like as JPB said, either the connetion is loose or the cap has failed.

[jswickhe]

With the four wire molex plug detached from the power module of both channels:

The capacitor which metered 50 VDC, meters also at 124.3 VAC.
The capacitor which metered 80 VDC, meters also at 178.5 VAC.

[Enzo]

I think your meter gets confused measuring AC on top of DC. Clearly you do not have 178vAC on that 80vDC. When you make an AC reading on top of DC with your meter, try adding a cap in series with your red probe. The value is not critical as these are not precise readings. I use a .047uf 630v cap because I have a lot of them handy.


In any case, I am convinced your 50v one is not doing its job, so tighten up the screws on top of it first, and if no help, it probably needs replacing.

[jswickhe]

Great advice everyone, thanks.

How likely is it that the faulting cap is the sole component damaged?

I've read of these being replaced with higher voltage and or higher capacity, but I haven't witnessed a scenario where the caps were each of different value.

What is the nature of replacement options for the faulty cap?

I'm under the impression that replacing the cap may resolve the hum buzz, but possibly reveal a damaged channel.

This is due to the difference in behavior between the channels when I defeat the DDT, on channel B the solidly lit led shuts off, on channel A it remains solidly lit. It makes sense that the led would shut off when defeating the DDT, the led being associated with DDT.

[Enzo]

One bad cap will be the cause of the problems shared by both channels. It will not be the cause of the difference between the two channels.

There is almost nothing in or out of the amp that can cause those caps to fail, they just do it on their own. It is most likely not related to anything else that is wrong. The large hum it causes might be stressful to an amp channel that tries to amplify it, but no direct cause.

The 15000uf value ought to be easy enough to find, so no reason to look for 18000uf or something. But if you did use 18000uf for the one side, no big deal. These are 90v caps, which is an odd value, you will probably find 100v caps much more easily. Modern caps are going to be physically smaller, but you can probably still find them with screw terminals like the originals. One VERY likely source would be the parts department at Peavey.

I doubt you NEED to replace both main caps, but doing so MIGHT make it more convenient to wire up. I don't recall if this amp has a metal bar connecting the common terminals of the two cps together, but if it does, it would be hard to mount if the caps were different heights.


SInce your two channels were behaving differently, aside from the hum in both, we must assume one of them has a problem. But until we have good power supply, that is moot. It may just be the two circuits reacted a little different to the bad power.