Crest Pro 8001 Power Amp - Bad Crossover Distortion On One Channel - Ideas?

[Enoch0)))]

I recently picked up a Crest 8001 for a mere $250 for my bands practice PA (yes, overkill). One channel has some gnarly crossover distortion. At first I thought the tweeter on the PA tops I was running were blown, but it indeed is channel A.

I threw it on my scope and there's a very pronounced crossover notch only on channel A.

I'm moderately adept with fx pedals and tube amps, but I don't know where to start with this beast, I figure I'd consult you guys, as this seems to be a rather common/popular amp, maybe someone has had one on their bench at some point and can point me in the right direction.

A quick call to my local tech shop and they said I should ship it to Peavey, but this thing is 80lbs, and that would likely cost too much.

Bias?

Many thanks in advance!

[olddawg]

I recently picked up a Crest 8001 for a mere $250 for my bands practice PA (yes, overkill). One channel has some gnarly crossover distortion. At first I thought the tweeter on the PA tops I was running were blown, but it indeed is channel A.

I threw it on my scope and there's a very pronounced crossover notch only on channel A.

I'm moderately adept with fx pedals and tube amps, but I don't know where to start with this beast, I figure I'd consult you guys, as this seems to be a rather common/popular amp, maybe someone has had one on their bench at some point and can point me in the right direction.

A quick call to my local tech shop and they said I should ship it to Peavey, but this thing is 80lbs, and that would likely cost too much.

Bias?

Many thanks in advance!

You are lucky. You have a stereo amp with one working channel. Without a schematic or the unit in front of me I would suggest this: With the power off locate the bias pot for the offensive channel. MARK IT somehow so you will know the original position. Make sure the bias pot is well soldered to the board. Clean the pot with Caig and rotating the pot back and forth. Put the pot back in the original marked position. Also make sure if the amp has a thermal tracking transistor and that it is correctly mounted on the heat sink. Power the unit up. If the problem improves. Order 2 new bias pots. Bias pots frequently fail do to corrosion. Set your bias. If a problem continues with that channel, compare DC measurements with the working channel to isolate the problem. Take your time and use the appropriate probe. You don't want 2 bad channels

[Enoch0)))]

This is great! thanks Olddawg! Sounds very reasonable, I will give that a shot. Cheers!

Oh, here's the schematic: http://www.peaveycommercialaudio.com...matic_set1.pdf

[km6xz]

There is a bias pot but as in any good design it has limited adjustment range.
How much of the wave form is missing? Does the notch occur as a dead band in the exact center of a test signal sinewave or is it offset to either rail. Does it develop full power into a load other than the distortion? If the dead zone is wide but when driven harder, clips both rails at at the same voltage peak as the good channel I would suspect the low voltage power supply. It has a commutator style output section where there is a mid voltage set of plus and minus rails and a high voltage set of rails For moderate signal peaks a set of parallel power transistors on each of the 2 low voltage rail form a conventional A/B amplifier and when the signal needs to increase about the low voltage rails on a louder signal, another set of transistors in series with the mid power transistors, are biased on. Those transistors are connected to a high voltage rail that allows the two sets of series power transistors to run up to the level of the high voltage rails. With a scheme like this, the percentage of waveform that is dead zone will give clues as to where the problem is. If the signal can drive to normal clipping levels and the dead zone is only a few degrees of the wave form, it is a minor problem in the bias level shifting stage in the predriver section. If it a lot more than a few degrees, it is likely driver circuit for the low voltage power output transistors.
Can you post photos of a 1khz test tone at 1/2 power and then just below high power clipping? Into 8 ohm dummy load and then with no load?

[Jazz P Bass]

These 8001's are a bear to work on.

If your only experience is pedals, it is my HO that you are in over your head.

[Enoch0)))]

There is a bias pot but as in any good design it has limited adjustment range.
How much of the wave form is missing? Does the notch occur as a dead band in the exact center of a test signal sinewave or is it offset to either rail. Does it develop full power into a load other than the distortion? If the dead zone is wide but when driven harder, clips both rails at at the same voltage peak as the good channel I would suspect the low voltage power supply. It has a commutator style output section where there is a mid voltage set of plus and minus rails and a high voltage set of rails For moderate signal peaks a set of parallel power transistors on each of the 2 low voltage rail form a conventional A/B amplifier and when the signal needs to increase about the low voltage rails on a louder signal, another set of transistors in series with the mid power transistors, are biased on. Those transistors are connected to a high voltage rail that allows the two sets of series power transistors to run up to the level of the high voltage rails. With a scheme like this, the percentage of waveform that is dead zone will give clues as to where the problem is. If the signal can drive to normal clipping levels and the dead zone is only a few degrees of the wave form, it is a minor problem in the bias level shifting stage in the predriver section. If it a lot more than a few degrees, it is likely driver circuit for the low voltage power output transistors.
Can you post photos of a 1khz test tone at 1/2 power and then just below high power clipping? Into 8 ohm dummy load and then with no load?

km6x: This is great. I've since dropped the amp off with a local buddy/tech who works on big SS amps often, but he would also appreciate any suggestions. Before I handed it off, I did throw it on the scope, although not at 1/2 power and full power though. My dummy load is only 200ishW. Each pic is a diff channel. Bottom is input, top is output. Sine was clean at the signal gen, but as soon as I connected it to the input of the amp, it showed a bit of crossover distortion. Maybe this will give you some clues? I should add I also (after checking for lingering voltages of course) removed the input board, sprayed it with DeOxit, worked the stereo/bridge/parallel switch a bit, and then disconnected checked for corrosion/burns and reconnected all the connectors I could easily reach without disassembly.

Test conditions were approx 1/2V sine input, sensitivity turned up to about 3, into an 8 ohm dummy load.

These 8001's are a bear to work on.

If your only experience is pedals, it is my HO that you are in over your head.

Jazz P Bass: I wouldn't say my experience is only pedals, but these definitely appear to be a bear to work on! Here's some of the gear I've built over the years. I've worked on countless other tube amps. The world of high power SS stuff though, I haven't delved into really.

Soldano SLO Clone. Mercury Iron, KT77's.


12AX7 preamp with 220V "switcher" supply. Even designed the boards.


20W "Cestus" 2x el84 4x 12ax7 high gain amp
50W modded Laney AOR pro tube (revoiced, added an additional gain stage)
SLO Clone


Hairball Audio 1176 Clone (from a kit)


Black Korina/Wenge/Ebony/Rosewood/Cocobolo Jass Bass


This is mine, but I didn't make it. I helped a bit though. 400WRMS/600P-P Bass amp made by Joe Frantzen. 6xKT90, 1x ecc99 driver, 3x 12ax7's. Near 800V B+ Not an electrolytic in sight. All Polypropylene filter caps (beer can sized) and no neg feedback. Lasercut chassis, Mercury Iron. Walnut cab. She's a beast. Still waiting on the faceplate. I built the cab. Walnut as well, with Faital 15PR400's.

[olddawg]

Hope you didn't spray the whole board???

[Enoch0)))]

Hope you didn't spray the whole board???

Oh, you're not supposed to give em a good soak in a bowl of deoxit? Haha, no, just the usual tiny spritz into the inside of the switch. Used a qtip with cotton removed, dipped in deoxit, to clean the xlrs. Pulled and reinserted the octal socket jumpers too. It tested the exact same on the scope before and after cleaning.

[Enoch0)))]

Well, the local tech's didn't want anything to do with it, so off to Crest I guess. : ( They said everything was just too packed in to even be able to probe it easily. What a PITA...