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Pet Peeve Peavey (GPS 3500)
Pet Peeve Peavey (GPS 3500)
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Old 27th February 2017, 11:49 AM   #1
Peter S is offline Peter S  Canada
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Default Pet Peeve Peavey (GPS 3500)

Hi Again Folks!
Well, I'm back at 'Step 1'; Admitting I'm powerless over the still suffering Amplifier! I have just purchased a Peavey GPS 3500, aware that channel A was not functioning. This is obviously not a particularly 'Hi Fi' amp, but at a rated 775 Watt/chan into 8 ohms, I thought it might be a worth-while project.

I have had great success on a number of projects, with the help of expert members of the Diyaudio forums, so I thought I would introduce the project before ripping into it, 'bull in china shop' style.

I tested the functioning channel, and saw that it could develop almost 745 watts across my 8 ohm furnace element. The torroid transformer seems to have separate windings for each channel, channel A was discovered (and remains) disconnected.
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Old 27th February 2017, 11:55 AM   #2
thanong is offline thanong  United States
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Very interesting, will follow the progress. Please update when available.
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Old 27th February 2017, 12:05 PM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Nice amp. Complicated driver stage but straightforward enough to fault find.
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Old 27th February 2017, 12:15 PM   #4
agdr is offline agdr  United States
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Pet Peeve Peavey (GPS 3500)
Subscribed! This will be interesting to watch.

Your Fluke 75 caught my eye. My dear old - still functioning - Fluke 77 here fixed 1000+ TVs and stereos in its day.
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Old 27th February 2017, 12:26 PM   #5
Peter S is offline Peter S  Canada
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The 'soft start' resistors show obvious signs of over-heating, so I thought it pointless to plug in channel A and attempt powering up the unit, even through a variac. Just wondering if anyone has any suggestions before I disassemble this amp. I have no experience with trouble-shooting class H (if that's what this 2 tier power rail amp is) but I thought I would start with checking output devices. I am hoping that 'in circuit' measurements, comparing Left and Right channels will give me some clues.

BTW, I have tamed a Crest CA-18 (1000 watt/chan, 8 ohms) for subwoofer use in my home theatre system, (a 39 page thread titled; "Toroidal Transformer Noise"). The Peavey seems to be a cheaper cousin to the Crest, with a one-peace PCB design, rather than modular sections, BUT, the torroid is dead quiet, at least with one channel connected, and the fans apparently stay off, rather than 'slow' until heatsinks reach 45 degrees C., making this amp a contender for (large) home subs.
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Old 27th February 2017, 12:51 PM   #6
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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These style amps look confusing. Think of it as a standard complimentary output stage and forget about the extra row of transistors, all they do is turn on when required to keep a good overhead voltage.
Looking at the NTC's, I'd say a dead short on an output transistor of the dead side is to blame.
Check with your Fluke, yes I have had one for many years and it has helped me repair countless items. Still in daily use.
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Old 27th February 2017, 01:50 PM   #7
Peter S is offline Peter S  Canada
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Thanks Jon, and all;
Good to know I have some 'back-up', from the start! First dumb question; What is an NTC? I assume you may be referring to the 2 damaged disc-like objects, pictured above. The schematic indicates these as simply resistors, and the parts list says they are 1 ohm 30 amp thermistors. BTW, If these thermistors are open or defective, would the amp still power on ( is the slow-start function timed, or do the power supply voltages have to reach a particular threshold?)
Also; a red LED called "DDT", (Peavey's protection system) is illuminated for the dead channel, I have not thoroughly studied the schematic, but I am guessing that it's power source is separate from the amp rails.
Thanks, I will begin disassembly, asap, Regards, Peter
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Old 27th February 2017, 02:22 PM   #8
indianajo is offline indianajo  United States
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NTC is negative temperature coefficient resistor.
They start off high resistance, to keep from dimming the lights in the room, then heat up & go low resistance as the rail capacitors fill up and stop drawing so much current. Newark sells GE, digikey sells some other brands with more variety. CL-21 is close for home use, 1.3 ohm cold, but in a PA venue you'd have to investigate more than 8 amps steady state current.
Also check for leaky filter cap on the dead side. Lacking an ESR meter, I like to charge them up to 2 v with ohms scale then see how fast voltage drops on 2 v scale. More than a decade of millivolts drop per scan, XXX. Compare to good caps depending on the size.
I had a 1300 w peavey that kept blowing outputs due to a dodgy solder joint on the feedback pin of the driver op amp. Kept whanging to plus rail when it heated up. Various techs didn't find it in 14 years. amp was so destroyed, outputs, emmiter resistors, VI limiter resistors, drivers predrivers, op amps, DDT stuff, 50v ceramic caps, coupla resistors, were all blown. 124 parts. A few traces were melted on the driver board. I used a room element heater for current limit on that one, the 100 W light bulb wouldn't allow enough current to let the +-15 v circuit for the op amp to function.
I put 5 amp fuses in the base lines of that one to limit the damage the next time an OT goes short. Also 2.2 v zener clamps to blow the fuses if anything gets out of spec, +-1 v on base line.
I found OT's with .450 v or below Vbe were stressed, would short out next time rail voltage got on them. The two that were .550 v or above I used as drivers. Probably not fast enough Ft for hifi, but this is PA level watts, bar customers aren't expecting hifi anyway.
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Last edited by indianajo; 27th February 2017 at 02:32 PM.
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Old 27th February 2017, 03:46 PM   #9
Peter S is offline Peter S  Canada
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Wow! Thanks Indianajo and Jon, for all the info! Sounds overwhelming, but I'm hopeful that it's nothing that complicated. The amp looks clean with no signs of over-heating anywhere but the thermistors! I appreciate your history with these amps.
I will get the main PCB out of the case as soon as I can steal some time. Seems like it will be a bit 'flimsy' with those large heatsinks on each side, and the narrow PCB in the middle. All info with Peavey or Crest class H amps would be helpful, I am trying to get familiar with these monsters!
Thanks, Peter
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Old 6th March 2017, 02:47 PM   #10
Peter S is offline Peter S  Canada
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Hi Folks;
This project is moving at a snail's pace! Work is becoming a real interference in life. I have only just disassembled the Peavey. The narrowed PCB seems to be a flimsy link between the heavy ends with the heat sinks, but I think this will be the least of my problems!
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