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Old 15th March 2015, 04:53 PM   #1
Seeber is offline Seeber  United States
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Default Peavey PV-1.3K power amp repair question

Hello, I'm new to this forum and have a question about repairing a Peavey PV-1.3K power amp. This is my first attempt at an audio power amp but I have done other electronic repairs for many years and the basics down. On this unit I have one good channel (channel A or L or 1) which is why I took on the project. Power amp block is good surprisingly and I have isolated the fault to pin 5 of U200 on the preamp board where it looks like most of the signal is clamped to ground looking with a scope. I've tried a few things but don't understand the circuit enough to proceed. Also have read quite a bit on this forum but still think I need help. Any help would be greatly appreciated. Thanks.
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Old 15th March 2015, 06:44 PM   #2
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The schematic is on eserviceinfo.com open 24/7. You don't have to be a member to download one.
Pin 5 of U200 gets the feedback from the output board, through pin 10 of the push on connector. Check the connector that the continuity is still good over to the output boards. Those push in connections are not entirely reliable. The drive circuit lines are probably high current enough in bar band use to keep the oxide burned off the wires and pins, but the feedback and ground circuits are low current/low energy. Another poster, a real vet, had the ground circuit go open on his similar PV4 and had a lot of trouble finding it. Note PV1.3k has "flying ground", the speaker return is not case ground, and goes up and down with the plus terminal. Makes for cheaper mono bridging. The way your clean oxide off tin connectors to copper wires is remove them and replace them. Also the sockets to the pin on the board.
Pin 5 U200 also gets clamp voltage from the jfet of the DDT circuit, the one that damps down the input if there is too much high freq (ie square waves due to clipping) that is going to tear up the speaker drivers. I had a blown up jfet in mine, but that was due to output transistor failure, driver transistor failure, predriver transistor failure etc etc, total of 124 new parts required. Jfets when good read like two diodes to the center, or a resistor from end to end.
I also had a bad solder joint on the other side of U100, the input amp side, that was causing the op amp to whang to 15v rail and the output to whang to 170 VDC anytime anything was brought near that input. It was socket to board poor solder joint, the socket to IC connection was good. That amp had probably been blowing up speakers with DC out since 1994 when it was built. When I bought it the crowbar triac had melted the trace to the output off the board, the power supply spade lugs were removed from that channel, and a label "do not use chan A" stuck on the input connector.
Notice you can swap A and B input boards in the screws on the side to work on the B one with it on top. The harnesses don't interchange, but they will stretch to reversed boards if you untie some wire clamps.
Wear safety glasses on turn on and use a series resistor in the AC to avoid the output transistors blowing the dies out through the transistor case to the ceiling. I used a 1500 watt heater element from a family dollar heater that had a failed tip-over switch, plugged right up to the 120 VAC harnesses with spade lugs. When I tried a light bulb box AC limiter, the +-15 v power supplies wouldn't come up high enough for anything to function.
If you put 25 A fuses in the power lines to the output board, the crowbar should blow them instead of melting the land on the PWB. I mounted four fuse clips to the screws on the top of the output board, and made new 10 gauge wires for the power. The scenario is, a speaker phone plug comes a little out, shorts the output, the output transistors heat up, one shorts, DC 170v of that polarity goes out on the speaker, 30 ms later the triac shorts the output to ground, then the triac melts the land instead of the breaker tripping. With additional fuses, the current through the triac will be limited to 30-35 amps. I used 1/4 x 1 1/4" automotive (32 vdc) fuses. They do actually blow when an output transistor faults out, I tried to save a couple of old ones that blew after a minutes service.
Good luck. plenty of capability in that amp, fans and e-caps may be reaching life, but the 5 output transistor section is extremely tough. Hint, use Double bananas for speakers instead of 1/4 phone plugs. The band I bought this one from (for $55) had.
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Last edited by indianajo; 15th March 2015 at 07:06 PM.
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Old 15th March 2015, 10:20 PM   #3
Enzo is offline Enzo  United States
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First, you have not told us the problem with the amp, you just told us what you think might be involved. What exactly is wrong about the amp?

You usually cannot read the input pins of an op amp reliably. But let's start with little signal at pin 5 of the IC. That pin receives feedback from the output. If anything between that IC and the speaker terminals breaks down so the thing can't amplify, then there will be no output to feed back from.

Do you have good signal at pin 1? Have we made sure the bridge switch is in stereo position?

I see no connection between pin 5 and the JFET. Look at the drawing, the path to pin 5 through R231 and C219 goes to ground. The JFET also goes to ground, but other than that they were drawn connecting to the same ground line on the page, they share no circuitry.
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Old 15th March 2015, 11:14 PM   #4
Seeber is offline Seeber  United States
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Thank you both and sorry for the omission. The bad channel does pass some signal but is weak and somewhat distorted and the volume pot shelves off quickly. Signal is good at pins 1 and 6. One other clue is that when I first powered on the amp the ddt led was stuck on. Not sure what I did to extinguish it but I had started by reseating and checking connectors, etc. Thanks.
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Old 16th March 2015, 03:29 AM   #5
Enzo is offline Enzo  United States
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OK, I'll wager the problem is over on the input side rather than the power sections. Have you swapped out the IC? Note the U200 is an old 4558, while U202 is a 4560. I am note sure what made them decide not to use the same IC for both, but I accept they did it for a reason, so I suggest we not try to use "something better" in place of the 4558.

The simple thing to do is swap that 4558 with the one on the other channel. The problem either moves with the chip or it stays in the channel. That tells you instantly if the IC is the problem or not.

By the way, you say you have good signal at pin 1, but is it about the same as the pin 1 signal in the other channel? Sometimes a signal looks OK but in fact is much reduced from normal, so check.

But if signal at pin 1 really is OK, then everything before it is OK. A couple notes in general though. Peavey amps are all similar, so the same techniques apply to many of their models. The compressor IC, U201 here, can be removed and the amp will function normally, but without the DDT. Its output pin connects to the U200 pin 3, and it drags down the signal there if needed, that is how it limits. JFET Q201 is not related to U201, but connects to the same place. It is a mute, and if it fails or the control at its gate fails. You can remove it from the circuit and the amp will function normally, you just won't have the power up mute, and so a thump might result. In both these cases, we remove those things for troubleshooting, but I expect them to be put back at the end of the repair.

Since it passes signal, I have to thing the current amplifier stages are OK, otherwise we'd probably have DC on the output. One thing I want you to do is power off, go in the rear and measure resistance across the speaker terminals. There is a triac across the output, and it should measure open. if you get a lowish resistance, the triac may be faulty, and that would squash your output.
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Old 16th March 2015, 03:35 AM   #6
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A couple of likely suspects for distortion after the twenty year boundary, are the Electrolytic NP cap on the input board at the back behind the input jacks,the electrolytic NP cap C203 on the input of U200, Electrolytic cap C207 on the DDT Jfet, electrolytic cap C206 on the output part of U100. Also the wiper on the input potentiometer can have an oxidation problem. I changed all these e-caps as a matter of course as part of the repair of my amp, and had trouble with one bad solder joint I made, that I found by tracing the audio through the amp with an analog voltmeter. I was using a transistor radio earphone outlet to drive the amp so the voltages were high enough to see at all points of the circuit with the 2.5 VAC scale of the Simpson 260-6XLPM analog meter I own. You can see the beats of rock music with the pointer of the meter, and be assured if you see them that what you are seeing is not an oscillation caused by a bad solder joint. Be sure to use a .047 uf cap on one input of the meter for AC, it will read AC on DC voltages. Scopes are nice, but the used ones I buy all blow up due to overage e-caps or something. I replaced the 2.2 uf non Polar e-caps with ceramic ones; John Curl would have a cat but if you buy the 50 v ones and run a 2 vac signal through them the higher voltage rating linearizes the curvy capacitance-voltage curve somewhat. Anyway, the 4.7 uf 50v caps I put on the input of another amp made it sound better than the tantalums I got at the local TV repair shop, which had popcorn noise from the day I bought them.
Reseating connectors is good, the ones between the input board and the driver board, and the ones between the driver board and output board all have those punch in wire connections that need their own reseating IMHO on the low level signals anyway.
Edit - because the op amp output is coupled to the input board by long parallel wires, I decided not to update the 4558 op amp U200 because later op amps are much faster and might cause oscillation. If I was really on a hiss campaign, I might try a slow but quieter 5532 insted of the 4558. The 4560 is necessary because it has the current necessary to drive the LED on the front panel. Again you can see the music flow into the op amp (before R109) and back out again with an analog VOM, but I would look at other components (or just change them based on the calender) first. Reseating the connector (socket) on the op amp is a good idea, any tin-tin connector is suspect of oxidation after twenty years. Because I had the bad solder joint problem, I replaced the socket on U100 (only) with a phosphor bronze contact one from TE connectivity that might be more corrosion free.
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Last edited by indianajo; 16th March 2015 at 03:51 AM.
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Old 17th March 2015, 03:41 PM   #7
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Thank you for posting great information and tips. I'm embarrassed to say before the last two posts I removed U200, powered on and blew the output. It actually smoked a trace right off the pc board. I hope this to be a lesson to newcomers and the less experienced that sloppy procedure and shortcuts cost time and money and needless irritation. The larger triac CR244 is gone, confirmed shorted after removal. The PNP busses are shorted (Q215 - Q219), so I'm removing the output transistors one by one and checking. Before this debacle I did switch U200 with U100 and problem persisted. I also swapped output channels, seeing that P110 and P210 had the same pinouts, and problem followed the preamp board. Thanks.
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Old 18th March 2015, 01:48 AM   #8
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Sorry about that.
Uh, yeah, see post #2 about putting in 25A fuses between the 10000 uf main caps and the output board to save the triac and the PC land to it from vaporizing. Bad design, smokes the board and triac every time. Took out one diac on mine, too. Enzo reminded me to disable the triac before fooling around, but I didn't either, and vaporized an 18 ga wire I had patched the output board with to the triac. The room heater in the 120 VAC wires in the back corner helps stop vaporizing so much stuff, though those 10000 uf caps store a lot of energy even at 75 v instead of the nominal 85. They sell insulated .250" flag terminals at O'brians auto parts. I use a Klein or Ideal crimp tool, and pull test them.
You can individually swap output transistors with MJ15024/25 because of the high emitter resistance (0.5 ohm) but you can't use something like a MJ21193/4 which is cheaper without replacing them all. You'll need smaller screws on the later transistors, #4 instead of #6 for the old parts. I bought #4 nuts with built in stainless star washers on them to be sure they touched the bottom of the transistor and carried the C current to the trace.
I used an old MJ15024 for a driver instead of the special cheap 15020, and converted the 20 outputs to the later parts at $1 cheaper per. But I'm having to fool around with the idle bias current.
Check your emitter resistors around any blown output transistors, I had a lot of those blown.
If any of the output transistors are reading 450 ohms C-b instead of 550 to 600, that was correlated to the ones that continued to blow up. The one that I salvaged for the driver transistor was the higher value.
Note if the Chinese 2SC# 2SA# predriver is gone, Peavey has changed to MJE15032/3 and changed R110 210 from 10 ohm to 33, and changed R102 127 202 227 from 1.5k to 4.7k.
This is in the idle current spreader circuit but still is not giving me more than 0.5 ma of idle current. I'm thinking of putting some potentiometers in to raise the idle current. Enzo told me to examine the crossover distortion with a scope, but I can't keep a used scope running. I don't do this for a living, I need a working car worse than a scope at this time.
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Last edited by indianajo; 18th March 2015 at 01:52 AM.
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Old 18th March 2015, 11:02 PM   #9
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Turns out I had one blown PNP output transistor on the B side, next to last in the chain. Now have parts on order and should arrive in a couple days. Looking forward to getting the output section back to where I started and then proceed with the preamp board. I did measure all the emitter resistors and all seemed to be fine. Will update as progress is made. Thanks.
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Old 19th March 2015, 03:31 AM   #10
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Hope you ordered fuse clips, along with the triac etc. I drilled a hole in the middle of my fuse clips and screwed them to the heat sink mount screw on the top of the output transistor section, 4 of them. Then I made new spade lug jumpers from the output of the fuse to the board input. This melting the speaker to triac land instead of protecting the speaker and blowing the main breaker seems to be a design fault of the PV-1.3k. My speakers cost 12 X what the amp cost, or 3 X with the new parts cost, so I need speaker protection. Used SP2-XT speakers were about $600 the pair, though they are getting more reasonable now.
You test the diac by putting 6v through a resistor to it , no current on DVM 200 ma, then putting 12 v through it get 10v/resistor value current through it. It drops about 2 v conducting and breaks about 8 v. I think Peavey calls it a SPS or SDS or something daft. Newark sold me some new ones as a Powerex BS08d.
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Last edited by indianajo; 19th March 2015 at 03:35 AM.
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