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Old 1st March 2012, 03:49 AM   #11
cwahls is offline cwahls  United States
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FWIW it was the most epic medium USPS flat-rate box I have ever shipped.
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Old 1st March 2012, 04:10 AM   #12
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Thanks for listening. this one has exploded 4558 op amps while I watch, driven by the O.T.'s. Hint- when newark is selling On semi O.T.'s for 1/2 price, and they have a 200 mw rating, that is not a keypunch error. 10 seconds at 3 ma idle current at 90 v and kerpow!! Thought I had found a bargain, then thought I had some assembly errors (not). Hint 2, old O.T.'s that pass the double diode test are not necessarily good. Even one that didn't leak at 18V ceo exploded. So, 20 new O.T.'s at $3.80 apiece. To stop the exploding driver transistors op amps, and ground trace burning, I have put 3.9V zeners on the OT base drive lines on the PWB, and 5 amp fuses on those lines, on the driver on + and on the OT on minus. Also a 5 amp fuse on the middle, allegedly "earth" on the Pwb. I had burnt that a couple of places before fuses. No telling how the zeners sound, nobody on here had an opinion.
Wish I could edit after 30 minutes. After buttoning up, pulled B driver PWB and looked at C206 carefully again. The PWB trace was burnt off under the nice shiny new c206 cap lead on the + side. So no connection. Resoldered, plugged back in and - - - big hum from transformer, music on A not B, +22 on B rail, so something else is wrong. Power off. Back to DC analysis.
I've found 25A @ 600 V Allen Bradley contactors that will fit in the box by the fan, and plan to put one in to disconnect the speakers instead of the stupid triac PCB burners. Replaces the stupid bimetallic thermal disconnects. I'm using the Peavey protection diac (8v)(replaced) but changed the resistor from 47k to 20 k and the capacitor from 2 uf to 4 uf to try to get 5 ma out of it to drive triac type optoisolators. Two optoisolators back to back (with diodes for voltage protection) will trigger a triac the same way even if the input is either +85 or -85 DC. Bought some schottky 100V diodes to back up the opto inputs. The relays are 120 VAC coil. Unfortunately, the relays are NO only, so I have to leave them on all the time and come up with an inverter circuit. Found a form C relay but it won't fit in the space. I might still have to put a 24 VAC CT transformer in there to make inverting relay logic isolated from the sound power supplies. I still have to disassemble the relays and see if I can coat the copper points with silver plumbing solder. But this beast may get under control, eventually. Thinking about putting hall effect magnetic sensors at the zobel coil to use the jfet to damp really big current spikes, but don't really understand how the jfet works. Thought the jfet pinched off the signal to plus rail with the gate at first, but the 3080 op amp drives the resistor leg of the jfet, not the gate. The DDT defeat switch drives the jfet junction I found today, defeat is switch closed. ???
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Old 1st March 2012, 08:13 AM   #13
Enzo is offline Enzo  United States
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When I see "resoldered xxx and plugged it back in," I want to run from the room.

Are you not at least using a bulb limiter when first powering up after ANY change? Or better yet, a variac and ammeter on the mains. Just plugging it in is a great way to blow up good new parts you just installed. And possibly cause even further damage.

Any time you make a change you must do this. You cannot say "Oh, I just installed those a minute ago, they must be good. ANy time you power up and something fails, you must reset the "Oh that's OK" list. A circuit fault can blow up other parts faster than any fuse, breaker, electronic measure, or your fingers. Always check.

We have a bit of shop philosophy here, "Never think up reasons not to check something."


Not asking you to change, but whenever I read "OT" it means "output transformer". I have to stop and realize you must mean output transistor. Perhaps that is just me. Or maybe that I spend a lot of time with tube amp people.


I'd also like to highly recommend making the amp work as stock first, and only then start modifying it. If you throw all your re-engineering into the middle of the repair process, it only will serve to confuse the effort. Some zener, for example, may limit some transistor, but it can also wind up masking the cause of the problem you are trying to solve.


Look at the compressor circuit again. The 3080 doesn;t drive the JFET at all. On the print, "pin 1" of the JFET is ground, the bottom pin is connected to pin 3 of U100, the amp's input. The output of the 3080 is ALSO wired to that pin 3. The 3080 and the JFET are wired in parallel, the 3080 doesn't drive it. JFET Q101 is a power up mute. It is driven - its gate is controlled by - the voltage at C107 as charged by R113 from the +16v rails, which ALSO powers the 3080. That is all the JFET does. the 3080 handles all the DDT work alone. They can interfere with one another, of course, but they don;t work each other. The DDT switch controls the 3080.
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Old 2nd March 2012, 04:26 AM   #14
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Cwahls, the transformer was nicely packed. The post office probably regrets their "it fits it ships" policy. Came on a Saturday afternoon, too, which was ultra cool for $12.
Thanks, Enzo. I'm using a 1500 watt room heater element in series with the 120 VAC amp, which cut the size of the O.T. explosions at installation. 100 watt bulbs are illegal now, and I never owned any for the reason they have been banned. With the amp dimming the lights at turn on, I was afraid to use any smaller bulb as a series limiter. The zener clamps behind fuses on the O.T. base driver line was self defense, I got tired of trying to figure out what trace the shorted O.T. burnt off the PWB this time. Was a few weeks before I figured out the 18V EB shorted O.T. leakage test found bad O.T.'s better than the double diode DVM check. Then I had to find out the newark NJW21194 O.T.'s with 200 mw power rating were rejects after the PNP original transistors stopped blowing up in pairs with them.
Thanks for the explanation on the JFET. I read in Mr Peavey's corporate history "white paper" that the DDT was invented to keep people from melting the speaker voice coils with square waves from their distorted guitar head. Nice, but I don't understand how these detect too much high frequency stuff (which is my understanding of a square wave). I don't see a RC filter for low frequencies. All I know at this point is the jfet and the 3080 have been blown on both sides, as I bought it (pretty destroyed). If I'm going to keep this thing, my worry is the lightning pop that comes at the beginning of a thunderstorm, not destroying the tweeters in my T300 HF projectors. Once I notice a storm is coming I turn stuff off (except tube amps) and this amp seems to be able to produce 1/2 watt with the input turned down, but all that energy is stored in there ready to come out at in inadvertant input. That is why I was musing about the zobel over current sense.
The different R & C are in there at the old diac circuit place for the relay disconnect, since I didn't want to take the O.T. assembly out again, and the triac traces were all melted off. The triacs are not ther so the diac and new RC are not doing anything to the circuit but driving a disconnected 2 wire cable at this point. When the amp works I'll look at putting the speaker disconnects relay drivers at the end of the cables. At this point (other than working on my failing internet connection today, and a bad hard drive) I'm going to replace the rectifier bridge on the B side that measures okay with the DVM, and doesn't leak current with the 18V supply. Something is collapsing the rail voltage on the B side when music happens, and with a different transformer in there, it is probably not the transformer. When I disconnect the B driver PWB from the O.T. assy, the rails go back to +-85, but when I put the 18V supply on the OT to driver collector inputs pin 3 and 6 (+-95 V nominal to driver) the driver is only leaking 0.3 ma. Thanks for listening.
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Last edited by indianajo; 2nd March 2012 at 04:35 AM.
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Old 2nd March 2012, 04:47 AM   #15
Enzo is offline Enzo  United States
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DO you have a variac and AC current meter?

The bulb thing works well enough, but if the amp is loo large for it, I have to wonder if putting some high wattage low resistance in series does the same thing. A variac and ammeter allows you to advance the mains to say 20v and see that current is already ramping up, so you turnit back off without ever stressing the stuff in the circuits.


Oh, and they phased out incandescent 100 watt bulbs in general, but decorative ones are still sold - you know the large round ones for vanity mirrors and such. And look at the dollar store.
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Old 3rd March 2012, 01:35 AM   #16
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Well, 100 W bulbs were back in the store today, now made in c****. Surprise. Now that I am not blowing up O.T.'s (output transistors) anymore. The room heater made a nice crackly noise when an O.T. blew, as the rails dropped to 40V or below. I got the room heater idea from Skippy/Dieter in Sydney, Aus. The last time I had B channel on music input with the resulting +3v -70V rails on the B side with 40VDC on the 10 ohm speaker simulator resistor, the heater was not crackling, and the 10 amp rail fuses (I installed) did not blow, so I am not blaming that on bad O.T.',s particularly as the +- rails went back where they belonged with the driver PWB unplugged from the OT assy. Have the generic replacement bridge out for installation but some PWB drilling required to wire in the + and - spade tabs. Had the 4560 op amp unplugged that time as I was under the impression it was just an LED driver for the front panel, but since + and - driver transistor input currents are inputs to the 4560, that was a stupid idea. Now that I traced the input board wiring this week. As the logic transistor on the driver PWB was also blown, and 3 driver transistors and both bias current clamp TO92 transistors were blown, plus a bunch of 50V capacitors and 1/4 w resistors, I'll have to suspect the 4560 is blown, too. I have some TI RC4560's. I think this band had a tendency to plug high power guitar amps into PA amp input phone jacks. As did the band that sold me the CS800s, also.
I realize, in full production, the rail fuses would have to be 25 amp, but 25 amp car fuses did not melt with shorted O.T.'s, so this whole fuse thing is a ****shoot. Pulling both rail voltages and ground center with a 3 pole relay on DC detection would be cool, and I have that many 25 A relays, but I can't make two of them fit in the box.
Time out for life. I have to drive out to north Clark county tomorrow and see what the tornado took away or dropped on my country property. It is south of Marysville between devastated Henryville and Chelsea, IN. Nothing to lose out there but a refrigerator stove and a freezer, a trailer with a new roof and rotten floors and walls, a couple of '98 model riding mowers with new motors and repaired decks, and a working Hammond H112 organ. Oh, a Peavey wedge monitor speaker that came with the rest of the bar band stuff. Hope my neighbors are okay. None of them had a basement or root cellar, as my place did once. Hope Cwalhs was not in the path, as this storm skipped across Illinois, also. However, I think I70 is a bit further north.
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Last edited by indianajo; 3rd March 2012 at 01:58 AM.
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Old 3rd March 2012, 05:08 AM   #17
Enzo is offline Enzo  United States
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Well, I wish you good fortune - that weather is pretty scary.
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Old 6th April 2012, 03:36 AM   #18
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Well, everything is okay at my country property. The damage was 2 miles north of there. I saw Michigan had some problems, I see you (Enzo) are active on Solid State thread so I hope everything was okay with you- I think the tornado was in Ann Arbor.
Anyway, on the PV1.3k on your schematic, what do you have for the value of R102? My schematic says 4.7k but I have boards with professional looking 1.5k resistors installed for R102. I've been merrily blowing 4558 op amps on channel B and I'm getting 9 ma into 10 ohms out of U100 pin 7 without the IC, with the output transistor board disconnected, so it is coming out of R102, not the NPN predriver transistor. The NJM4558 datasheet shows output swing voltage degrading at 2000 ohms load, so I'm wondering if 1.5k is a little low resistance. If U100 was actually NJM4560 this much current wouldn't be a problem but they obviously got away from the 4560 for a reason. I suspect it is because 4560 might oscillate with that long run of parallel wires between input and output running over to the input board. No telling what IC was in this unit originally, the 4558's weren't NJM so probably were repairman installed. I got my "new" 4558's from mcmelectronics.com, so they should be "new" and the one on driver board A works okay with the 1.5k resistor. I'm getting 14V out 7 of this IC installed with 5 and 6 0V, which was new not too long ago. Trying 2.2k for R102 before I hear from you.
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Last edited by indianajo; 6th April 2012 at 03:41 AM.
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Old 6th April 2012, 04:31 AM   #19
Enzo is offline Enzo  United States
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R102, 1.5k or 4.7k? See the service bulletin I attached. Looks like your schematic is newer than your board, so make the changes to your board.

But I seriously doubt that change is the cause of your blow-ups.


If the schematic says 4558, then that is what belongs there. Especially since the U102 is specified as a 4560.

NJM doesn't matter, any 4558 will be fine. Leave the IC brand snobbery to the distortion pedal guys. These circuits are not that high-strung.


Let me get this straight, you are operating the amp with the power board disconnected? The 10 pin ribbon is off? DOn;t do that, the circuit depends upon that other board being there. You have removed some of the current paths, not to mention the dual diode between pins 1 and 2 that complete the bias string.
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File Type: pdf PV PV1.3K predriver update.pdf (73.5 KB, 128 views)
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Old 6th April 2012, 06:59 PM   #20
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Thanks for service bulletin, thought the change might include changing 4558 to 4560 but I see not.
With the room heater in the 120 VAC line, I'm still getting collapse of rail voltages on B side driver board plugged in to +1.5V -80V. It is not blowing the 10 amp rail fuses, but the output transistors hold off the +-80V fine without the B driver board plugged in. I've already put stainless star washers on stainless screws between the speaker negative and the heat sink (ground) replaced the 10000 uf caps, the transformer and the diode bridge. No change. I'm trying to fix the driver board, So I try it with the output board unplugged for a second or two while I make measurements. With or without the output transistors board plugged in to the driver PCB, I get 14 VDC out of pin 7 of U100 on my second 4558 with 5 and 6 at 0 v. I've had 3 different transistors in the Q100 spot and none fails my 18vDC leakage test backwards. I get 8 ma out of pin 7 U100 without the IC or OT board link, which I thought was excessive. I have no 33 ohm resistors, and there is a stack of capacitors over R110 (10 ohms) simulating a 0.015 uf C120 that was blown, so there will have to be a parts order before I resume. I'm using TIP41C and TIP42C for Q100 & Q104 because they have the same safe operating area at 80V as MJ15032-33 at $.38 , but I guess I'll go ahead and buy some $3 MJE15032 and 15033. Thanks. Will be a while, I have to buy lawnmower tires and 23 acres of fuel this month.
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Last edited by indianajo; 6th April 2012 at 07:02 PM.
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