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Old 14th April 2012, 08:49 PM   #21
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Its a rainy day, time for inscrutable problems. I have changed the resistors in the service update diagram on the early PV1.3K amp to the indicated values, and the TIP41-42 TO220 drivers I've picked are 6 amp transistors instead of 4 amp for the oriental OEM drivers so maybe they will be okay for a while. The new resistors cause the load on the U100 op amp pin 7 to be 3 ma (op amp removed) instead of 7ma, which should be okay.
My apologies to cwahls, what I thought was the plus power supply rail collapsing, +1.5, -85, was cause by my 10 ohm load resistor for the speaker interacting with the room heater I have in series with the primary of the transformer. With the 10 ohm 450 W load replaced with a 2kohm 5 W load, the rail voltages are +1.5, -180V, referred to the heat sink which means that the output transistors are doing their job moving the power supplies around against the 4k resistors that center them on the bridge-capacitor PWB.
So how are the NPN output transistors pulling the + rail down to 1.5 without generating any voltage on the OutputTransistor emitter current sense pin 5 of the interboard connector? I replaced some 2.2k sense resistor, I now have 730 ohm between pin 5 (plus emitter current sense) and 10 (chassis ground and speaker negative). My U100 4558's, two different of them, put out 14 V on pin 7 to drive the NPN output transistors hard, caused by -15.5 V on pin 5 of the op amp and -5.7 on pin 6. I spent last night and today verifying continuity from pin 9 interboard connector (speaker + feedback) through 61k to op amp pin 5, and speaker ground InterBoard pin 10 to op amp pin 6. So I think if this happens, Q102 is supposed to throttle back the "cold bias" current to something reasonable based on the voltage coming in from InterBoard pin 5 the emitter current sense. But that is 0 V. I've actually got terminal strips up on the output transistor assembly relaying the emitter resistor top up to where I can reach it on one output transistor, and it is also reading 0 V. So are the interboard pin 8's. I replaced a lot of 1 ohm emitter resistors when I replaced the output transistors, and haven't blown up any OutputTransistors since I checked all the emitter resistor pairs at <1 ohm.
Actually, the cold emitter bias current on both channels is running 0 V, even though channel A makes music of a sort in a car radio speaker. I had hoped changing R210 to 33 ohm would push out the OT base voltage spread a little and make some cold bias emitter current, but I am running 0V on pins 5 and 8 InterBoard connector on both A & B channels. Do I read this schematic incorrectly? aren't IB 5 & 8 sense lines for idle OutputTransistor bias current? Don't Q102 and Q103 throttle back the OT bias current in a sort of negative bias control setup? I've just spent some time tracing through all the components from InterBoard connector pin 5 to Q102 base, and everything reads okay with the ohmmeter, as does the BE junction. Q102 is now a MPS8099 on NPN and MPSA56 on Q103 PNP.
Thanks for looking.
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Last edited by indianajo; 14th April 2012 at 09:02 PM.
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Old 15th April 2012, 06:45 PM   #22
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One thing this exercise makes blatatantly obvious is that the floating power supply rails of the PV1.3k and the PV2000 both, subject the driver transistors to 190V! With the B board stuck with the minus rail at -190VDC, it is hard to avoid this teaching point. The TIP41-42C predriver transistors and the motley crew of Motorola 2n6030 (PNP) and NTE60 (NPN) driver transistors are manfully standing the voltage, but if I get the power above 1/2 W like it is now I'm sure something will break eventually. I've got exactly two of On Semi MJE15033s PNP TO220 but for NPN TO220 rated above 200 VDC I've only got some GE Semi D44R1's which have the voltage but are only rated 1 A. That will probably do at 1/2 W, but not 650W. More $$$.
That is not going to solve my stuck B board. Further checks show the quad diode stack splitting the plus and minus predriver base drives from the U100 op amp is okay, and polarized the right direction, taking plus current from Interboard Connector pin 2, returning it on 1. The 400 ohm resistor connecting the + and - driver transistor bases is okay, also, with both TO3 driver bases at 1.1v. With the PNP predriver base at +9.3 V, no wonder no cold output transistor emitter bias current is flowing, the predriver Q204 is stopping everything dead. ????
One is tempted to drill the rivets tying the two emitter lines of the output transistors of the two channels to chassis out, float the speaker minuses from chassis, and connect the center taps of the two power transformer halves together on the capacitor PWB to reduce the 190 volt rails to the 95V shown on the print, but I'm too green to fully understand all the implications. For certain it would mess up the bridged output terminals mode of the amp, which I won't use, but I don't know what other problems it would cause.
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Last edited by indianajo; 15th April 2012 at 07:00 PM.
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Old 15th April 2012, 11:06 PM   #23
Enzo is offline Enzo  United States
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Keep in mind that flying rail amps may have funny voltages to ground, but the amp circuit itself is moving with those rails. In other words, in a "conventional" amp, if there were 50v rails, then drivers and outputs can be subjected to 100v - the sum of the two rails. The flying rail is no different. Parts in the drive train can be subjected to the total of the two rails. Those rails are not referenced to ground, they are referenced to the load.

If the output is pulled over to one rail, then the opposing rail will have been pulled along with it.


DOn't think of redesigning the circuit, it is fine like it is, at least once it is repaired.
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Old 17th April 2012, 02:28 AM   #24
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Quote:
Originally Posted by Enzo View Post
Keep in mind that flying rail amps may have funny voltages to ground, but the amp circuit itself is moving with those rails. In other words, in a "conventional" amp, if there were 50v rails, then drivers and outputs can be subjected to 100v - the sum of the two rails. The flying rail is no different. Parts in the drive train can be subjected to the total of the two rails. Those rails are not referenced to ground, they are referenced to the load.
If the output is pulled over to one rail, then the opposing rail will have been pulled along with it.
DOn't think of redesigning the circuit, it is fine like it is, at least once it is repaired.
Thanks for the extension of my limited knowledge. I took money to the bank account with the debit card today to order MJE15032, and am dutifully putting the last MJ21195 and MJ21196 on the TO3 driver spots tonight. Both PCB's, even the one that works.
My only previous transistor amp repair was a dynakit ST120, which had 80V single rail and quasi comp allegedly 2n3055 output transistors- 60V rated in 1966. They got away with it, right? Probably not, they were probably specially selected 2n3055 transistors - the burned TO3 hulks had an RCA 5 digit number on them.
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Last edited by indianajo; 17th April 2012 at 02:31 AM.
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Old 17th April 2012, 04:07 AM   #25
Enzo is offline Enzo  United States
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I have seen any number of amps with 2N3055 selected for 100v curves.


There are certain old Fender models with a schematic specifying 50v filter cap in the bias supply, and in fact installed in the amp, with the same schematic calling for -55v on the supply. But i am not aware of any glaring errors like that in the Peavey amps.
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Old 18th September 2012, 01:13 AM   #26
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Well, I figured out some things on my own but I need help again on the PV1.3k with bad channel B. I actually had it running playing music from a radio for half an hour on both channels. I had 32 ohms on the 120 vAC (+-75 rails), 10 ohms@450W on each output, with car radio speakers clipped across 5 of the 10 ohms on each side(e caps in series with car radio speakers). Then Pop! back to -93v, +2V rails. I've got the triacs removed until it works a couple of hours, since all they do is burn lands off the OT board anyway, they don't blow the breaker.
When stuck with DC out on speaker +, op amp U200b (4558) is putting out 6.2VDC into the diode stack and 4.2v into the base of the NPN pre-driver, no wonder. The output transistors are fine, with the driver board disconnected the rails are +-75 at the OT's and the emitter current on the one I have test points run out on is 0.00V. As soon as I plug the driver board in, back to -2+93 rails. The op amp has -15 into pin 5 and -6 going into pin 6 (minus) So V+-(V-) should be -15v-(-6v)=-9v? * A factor =minus 15 V out, right? As soon as I pull the op amp, the rails go back to normal. changing the op amp doesn't help. The capacitor between pins 5 & 6, C208 was changed to a 1000 v rated 22 pf cap, which made it start working before. Q200 & Q204 predrivers are ON semi MJE15032-33. Drivers are survivor MJ15024-25 from the old OT's. I did have it working with a MJ15025 and a MJ21196, but pulled the 21196 to not violate your "matched technology" up and down transistors rule. OT's are all MJ21195-96, tested for Vce and the odd one rejected.
Ouput Connector pin 9 is -43, pin 10 is 0, Op amp pin 8 is +15.5, pin 4 is -15.5. I've checked the current out of pin 7 of the op amp to make sure no predriver transistor is blown, and I'm only getting 3 ma into a 1000 ohm resistor then the ammeter then emmiter line of OT ground. The predrivers read okay for the double diode test with the ohmmeter, anyway. I've traced R235 (4.7k) and CR 220 out to the ground at pin 10, they are okay. C208 is still reading infinity on the ohmmeter (@2v). The 61k resistors read right. All the electrolytic caps are new, and I had AC radio beats going into op amp pin 6 when it was working. Please ???????
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Last edited by indianajo; 18th September 2012 at 01:31 AM.
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Old 18th September 2012, 04:05 AM   #27
Enzo is offline Enzo  United States
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Your output stage is working. You disconnected the driver and it centers OK, right?

You replaced the op amp, so we assume it is OK. The poor little op amp is trying its heart out to correct a problem in the system loop.

Remember, this amp has flying rails, the center tap of the power supply is NOT ground. What happens is the system more or less drives into ground, pushing the rails back and forth, and the center tap moves with them, feeding the speaker. So when the rails go off the end, it means the amp is skewed over to one side. It is the same thing as putting DC on the output of a conventional amp, we are just looking at it from the other direction, so to speak.

You gave us a few op amp voltages on pins 5 and 6, but you left out the MOST important ones. What voltages are on the IC power pins, pins 4 and 8? Note that +16 and -16 come onto the board left side driver page, but the op amp in question runs on +16A and -16A. Look closer on the left of that oage and see that the 16A rails are simply the 16v rails run through isolation diodes. SO verify that even during failure mode you still have good +16A and -16A, AT THE IC. If it is going away, then look back at the plain 16s coming on the board, and if not right, back to the small power supply board.

Look at your circuit, you have -6v on pin 6? How did it get there? Pin 6 is a resistor to ground, isn;t it? Pin 5 is a sample back from the speaker output. Offhand I see no other places for any voltage to get on pin 6 other than through the IC. All the more reason to look for missing or funny power rails to the IC.

You pull the op amp and the amp instantly settles where it ought to be, so again, the amp is working, it is being TOLD to more off center. To me that means everything to the right of the op amp is probably OK.
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Old 18th September 2012, 06:35 PM   #28
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Thanks for listening. This morning I pulled the 4560 op amp and the 3080 transconductance amp, no improvement. Then I changed the TO3 NPN driver, Q207, worked okay for 1 second then pop! rails off center.
Then I checked the U200 pins 4 & 8 without op amp, -15.1, 15. Then with op amp, installed and and rails off center, pin 4 -15.1, pin 8 +9.4V . So bingo, you're right as usual. Will check the voltage regulation stack again. With the + rail at 2.1 V I'm a little vague how the op amp V+ will regulate to + 15, but will study the schematic.
The minus of my meter is on the emitter line on the O.T. assembly, I put a solder terminal strip over there to measure the emitter resistors of a PNP and NPN resistor directly against the emitter ground line. That terminal reads 0 ohms to the pin 10 output connector but is easier to clip on.
Update the 47 ohm resistors from the +- rails go to CR200 and CR216 the B 16v diodes, and those diodes read 560 ohms forwards and build up backwards to 1999 ohms as the capacitor charges up. Is there supposed to be some connection from the C16v supplies (from the power board through P204) to the B16V supplies? That power board 16v stuff was all blown up when I bought the amp and I replaced it, but I read the schematic that the supplies for the 4560 from P204 and the supplies for the 4558 from the OT rails are separate.
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Last edited by indianajo; 18th September 2012 at 06:53 PM.
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Old 27th September 2012, 08:01 PM   #29
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****, C218 on B driver PWB, the -16V supply filter cap, was upside down. Just like it was when I bought the amp, and upside down from C118 the analogous cap on A driver PWB. Was working a while since it was a new rubicon 50v cap, was always supplying the 4558 op amp - rail (or the short through the op amp from output to rail was from the 61k feedback resistor at -90 v), but the -16 supply was starving Q204 the PNP predriver apparently. What was collapsing was the op amp plus supply, because the main supply rail was at 2v with the NPN trying to make a little bias current while the the PNP side was dry of supply current. I was wondering why I was getting only 0.1 V emitter current output transistor voltage on the NPN output transistor side, with the + output transistor rail collapsed to +2.5 V. Has worked on both channels a whole 5 minutes now with plus and minus rails at +-75v (with the room heater still on the AC line) . Bias current OT connector pin 5 is -.04 v, pin 8 is .06v. Seems safe. Now on to producing a safety circuit that really works instead of burning the lands off the output transistor board to the triac!
The safety circuit is going to use the diacs, or bidirectional switch as some people call it, to turn off the triac that hold a normally open speaker relay closed. I was going to use 2 triacs to do a logic inversion so I don't need a 24 VDC supply (and transformer) but I can't really figure a way to do inversion in 120 VAC logic. So there are going to be 4 optoisolators from the diacs on the "flying speaker ground" to the relay turnoff logic on the wall plug neutral. The relay may be too slow to prevent speaker fires, but I have also put 25 amp DC fuses in the supply rails. This will cause DC on the speaker if one fuse blows due to a bad output transistor, but the relay will come along and clean up the mess in a quarter second or so if the other fuse doesn't blow too.
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Last edited by indianajo; 27th September 2012 at 08:17 PM.
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Old 4th January 2013, 06:02 PM   #30
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Anybody bored? Waiting for the customers to shovel out the snow and get out?
I got both channels of the PV1.3k working, and figured out why channel B had no power to it (wires tied off by repairman) and printed stickers saying "Do NOT use channel B". The output kept whanging to +75 V on the output with any op amp in the U100 spot (4558) any time I touched anything on the input circuit with a meter. But stable as a rock at 0 v with no op amp installed. Turns out the pin 6 of the U100 socket had a bad solder joint, probably left the factory that way.
So anyway, with the output at +75v and a room heater in series with the wall AC, Q102 doesn't do anything. That is with a 10 ohm resistor on the speaker output, with a 4 ohm speaker in series with 3000 uf AC capacitors bypassing 5 ohms of it. So if Q102-103 don't limit DC idle output transistor current, what do they do? The sense path from Q102 base to the emitter resistors is continuous, I probed the trace on the output board. The several of the burned 2200 ohm sense resistors were long replaced, the sense line to emitter line is 700 ohms.
I see from this thread on an Ampeg bass amp Ampeg svt 3 pro Power Amp that Q102 and Q103 might be "VI limiters". So what does it take to get the base of Q102 up to 0.6 v and actually limit the drive current to the driver BJT's? Maybe I still don't have enough load resistors on the speaker jacks to test out this part of the circuit?
On an aside, I've traced out the AC power scheme enough to figure out that the triac on/off "relay" is switching the neutral side of the AC power, and the circuit breaker may be on that leg, also. No wonder there is no CSA rating on the backplate. Anyway, I wanted to put a MOS spike supressor on the AC so I don't have to unplug it everytime I leave the house when it might storm. A poster on another thread warns me these fail shorted. SO, to protect the hot line from 1000 V spikes from lightning, I have to install another fuse, on the hot input wire. Fortunately, there is room by the input splice board. Maybe units sold in Canada got an extra circuit breaker up in that dead area? I'm trying MDL30 at this time because I have some, but will put some MDL20 fuses on my next out of town order.
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Last edited by indianajo; 4th January 2013 at 06:07 PM.
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