PL700II blues

The Rebel

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#1
I have a PL700 Series Two, that I bought from a gear buddy of mine, that is having output issues. Back story goes like this; He bought two original 700IIs loooong ago and played them for years until he sold them to a gear buddy of his that bought them, played them for a while until he didn't. They sat in his house for years. So my buddy buys them back with intentions of upgrading them with WOA control boards and wattsabundant DC protect circuits. He found a tech that would do this for him so he hands over his two 700IIs to be upgraded. Tech guy proceeds to do his thing to the amps. When the first one is done he records a video of it playing and sends it to buddy. Buddy says keep it until both of them are ready. Well, that day never came, as the second amp never came back to life. So, he picks them up as is, thinking he has at least one working amp out of the two. Gets them home, the first one(from the video) now has bad distortion and very low output, with the right channel outputs getting very hot to the touch. Tech guy never says a word about that, and at this point there is no communication between the two of them any longer, so there isn't any way for me to ask him what all he did to either amp. Not that I feel like he would tell me straight.
So ff to a couple weeks ago; I buy the first(video) amp from him with intentions of troubleshooting the control board to see if I can get it going. It has the WOA Rev. E control board in it that the Tech guy built. From this point I'll refer to him as Tech guy, I don't know him. It has a wattsabundant DC protect board in it. All new ON MJ15024G(BM1811) outputs, and 4 RCA 66546 drivers with silpads mounted to stock back planes. New RCA inputs, and new outputs. Various new wiring throughout the amp. The PS filter caps look original. All fuses are correct and good. One 10A main, and 4 AGX5 . This amp looks immaculate inside and out, save for a little dust from sitting out of his cabinet all year long.
On a suggestion from a well known WOPL builder friend of mine, I was told to remove the outputs from that channel and power it up on a DBT to check voltages. I did that, and no shorts on DBT powered up with my variac. Relay clicks in as normal. So I decide to hook a pre amp to it and a speaker to the left channel to see if it is distorted. It played clear, but after about 30 seconds I see magic smoke appear from the back plane on the right channel, so I switch it off.
I've been able to switch it on for a few seconds at a time to check some voltages to compare left and right channels on the control board. After about 10-15 seconds I can smell something getting hot so I switch it back off. I am getting different voltages from some of the points on the lugs at the bottom of the board, and also at the two op-amps at the top of the control board. Just checking the outputs in the diode mode on my DMM, all of them are checking good on the meter, but I haven't checked any of them in circuit.
I can get some more voltage readings and whatever else I need to do to check the control board for proper operation with recommendations from the group if anyone wants to help diagnose this problem. I know it's hard to do long distance. Thanks for any help or suggestions.
 

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#5
Looking at the pictures, the relay board is wired according to instructions which focused on minimizing rework. I have acknowledged that it is not the ideal grounding scheme and recommend following the White Oak grounding scheme when installing the driver board. I don't think the grounding scheme is the source of trouble here.

Generally speaking, amplifiers don't make a burnig smell very long before catastrophic failure. Was anything done to the display such as changing caps? It's possible that an electolytic cap installed backwards on a display board is the source of trouble.

Assuming the display boards are ok, it's necessary to divide and conquer by separating the output stage frpom the driver board. I would suggest pulling the AGX fuses and with a 60 watt bulb in the dim bulb test set, see if the light dims without the burning smell. A 60 watt bulb provides a little more protection than the standard 100 watt bulb. If the light doesn't dim it suggests the problem is in the driver board which is what I suspect. Output stage issues produce heat for a short time before transistors fail and fuses blow.
 

The Rebel

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#7
No, I'm talking about the DCP
Exactly. Two wires soldered to the one that is in the lug socket. What's your suggestion if this won't fly?
Looking at the pictures, the relay board is wired according to instructions which focused on minimizing rework. I have acknowledged that it is not the ideal grounding scheme and recommend following the White Oak grounding scheme when installing the driver board. I don't think the grounding scheme is the source of trouble here.

Generally speaking, amplifiers don't make a burnig smell very long before catastrophic failure. Was anything done to the display such as changing caps? It's possible that an electolytic cap installed backwards on a display board is the source of trouble.

Assuming the display boards are ok, it's necessary to divide and conquer by separating the output stage frpom the driver board. I would suggest pulling the AGX fuses and with a 60 watt bulb in the dim bulb test set, see if the light dims without the burning smell. A 60 watt bulb provides a little more protection than the standard 100 watt bulb. If the light doesn't dim it suggests the problem is in the driver board which is what I suspect. Output stage issues produce heat for a short time before transistors fail and fuses blow.
Sounds good. I'll look into the White Oak grounding scheme and work on that, even though I'm with you on the chances of the ground being a problem here is slim and none. I'll pull the AGX fuses and swap in a 60 watt bulb. I want to be able to keep it powered on for more than a few seconds to check more voltages for comparison between the two channels. I do not know if anything was done to the display section.
FWIW, I have a 100 watt bulb in it now, it dims fine, and never brightens back up once the caps are charged.
Another thing, I found this on the right channel driver board that looks like it's been hot. The one on the driver next to it checks 190 ohms, this one is up to 219 ohms.
 

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The Rebel

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#8
The picture of the xfmr & dc caps shows a display board resting on top of the chassis. Put a piece of cardboard under the display board so that it does not short to the chassis.
I have the display board on a piece of cardboard when I powered it up. Thanks.
 

The Rebel

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After pulling the AGX fuses and using a 60 watt bulb, there is no more smell coming from anything on the back plane. Voltage at the PS caps is 87V, and - 87V.
 

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#11
On the right channel I have no idea why a white wire is going into the PNPR connection point on a quasi comp amp. There should be NO connection to PNPR or PNPL for quasi comp builds. That white wire is probably a ground wire that should have gone to 5R, not PNPR but that should be traced and figured out where it should be going.

With out a ground connection going to either the 5L or 5R connection pad on the control board (they are connected together on the back of the board with a bus wire jumper) there is no ground to the control board. I can only speculate as to what might happen without a ground connection.
 

The Rebel

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On the right channel I have no idea why a white wire is going into the PNPR connection point on a quasi comp amp. There should be NO connection to PNPR or PNPL for quasi comp builds. That white wire is probably a ground wire that should have gone to 5R, not PNPR but that should be traced and figured out where it should be going.

With out a ground connection going to either the 5L or 5R connection pad on the control board (they are connected together on the back of the board with a bus wire jumper) there is no ground to the control board. I can only speculate as to what might happen without a ground connection.
Joe, that is one of the three white ground wires on the DCP board. I pulled it out of PNPR and will put it under 5R. Thanks for looking at that for me.
 

The Rebel

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Joe, at this point, I have put the ground under 5R, and put the AGX fuses back in the right channel. All is well, no more hot smell. Should I be getting the same voltage readings across the lugs from the L to R channels on the bottom of the control board? Is it time to put the outputs back in and test the amp, or is there other voltages I need to check before I do that? The voltage at the caps is now 77V and -77V.
 

Gepetto

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#14
They should be close but may not be exact, except for 12L/R, 11L/R, 5L/R and 2L/R which should be identical.

You should check output offset voltage for near zero on pin 6 and you should check the bias setting to be 0.350V on each channel. If all is good there then you should be able to put in outputs starting at the bottom and working your way up a row at a time, waiting for the bulk caps to discharge in between of course.
 

The Rebel

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#15
They should be close but may not be exact, except for 12L/R, 11L/R, 5L/R and 2L/R which should be identical.

You should check output offset voltage for near zero on pin 6 and you should check the bias setting to be 0.350V on each channel. If all is good there then you should be able to put in outputs starting at the bottom and working your way up a row at a time, waiting for the bulk caps to discharge in between of course.
That's what I'm getting, close matches. Checking bias on 10L/R(?), I am getting 350.0mV on L. and can only get 62.0mV on the R. Offset is 0 on both L/R.
 

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#16
Check the resistors on the right channel to ensure none got damaged due to the miswire on PNPR. R35R (22), R36R (56), R37R (22), R39R (1). Also check D13R. Also check that the bias transistor mounted to the chassis is a good transistor and not shorted from C-E.
 
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WOPL Sniffer

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#17
Exactly. Two wires soldered to the one that is in the lug socket. What's your suggestion if this won't fly?

Sounds good. I'll look into the White Oak grounding scheme and work on that, even though I'm with you on the chances of the ground being a problem here is slim and none. I'll pull the AGX fuses and swap in a 60 watt bulb. I want to be able to keep it powered on for more than a few seconds to check more voltages for comparison between the two channels. I do not know if anything was done to the display section.
FWIW, I have a 100 watt bulb in it now, it dims fine, and never brightens back up once the caps are charged.
Another thing, I found this on the right channel driver board that looks like it's been hot. The one on the driver next to it checks 190 ohms, this one is up to 219 ohms.
Yep, check them back plane resistors.... All of em.
 
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Gepetto

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#19
You will have a hard time measuring the 0.33 ohm emitter resistors unless you have a 4 wire DMM. You will be looking for open 0.33 ohm resistors which is easy to detect. The 10 and 180 ohm backwall resistors are easy to measure in circuit.
 

The Rebel

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#20
You will have a hard time measuring the 0.33 ohm emitter resistors unless you have a 4 wire DMM. You will be looking for open 0.33 ohm resistors which is easy to detect. The 10 and 180 ohm backwall resistors are easy to measure in circuit.
Everything on the back wall is measuring good. The only thing suspect is the one on that far right driver that I posted a pic of. It's not reading the same as the other 3 that didn't get hot.
I need a picture of a bare PL 14-20 Rev. E control board or a schematic so I can check the resistors you listed. I can't see the numbers on my board.
I do know that D13R is broken in half. Have not checked any of the resistors you listed yet.
Is there a check on the two op amps at the top of the board? I was getting different voltages between L/R when I had it powered up the last time.
I have the control board off now.
 

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