PL700II blues

[Gepetto]

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.

PM me your email and I will send you the complete doc pack since you acquired these from another. He got the doc pack, you didn't.

One of life's mysteries to me is the Phoenix connectors. When I ship them with the kits, I interlock the tiny dovetails so it is like one solid connector. This must have been an aftermarket procurement and the guy did not know that they interlock. These connectors are a thing of pure German precision the way they lock together.

On the drivers, you only need the special drivers in the second and fourth column when counting starting at the transformer end. They are needed in the PNP quasi configuration only for stability. The first and third columns can be MJ15024 or MJ21196 transistors since that configuration is straight NPN emitter follower. The broken D13R is not doing you any good and is likely an artifact of the misplaced ground wire, too much current flowing through that poor small signal diode.

There is no check you can perform on the op amp, they are seldom a problem.

 

[The Rebel]

PM me your email and I will send you the complete doc pack since you acquired these from another. He got the doc pack, you didn't.

One of life's mysteries to me is the Phoenix connectors. When I ship them with the kits, I interlock the tiny dovetails so it is like one solid connector. This must have been an aftermarket procurement and the guy did not know that they interlock. These connectors are a thing of pure German precision the way they lock together.

On the drivers, you only need the special drivers in the second and fourth column when counting starting at the transformer end. They are needed in the PNP quasi configuration only for stability. The first and third columns can be MJ15024 or MJ21196 transistors since that configuration is straight NPN emitter follower. The broken D13R is not doing you any good and is likely an artifact of the misplaced ground wire, too much current flowing through that poor small signal diode.

There is no check you can perform on the op amp, they are seldom a problem.

Ok, sounds good. PM sent.
So far the only things I found bad on the control board is the D13, and one 56 ohm resistor at R36. Everything else checks good.
Thanks for the doc pack, this is what I have been needing the whole time.
One more question on the control board, and this is just me thinking out loud. In the future, when I'm able to buy your back plane kit and all new outputs, does the control board need to be reconfigured to run full comp back planes? Is this something that can be done to this board, or do I start over with a new control board?

 

[Gepetto]

Ok, sounds good. PM sent.
So far the only things I found bad on the control board is the D13, and one 56 ohm resistor at R36. Everything else checks good.
Thanks for the doc pack, this is what I have been needing the whole time.
One more question on the control board, and this is just me thinking out loud. In the future, when I'm able to buy your back plane kit and all new outputs, does the control board need to be reconfigured to run full comp back planes? Is this something that can be done to this board, or do I start over with a new control board?

Yes Kevin, the doc pack has a conversion document that describes the few changes to convert to full comp

 

[Gepetto]

The toasted 56 ohm and blown diode can explain low bias

 

[The Rebel]

Update on amp repair. I've replaced toasted 56 ohm resistor at R36R, and the blown diode at D13R. Replaced the toasty 180 ohm resistor on the back plane. In the process of checking the bias transistors mounted to the back wall, I found the left channel transistor dead so I replaced both of them with 5088s. All seemed to go well. Except that the bias reading is still low in the right channel.

 

[wattsabundant]

That burnt resistor suggests an open base/emitter connection on the corresponding transistor. Possible bad socket/broken wire. An open base emitter junction, although not impossible, is a rare condition on power transistors. The service manual is attached. Page 20 provides an excellent procedure for checking outputs.

 

[Gepetto]

Or a severely reversed bias B-E junction which is possible (and created a walking wounded) given the miswire that existed before we caught that. In any rate, the drivers on the right channel could be defective and should be looked into.

 

[The Rebel]

Update on amp repair. I've replaced toasted 56 ohm resistor at R36R, and the blown diode at D13R. Replaced the toasty 180 ohm resistor on the back plane. In the process of checking the bias transistors mounted to the back wall, I found the left channel transistor dead so I replaced both of them with 5088s. All seemed to go well. Except that the bias reading is still low in the right channel.

Ok, another update on the low bias reading, it sure helps when the drivers are in the sockets. D'oh!! I didn't realize I needed them in the sockets to check bias, still learning. Bias adjusted, amp is running cool, all voltages are correct. Will hook a pre amp and speakers to it shortly.
Thanks for all the help guys. Joe, thanks for the resistors and pointing me in the right direction.

That burnt resistor suggests an open base/emitter connection on the corresponding transistor. Possible bad socket/broken wire. An open base emitter junction, although not impossible, is a rare condition on power transistors. The service manual is attached. Page 20 provides an excellent procedure for checking outputs.

Don, I've checked all the outputs on the right channel, both in the sockets as per the manual, and out of the sockets individually. I haven't found a bad output yet. I did replace the 180 ohm resistor on that socket. Now ready to hook it back up and power it on for a quick test. Thanks for bringing that up.

 

[The Rebel]

Ok, that went well until it didn't. Hooked it up to a known good pre amp that I use daily, hooked to test speakers, turned it on, sounds good. Listened for about 5-6 minutes. Just when I was getting ready to snap a picture of the stack the left channel decides to head out. Distorted sound coming from left channel, and the outputs are between 90-98F on the infrared thermometer, while the outputs on the right channel are sitting around 78-85F. really kind of cool to the touch. Fins on left channel are slightly warm, nothing hot, just knew something was amiss. Offset is 0-0.1mV on both channels.
So, I got the right channel running good, but now have the distorted sound back in the left channel. The only thing I did to the left channel was to check all the outputs, and replace the bad factory bias transistor with a 5088. Back to the drawing board on this one.

 

[WOPL Sniffer]

Precisely why I don't rebuild the old junk and do only complete gut jobs on these. They may last 6 minutes, they may last 6 months.....

 

[The Rebel]

Precisely why I don't rebuild the old junk and do only complete gut jobs on these. They may last 6 minutes, they may last 6 months.....

I see your point, and hear you loud and clear, Perry. For some reason, seems like I'm always the one that pulls in the strays and brings them back to life.
Just talked to my gear buddy about the two amps again. Found out this one had the distortion issue when he got it back from his friend before it went to tech guy for the new outputs and the WOA PL14-20 E control board. So what ever was ailing it before wasn't addressed when tech guy went through it.
Could a bad PS cap do this, cause low, distorted output from one channel? I planned on new caps soonly, just gotta get past Christmas season before dropping any coin on an amp. Even if it is a PL 700!! lol

 

[Gepetto]

I see your point, and hear you loud and clear, Perry. For some reason, seems like I'm always the one that pulls in the strays and brings them back to life.
Just talked to my gear buddy about the two amps again. Found out this one had the distortion issue when he got it back from his friend before it went to tech guy for the new outputs and the WOA PL14-20 E control board. So what ever was ailing it before wasn't addressed when tech guy went through it.
Could a bad PS cap do this, cause low, distorted output from one channel? I planned on new caps soonly, just gotta get past Christmas season before dropping any coin on an amp. Even if it is a PL 700!! lol

Anything is possible but extremely unlikely. Measure your bulk voltages when you have the distorted output.

I would examine the control board very carefully under magnification to see if there are any missed solder joints on the offending channel. There are many solder joints, easy to miss one.

 

[laatsch55]

YEP!

 

[The Rebel]

Anything is possible but extremely unlikely. Measure your bulk voltages when you have the distorted output.

I would examine the control board very carefully under magnification to see if there are any missed solder joints on the offending channel. There are many solder joints, easy to miss one.

I got news today that this was going on before the new control board came into play Joe. But, yeah, everything I look at is under a lighted magnifier.

 

[Gepetto]

You still have all 4 of your rail fuses intact? Or have a ratty connection to one of the rail fuses? Including a possible poor connection inside the fuseholder?

 

[wattsabundant]

Is the offending channel the same one that had the burnt base/emitter resistor on the driver transistor?

 

[Gepetto]

Is the offending channel the same one that had the burnt base/emitter resistor on the driver transistor?

No Don, the other channel is acting up. The right channel had the burnt components. Right Channel now all good.

Problem is on Left channel now.

 

[The Rebel]

You still have all 4 of your rail fuses intact? Or have a ratty connection to one of the rail fuses? Including a possible poor connection inside the fuseholder?

Yes all rail fuses are good, and connections are nice. But I agree, this seems like low, or no voltage to the outputs.
I will pull it back apart and start checking closer. I'll check voltages then. Thanks.

Is the offending channel the same one that had the burnt base/emitter resistor on the driver transistor?

Don, the right channel had a straight short to the control board that caused the burnt resistors. All that has been taken care of and the right channel sounds good, so far. The left channel has low/distorted output, I didn't know it until today, but supposedly had the distortion issue before the tech guy did his thing. I suppose he replaced all the outputs thinking that was what was causing the distortion issue.
What's kind of weird is that it played great for 5-10 minutes, then decided to head out. It did the same for the tech guy before the owner picked it up from him.
I still think it's something simple causing it.

 

[wattsabundant]

I'm guessing an oscilloscope is not an option. That would likely turn a difficult problem into a simple one. What is being described is an intermittent component that heats up and causes the bias transistor to turn full on. Whcih would rop the bias to nearly zero. An open bias pot would do that as would a number of cracked solder joints. The entire channel needs to be inspected with a magnifying glass to look for a cracked joint. Also an open zobel network would cause oscillations that a scope would confirm.

 

[The Rebel]

I'm guessing an oscilloscope is not an option. That would likely turn a difficult problem into a simple one. What is being described is an intermittent component that heats up and causes the bias transistor to turn full on. Whcih would rop the bias to nearly zero. An open bias pot would do that as would a number of cracked solder joints. The entire channel needs to be inspected with a magnifying glass to look for a cracked joint. Also an open zobel network would cause oscillations that a scope would confirm.

Don, the bias transistor was bad on that channel(broken legs). I replaced right and left transistors with 5088s. They're both checking good now. Yessir, I do need a scope. I plan on keeping this hobby alive.