Too much bias on PL400 series 2

[Dr12volt]

I have a PL400 series 2 that runs very hot...infact it shut off earlier tonight. I knew it was running warm and planned on checking bias when necessary...it was necessary. I checked the voltage across the 10Ω resistor as needed and it was over 550mV on left and 510mV on the right. I adjusted the potentiometers but could only bring both sides down to around 470mVs. It works and sounds good but should I be concerned?

any suggestions?

thank you in advance....

John W Myers

 

[Gepetto]

Yes you should be John. At over 550mv on that one channel, you are on the edge of having the amp go into cross conduction.

Bias should be 0.35-0.38

 

[Dr12volt]

any suggestions on what i should look for to get it back into proper bias? I have the pots turned all the way down and still have about 470mV across each resistor when checking bias. thanks.

 

[Gepetto]

any suggestions on what i should look for to get it back into proper bias? I have the pots turned all the way down and still have about 470mV across each resistor when checking bias. thanks.

Hi John
Lets start with, do you have pics of what output transistors are mounted on the heatsinks?

 

[Dr12volt]

Here is a pic of the output devices on the back of the unit.

 

[Gepetto]

Here is a pic of the output devices on the back of the unit.

The lowest row should have RCA 410 or 66546 in columns 1 and 3 with column 1 being closest to the output binding posts. Should not have MJ15024 in those locations. And what is manufacturer QS2? Looks like knock offs. Should be ON Semi or Motorola for MJ15024 devices. Refer to page 1.

 

[Dr12volt]

The lowest row should have RCA 410 or 66546 in columns 1 and 3 with column 1 being closest to the output binding posts. Should not have MJ15024 in those locations. And what is manufacturer QS2? Looks like knock offs. Should be ON Semi or Motorola for MJ15024 devices. Refer to page 1.

These came in the unit when i purchased it. I am not sure of the maker of the QS2 TO3s. Oddly enough, the left channel, the one withe the RCA drivers is much hotter than the other side with the QS2s. Amp seems to work and sound fine. It just gets too hot to touch even with no signal on the input. thanks.

 

[Gepetto]

These came in the unit when i purchased it. I am not sure of the maker of the QS2 TO3s. Oddly enough, the left channel, the one withe the RCA drivers is much hotter than the other side with the QS2s. Amp seems to work and sound fine. It just gets too hot to touch even with no signal on the input. thanks.

You could either have a leaker or a defective bias transistor. A leaker is one of the 66546 or MJ15024 devices, meaning it has a defective C-E junction.

You will have to debug step by step. Has it been messed with internally?

 

[Dr12volt]

I am not sure if it has been altered internally. So do i need pull all of outputs and check them all? I dont mind doing that, just want to make sure. Or do I just need to check the bottom (dirves) in the columns?

Thank you for your patience and help. I know a quite a bit about electronics, just not servicing or repair. I am trying to learn so I can service these units myself if necessary...I have 4 of them that I am incorporating into a active 4 way home system.

 

[WOPL Sniffer]

Those things are an accident just waiting to happen. If you are just going to service them and leave 40 year old junk inside, you better up your insurance and and make sure your smoke detectors work. Somebody cobbled them together with crap/wrong parts and they are dangerous as they sit.

 

[Gepetto]

I am not sure if it has been altered internally. So do i need pull all of outputs and check them all? I dont mind doing that, just want to make sure. Or do I just need to check the bottom (dirves) in the columns?

Thank you for your patience and help. I know a quite a bit about electronics, just not servicing or repair. I am trying to learn so I can service these units myself if necessary...I have 4 of them that I am incorporating into an active 4 way home system.

A properly operating 400 will have barely warm heat sinks at idle. The warmest ones will be the ones nest to the transformer, they are warmed by the slight warmth of the transformer

 

[WOPL Sniffer]

Don't forget the Dirves

 

[J!m]

5-1/4 floppies.

 

[WOPL Sniffer]

5-1/4 floppies.

HUH????

 

[Gepetto]

John
A bias trick to debug is to, with the power off and bulk capsfully drained, put a short wire jumper wire from C-E of the bias transistor. With this in place, the measured bias should drop to zero. If it does not, you have a leaker or two. If it does drop to zero, your heat should subside. It is safe to run the amp with zero bias, in fact, much safer than your current condition.

 

[WOPL Sniffer]

Yep, instead of properly fixing the amp, cobble it a bit more..... HA.....

 

[Gepetto]

John, you should have a DBT to be working on your amps. If you do not, recommend you construct one before going any farther.

 

[George S.]

John, are you going to rebuild your amps to original or upgrade them with WOAD components?
What sort of active crossover are you going to use and what speakers?
Tell us more about yourself and what sort of system your planning.

 

[Dr12volt]

John, are you going to rebuild your amps to original or upgrade them with WOAD components?
What sort of active crossover are you going to use and what speakers?
Tell us more about yourself and what sort of system your planning.

The system I am building up to is for my office and listening room. I will have a set of A/D/S/ L1230 cabinets that my buddy bought new in 1984 and then sold to me. I will be wiring them all active and then will build a pair of 10" large sealed subwoofer enclosures for the bottom end. For crossovers I will be using Behringer or possibly a Mini DSP. I know that 4, even 3 of these amps are way overkill but i am building this system for me, not for anyone else and I am not as worried about the cost as well as the effort.

I have a SONY PS-X60 turn table with an Audio Technica cartridge.

I will be mosty streaming music from Tidal with the higher bitrate through my computer with a Topping D/A. I also have about 2.25T of music at as high of a bit rate as possible for listening.

I have the speaker protection circuits and the upgraded capacitor sets from White Oak as well as the new output devices. I will most likely replace the driver boards in the near future. I want to get this system up and running before i get to that expense.

 

[Dr12volt]

Those things are an accident just waiting to happen. If you are just going to service them and leave 40 year old junk inside, you better up your insurance and and make sure your smoke detectors work. Somebody cobbled them together with crap/wrong parts and they are dangerous as they sit.





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pretty interesting rely considering this is a DIY Forum for helping people to get the most out of their electronics. You would expect to get some assistance and a direction to start but I guess thats not why you are in the forum. It would be so much easier to just go buy all new gear but I love these amps for the nostalgia. I plan on rebuilding them whatever the cost. To just throw them out just seems way to harsh. There are good people with good intentions that can help and if I can do it this way I will have no problem taking them to someone that can. I have been in the electronics industry since 1979 just not in the service/repair end but would like to learn how to service and repair vintage electronics. There seems to be a shortage of people that really do care to preserve some of this older gear. My job has me working with people that spend hundreds of thousands of dollars on cars that are 50+ years old. If it were not for people like that I would no be doing my dream job and designing and outfitting them with audio systems. almost none of them are totally original and have had many different people work on them over the years and may have not gotten it right. That does not mean they are a lost cause and just require more attention and research to make them what the owner desires.