So I got a broken PL400 series two for not that much.
Pictures are attached, I think it's a series two but it doesn't have the nice frontpanel, looks like a special version, haven't seen it before. If anyone can give me more info about this please do.
So My plan is to rebuild it from the ground up, change the front so it looks like a normal series two (altough I like the VU meters on the series one), and maybe add two fans blowing at half speed over the cooling fans.
But first things first:
-Replace the Drive board with the board from white oak audio. I have read about it and it seems that this would be the way to go
-Replace output transistors, don't know what would be the best choice. MJ15024 or MJ21196? I've read MJ21196 would be the toughest one
-Power supply caps replacement boards from white oak audio
-Add a DC protection output relay
I think that would be good start.
However I have a few questions:
-for the DC protection, any good recommendation kits?
-Best choice for output transistors and should I go for full complementary output?
Then next up will be asthetics. Gonna get a frontpanel like a series two, I think I saw them on the white oak audio website.
Then the question will be if I can find a solution for the Led VU meters. It's based on the UAA170, Worst case would be to make my own PCB's.
Have to open it up, maybe the PCB's are there but with components left out.
For the fans, I hope that I can add 2 120mm Fans on the heatsinks. Put them in series so they turn slowly and don't add much noise but will keep it cooler.
Any other tips are welcome ofcourse.
Pictures are attached, I think it's a series two but it doesn't have the nice frontpanel, looks like a special version, haven't seen it before. If anyone can give me more info about this please do.
So My plan is to rebuild it from the ground up, change the front so it looks like a normal series two (altough I like the VU meters on the series one), and maybe add two fans blowing at half speed over the cooling fans.
But first things first:
-Replace the Drive board with the board from white oak audio. I have read about it and it seems that this would be the way to go
-Replace output transistors, don't know what would be the best choice. MJ15024 or MJ21196? I've read MJ21196 would be the toughest one
-Power supply caps replacement boards from white oak audio
-Add a DC protection output relay
I think that would be good start.
However I have a few questions:
-for the DC protection, any good recommendation kits?
-Best choice for output transistors and should I go for full complementary output?
Then next up will be asthetics. Gonna get a frontpanel like a series two, I think I saw them on the white oak audio website.
Then the question will be if I can find a solution for the Led VU meters. It's based on the UAA170, Worst case would be to make my own PCB's.
Have to open it up, maybe the PCB's are there but with components left out.
For the fans, I hope that I can add 2 120mm Fans on the heatsinks. Put them in series so they turn slowly and don't add much noise but will keep it cooler.
Any other tips are welcome ofcourse.
As you know, the PL is a can of worms. To do it right, check with Phoenix Audio Community Forums. They can fix all PLs. They love PLs.
SAFETY WARNING!
The repair of these amplifiers require working with LETHAL VOLTAGE LEVELS THAT
CAN AND WILL KILL. The voltage levels in the PL400 excede 140 Volts across the
powersupply rails. The Dual 500 and PL700 is higher still. If you are not
comfortable working with high voltages and have no technical experiance in doing
so, DO NOT ATTEMPT TO REPAIR THESE AMPLIFIERS!
In each step of the repair, you must discharge the powersupply caps to protect
yourself and to prevent damage to the amp while you are working on it. Use a 10
watt 10-25 ohm resistor to discharge the two powersupply rails together before
you put your hands inside the amp. DO NOT SHORT THEM WITH A SCREWDRIVER!
The repair of these amplifiers require working with LETHAL VOLTAGE LEVELS THAT
CAN AND WILL KILL. The voltage levels in the PL400 excede 140 Volts across the
powersupply rails. The Dual 500 and PL700 is higher still. If you are not
comfortable working with high voltages and have no technical experiance in doing
so, DO NOT ATTEMPT TO REPAIR THESE AMPLIFIERS!
In each step of the repair, you must discharge the powersupply caps to protect
yourself and to prevent damage to the amp while you are working on it. Use a 10
watt 10-25 ohm resistor to discharge the two powersupply rails together before
you put your hands inside the amp. DO NOT SHORT THEM WITH A SCREWDRIVER!
If you really are going to completely rebuild it the MJ2119x is the best transistor to use, although I’d probably let availability dictate whether to use those or 15024. Either is sufficient in the 400, but ONLY the 21196 should be used in a 700. Whether to replace the drivers depends on if you’re converting to full complementary or not. If not, leave the 66546. A good reason TO convert to full comp is being able to bias the amp at the best place to bias a PL - 3 mA per output transistor. The QC is often unstable there - giving bursts of oscillation that aren’t destructive, but annoying. The recommended class B prevents this, and biasing too hot for the heat sinks also does. When you use the PNP negative side, you can bias it where all quality PA amps are, keeping the heat down but having enough bias to rid yourself of the roughness of the original PL.
If you do want original sound (two of my 400’s are) you can safely leave in Fairchild PL909 outputs if you don’t thermal cycle the heck out of it. They have sufficient SOA, but all aluminum TO-3’s are subject to wear out from thermal cycling. This is only an issue in heavy PA use. All SJ6343’s in the CS800 will eventually go because of this. Also, if they have been replaced at some point with 2SD424 or 2SB555 (common for repair shops of the day) those will handle even somewhat heavy use. If you have RCA1B05’s in there get them out - they have sufficient SOA but the gain is too low and sound terrible. The other “PL909” from the series 1 is a ticking bomb. Most of those bombs have long since gone off.
The meters on the series 2 are interesting - one of the most endearing features IMO. At one point years ago, I kluged up a look alike meter (probably not the same calibration, but worked exactly like it from a distance) using four LM3914’s (not the 16’s) per meter. I eventually built it into part of a disco light show connecting it to a bunch of triacs. Only populated 15 lamps on each side. Think of the cool factor of those overhead with red PAR38’s. Especially when the clip indicator comes on. One of my projects on the bucket list is to convert (or build new) a 700 S2 to class H. It will require the full comp conversion - I’ve tried to run QC with rail switches and they break into uncontrolled oscillations. If I have trouble obtaining the meters I’ll start there again - I know I can make it work. Good to know there is a potential source of front panels.
If you do want original sound (two of my 400’s are) you can safely leave in Fairchild PL909 outputs if you don’t thermal cycle the heck out of it. They have sufficient SOA, but all aluminum TO-3’s are subject to wear out from thermal cycling. This is only an issue in heavy PA use. All SJ6343’s in the CS800 will eventually go because of this. Also, if they have been replaced at some point with 2SD424 or 2SB555 (common for repair shops of the day) those will handle even somewhat heavy use. If you have RCA1B05’s in there get them out - they have sufficient SOA but the gain is too low and sound terrible. The other “PL909” from the series 1 is a ticking bomb. Most of those bombs have long since gone off.
The meters on the series 2 are interesting - one of the most endearing features IMO. At one point years ago, I kluged up a look alike meter (probably not the same calibration, but worked exactly like it from a distance) using four LM3914’s (not the 16’s) per meter. I eventually built it into part of a disco light show connecting it to a bunch of triacs. Only populated 15 lamps on each side. Think of the cool factor of those overhead with red PAR38’s. Especially when the clip indicator comes on. One of my projects on the bucket list is to convert (or build new) a 700 S2 to class H. It will require the full comp conversion - I’ve tried to run QC with rail switches and they break into uncontrolled oscillations. If I have trouble obtaining the meters I’ll start there again - I know I can make it work. Good to know there is a potential source of front panels.
Thanks for the replies.
I checked the transistors and they are 2SD555 and 2SB600. So it's already a full complementary design.
Gonna check how many of them are good and if not it will be MJ2119x. RS has them so no problem.
The meters I still have to check what's inside
Your Class H idea sounds good. I like the simple topology yet the slew rate is high enough and distortion is quite low.
Class H will save some dissipation and a lot of power will be possible as the outer rails could be higher. Could make a serious monster amp.
I wonder if it would be possible to use the same topology but change the output to something like a bryston output (quad complementary) using both NPN and PNP either side.
And about the opamp at the input. Any modern day upgrades that would make sense?
Ideas enough to tinker with when it is running and safe (relay)
I have experience repairing things with high voltage (QSC pl6, PL380 etc and some KV tube transmitter power supplies) but thanks for the warning. It's indeed dangerous if you are not carefull
I checked the transistors and they are 2SD555 and 2SB600. So it's already a full complementary design.
Gonna check how many of them are good and if not it will be MJ2119x. RS has them so no problem.
The meters I still have to check what's inside
Your Class H idea sounds good. I like the simple topology yet the slew rate is high enough and distortion is quite low.
Class H will save some dissipation and a lot of power will be possible as the outer rails could be higher. Could make a serious monster amp.
I wonder if it would be possible to use the same topology but change the output to something like a bryston output (quad complementary) using both NPN and PNP either side.
And about the opamp at the input. Any modern day upgrades that would make sense?
Ideas enough to tinker with when it is running and safe (relay)
I have experience repairing things with high voltage (QSC pl6, PL380 etc and some KV tube transmitter power supplies) but thanks for the warning. It's indeed dangerous if you are not carefull
If you’ve got enough D555/B600 to do a channel, then you only need one channels worth of 2119x. Alternatively, you could use the NECs for drivers since they are a 10 MHz device and use all 2119x for outputs.
The 400 is kind of on the edge as to whether to class H it (although I’ve built smaller class H’s) but the 700 really needs it. The rails are just too high unless you go series-paralleling them and you’ve still got a massive heat problem. I have two pairs of 0-20-40-80 EIs each almost exactly the same size as the trafo in the 400. I could do 3 step and make it so it couldn’t be burnt out - while running the rails a bit higher at +/-115.
The idea of a quad complementary output is interesting - but you need an even number of outputs. You’d have to use all 16 positions for outputs and put in flatpack drivers somewhere. I’m pretty sure MJE1503x have been used in PL rebuilds before, giving you 4 outputs instead of 3.
The 400 is kind of on the edge as to whether to class H it (although I’ve built smaller class H’s) but the 700 really needs it. The rails are just too high unless you go series-paralleling them and you’ve still got a massive heat problem. I have two pairs of 0-20-40-80 EIs each almost exactly the same size as the trafo in the 400. I could do 3 step and make it so it couldn’t be burnt out - while running the rails a bit higher at +/-115.
The idea of a quad complementary output is interesting - but you need an even number of outputs. You’d have to use all 16 positions for outputs and put in flatpack drivers somewhere. I’m pretty sure MJE1503x have been used in PL rebuilds before, giving you 4 outputs instead of 3.
Using the NEC's as driver is indeed a good idea I didn't think of. The higher Ft will probably have some effect on THD and TIM.
I'm first gonna make it normal complementary using White Oak Audio boards. If this works well and sounds good I'm gonna try the quad Complementary but this will be a new Amplifier project then.
Gonna Leave the PL400 original with safety upgrades that are needed.
I'm first gonna make it normal complementary using White Oak Audio boards. If this works well and sounds good I'm gonna try the quad Complementary but this will be a new Amplifier project then.
Gonna Leave the PL400 original with safety upgrades that are needed.