If this is all original and otherwise ok, check the main power supply filter capacitors.
Also, is the output Zobel resistor still ok? Some burn up if abused.
Also, is the output Zobel resistor still ok? Some burn up if abused.
85VRMS? 40VRMS is 200W the normal output of a PL400. Many PLs didn't like the use of SilPads on the output devices, do you have SilPads or mica/goop.
Craig
Craig
Maybe 85 millivolts? There are probably 15000 reasons why it could be doing this. Main caps going south, bad feedback electrolytic, bad/burnt zobel, incorrect bias, just enough drift in parts over time, to name a few.
If you were getting a full rail to rail oscillation it would be pulling some serious current. At 85 mV it may be sort-of behaving normally otherwise. Other than making the crossover distortion intolerable. When one suddenly starts to sound “crackley” on piano notes and it didn’t before is the clue that it started oscillating. Put a scope on it and yup - there it is.
If you were getting a full rail to rail oscillation it would be pulling some serious current. At 85 mV it may be sort-of behaving normally otherwise. Other than making the crossover distortion intolerable. When one suddenly starts to sound “crackley” on piano notes and it didn’t before is the clue that it started oscillating. Put a scope on it and yup - there it is.
Frequency of oscillation could possibly give some clues what's going on, and if you also post schematic. Is it constant, temperature-related etc? Have components been replaced? One or both channels? Is the oscillation amplitude actually V not mV?
We saw so many of those in the shop in the day, they were so unstable we quit taking them in. I would almost keep it original as a show peace than try to make it functional. Sorry to be so pessimistic but you could spend as much time chasing your tail on it as build new from scratch.
A very long time ago, like 1980, I was fixing amps at a sound shop, and I had to fix a couple of those beasts. The problem was that they used an op-amp IPS with way too much gain and therefore terrible stability, so yes, they were always oscillating. I found a "fix" by adding feedback around the op-amp, but today, I would use a simulation to hopefully find a better solution. Theoretically, they worked when they were new, so maybe there was a problem with aging electrolytics, like maybe the feedback decoupling cap. But those amps were so notorious for stability problems that I think it was just a bad design. I think there was a discrete version, which should be better. But it's probably futile to just replace parts.
There are plenty of modern amplifiers with op amp front ends, boatloads of NFB and stay stable for decades. There are design flaws, only some of which can be addressed. The lead compensation cap value is dead wrong. Fixable. Local feedback around the rest of the amp is a good idea and wasn’t originally implemented but could be. The whole output stage unfortunately, cannot be properly decoupled. It’s too spread out and there is no way to implement a low inductance bypass to where high frequency ground needs to be returned (A cap to supply ground thru an 8” piece of wire is as useless as tits on a boar hog). The point to point wiring scheme just has its limits. Some transistor combinations are ok with it, others simply never will be. The most stable output transistor combo was the full complementary using MJ15024/5.
Thanks for everyone's reply. I am helping a friend who purchased four of these amps, two of which are not working. The one amp which is the subject of this thread, is one I have not seen, and the 85vrms ( which I thought was odd) was not correct and is in fact 85vdc. This amp has shorted (all around) TP9054 in one channel and a DB15025 in the other channel. I'm guessing these are the drivers in a full complementary output amp, which I will confirm once I've seen the amp. Sorry for the misleading info, I should have asked him to verify AC or DC.
My main goal is to help him get these amps working and teach him how to work on audio gear, I have no monetary incentive, just helping a friend. He has worked in audio for most of his career and has always wanted to have the Phase Linear 400's in his system and is willing to spend the money to have them in working order. I have worked on a vintage audio in my spare time and run the electronic repair shop for a mobile audio manufacturer.
This is a long term goal and I may post from time to time to upgrade our progress and ask for advice. This may be a futile exercise, but so be it. If you care too indulge me great, I'm open to any suggestions.
My main goal is to help him get these amps working and teach him how to work on audio gear, I have no monetary incentive, just helping a friend. He has worked in audio for most of his career and has always wanted to have the Phase Linear 400's in his system and is willing to spend the money to have them in working order. I have worked on a vintage audio in my spare time and run the electronic repair shop for a mobile audio manufacturer.
This is a long term goal and I may post from time to time to upgrade our progress and ask for advice. This may be a futile exercise, but so be it. If you care too indulge me great, I'm open to any suggestions.
Whatever you do, don't touch either of the two that are working, until and unless you are able to get
the other two working properly first. Not even to replace a capacitor or connector.
the other two working properly first. Not even to replace a capacitor or connector.
85 VDC at the output is the typical output-stuck-to-the-rail issue. Often (but not always) very easy to diagnose as it indicates a complete loss of DC feedback. Can be anything obvious like a cracked solder joint on the 40412 VAS heat sink/collector to insidious like a bad bootstrap capacitor (Either will stick you to the + rail, and in the latter case it’s not obvious why). I’ve had both, and there are at least a dozen other possible causes.
Speaker relays really need to be added with these amps.
Speaker relays really need to be added with these amps.
