JL 300/2 excessive power supply duty cycle

[miniman82]

Ok, I’ll look at it.

 

[miniman82]

The problem inexplicably went away
Today I powered it up to trace the circuit you posted, and there are indeed a few transistors and some diodes going back to the rails. But I noticed the power supply was actually stable and not hissing anymore, so I don’t know if I touched something and made a connection stable or I knocked away a randomly bridged component but it’s behaving for the moment. If it comes back I’ll return to this thread, I’ll start another post to repair the DC offset in the left channel.

 

[Perry Babin]

Re-take all voltage readings while it's operating normally.

 

[miniman82]

Returning to this amplifier.

I went and rechecked pin 1 on the PS card and it’s rising slowly as the amp boots up, leveling off at around 2.9v when fully powered up meaning regulation is still working correctly. I have +/-53 on the rails.

There was +1.4v of DC offset in the left channel at powerup, so since the power supply was behaving I left it running to investigate. While checking voltages between driver cards looking for something abnormal, the DC offset disappeared and hasn’t come back since. The driver cards get pretty warm if you leave it on, so I think this is going to end up being either temperature related or possibly tin whiskers. The bus bars have visible tin whiskers on them, so it’s possible one migrated and was touching something it shouldn’t have been touching.

I’ll power it up again tomorrow and if I don’t find anything, it’s gonna get a good scrubbing and go back into the case.

 

[Perry Babin]

Isn't the ±53v for the driver voltage? The rail voltage should be lower.

 

[miniman82]

Not a clue, there’s test points for 53 and 15v and they both check out. If there’s another supply for the output stage I have no reason to believe it’s out of range, but I can look into it if you think it’s significant.

 

[Perry Babin]

The rail voltage can be found on the output transistors. I think it's about ±34v.

The ±53 goes to pins 1 and 2 of the driver boards.

It was simply clarification. The threads are not only for the OPs.

 

[miniman82]

Yes, I have around +38 and -35 on the output stage.

Tried to pass audio but nothing came through, do these have muting circuits?

 

[Perry Babin]

Yes but if it has comax switches, they are more likely the problem.

The muting transistors are the J108/J111 transistors behind the driver boards.

 

[miniman82]

I’ve noticed that the V2 amps tend to have the crappy Comax switches in them, V1 stuff tends to be a bit better on the switch quality but I’ll check anyway.

 

[Perry Babin]

They used the comax for a long time. The V2 didn't comes out until about when I quit and I saw a LOT of comax problems.

 

[miniman82]

These don’t say what brand they are but they seem to be working fine, the headers to the pre card we’re all flaky so I cleaned them and now it passes audio just fine. I think I’m just going to clean this thing really well and leave it idling for a day or so. If it doesn’t do anything crazy during burn in, I’ll call it good.

 

[Perry Babin]

I think you have an intermittent problem. To have bad connections on 2 rows of independent header pins that produce the exact same symptoms is lottery winning odds.

 

[miniman82]

Could be, I only know deoxit seems to have it under control for the time being so I’ll watch it closely and if it gives me issues again I’ll replace all the headers. Wouldn’t be the first time, I have a 250/1 that did the same thing to me.

 

[miniman82]

These the muting transistors?
(Q306 & Q406)

 

[Perry Babin]

Yes. The gate voltage will be negative 10-15v to allow audio to pass.

 

[miniman82]

Ok, I’m gonna swap these with the ones from the pre card since it looks like they muted the output section as well. If that fixes it, I’ll order up a pair of J111’s and put those on the pre card when they come in.

 

[Perry Babin]

It's rare that they fail on the main board. Many people replace them because they think they're leaky but that's how they read without any gate voltage.

The ones on the preamp board are likely good but more likely to be damaged because they're connected to an outside connection

 

[miniman82]

I wouldn’t think they’d fail either, but then again the muting FET on my PPI 2075AM failed for seemingly no reason as well. If I had to guess the wonky power supply probably killed it when it was malfunctioning, only time will tell if it’s gonna happen again.