Hartke A100 Bass Amp - Strange reset

[saint gillis]

  Hi I'm working on one of these combos. There is a strange issue.
  At first the amp works fine. Then if we switch it off, the caps discharge, we hear "plik", then if we switch it on again imediately, the positive PSU rail becomes someting like 10s of volts lower than expected, the lm7815 doesn't have enough volts to generate its regulated supply and everything is stuck. If we switch the amp off, we wait untill we hear "plik" in the speaker, we wait few seconds, we switch it on again, and then it's fine again.
  When there is no speaker loading the power amp, there is not such behaviour.
  Any idea about what's happening?

 

[saint gillis]

Schematic : https://groupdiy.com/index.php?action=dlattach;topic=72271.0;attach=67000

 

[JohnRoberts]

I didn't check the schematic because its after beer o'clock here... but sounds like regulator latch up...

Like the old doctor said, if it hurts when you do that, don't do that.

JR

 

[saint gillis]

At the beginning I thought it was simply a regulator problem.. but it seems to be something at the power amp that loads the +50V

 

[saint gillis]

Beer o'clock, lol

 

[Gene Pink]

Beer o'clock, lol

 

[saint gillis]

Alright, I investigated more. I thin the power amp is fine, and John is right (as always). The +/- 50 rails are good. So it looks indeed like a 7815 latch-up thing...
Its latchs up only under this condition : we switch off the amp, the caps discharge, and then we switch it on again immediately after the caps has discharged. And also it doesn't fail if the amp is not loaded by a speaker.

  I tried to add a reverse diode on the output of the 7815, didn't solve the pbm.
  Then I tried to put a 24V zener after the limiting resistor R72. I thought the 7815 accepts +35V max at the input, maybe  the 50V through the limiting resistor could cause some strange stuff at start-up. But it didn't help either...

 

[pucho812]

Like the old doctor said, if it hurts when you do that, don't do that.

JR

listen to the doctor

 

[saint gillis]

Yes but I want to know what's really happening.
So I've painfully adapted an LM317, and it absolutely does not do the latch-up, so that's a solution.

Now for the sake of science I'd like to understand why and how this latch-up occurred with the LM7815...