Pretty dumb thermal protection question

[xyloeye]

I just fixed an intermittent problem with my Eden bass amp. It was cutting out randomly with what seemed like a thermal shutdown. Problem was, it wasn't hot, but each time it shutdown a red LED on the power supply board would go out. The rail voltages were still present, output trans voltages OK, etc., but no output until a wait of about 10 minutes. This thing has a thermistor controlled fan which works but wasn't coming on as a result of overheating. Sorry, this is a really long explanation....

Schematics are hard to get from Eden, although I finally got one of the power supply from a similar Eden amp. I fixed the problem by simply resoldering the LED. My question is why would a series LED be responsible for the shutdown and would it be limiting output to the preamp or what? I can upload the schematic if necessary. Thanks for any help in updating my limited knowledge.

[J M Fahey]

Hi xylo.
Your question is not dumb, but intriguing.
Post what you have.
Obviously that LED was no there (hidden) for illumination or display, but for some other function, probably a 1.9V reference voltage.

[xyloeye]

Thanks for the interest

Here's what I have. The LED is in series with another diode. I just can't understand how it would cut the output (preamp input?). Eden has pretty mickey mouse schematics. Some of them are hand drawn!

[J M Fahey]

The probably 40V AC from the power transformer pass through the 3K9 resistor, get rectified by the 1N4004 diode, pass through the Led and charge the 10x35V Electrolytic to around -24V DC in about 0.5 second.
I think this slightly delayed voltage turns an output relay on.avoiding the initial thump or at least lowering it substantially.
When the amp is turned off, the output relay turns off also in around 0.5 second, too late to catch the switch "pop" (which is dealt with in other ways) but fast enough to shut off the general mayhem that appears when op amps starve with the last 2 or 3 volts of power..
Had to guess a lot, of course, we lack 99% of the schematic, but that's what I would use those parts for.
Of course I may be wrong, only David (Eden) knows for sure.
An open LED, of course, would not let the output relay turn on and your amp would be eternally mute.

[xyloeye]

Thanks again for your analysis. I kind of get it, I think. I can't find a relay anywhere, so it must be turning it off somewhere else. It does seem to be working well now though.

[J M Fahey]

Well, a relay comes to mind first, they may use a FET or a gate there.
A negative voltage is consistent with turning an N Channel FET off.

[xyloeye]

Oh, yeah. That makes sense. I have a fairly complete schematic from a similar Eden. It has two LEDs in series in the power module for some reason. There is a connection to an FET that looks like it cuts off the input signal. I think you're right. I'm thinking that there never was a thermal protection issue, just an intermittent heat sensitive solder joint on the LED. Not that I'm in the repair business, but I have a reasonable understanding of audio circuits. Never thought a lonely LED could do that....

[J M Fahey]

I have a fairly complete schematic from a similar Eden

OK, post what you have, it's always useful for fellow technicians.
Post it into the Schematics section labelled "Eden such and such" so it's easy to search for.
Thanks.

[xyloeye]

I'm at work right now. I'll do it tonight. Thanks

[xyloeye]

Here's what I have

Here's the rest of what I have.

[jordileft]

Here's the rest of what I have.

Hi! Could you upload again this schematic? I can't open it. Thanks!

[xyloeye]

Hi! Could you upload again this schematic? I can't open it. Thanks!

Sure, but I'm going to have to dig it out. I'll upload asap.

[jordileft]

Sure, but I'm going to have to dig it out. I'll upload asap.

Thanks! It would be really appreciated.