H&K Zenamp pilot light issues

[ken]

I know this sounds weird...

I have a Hughes and Kettner Zenamp head on my bench. It works OK, but for some reason the pilot light will flash on when you first start up the amp, and then go out and stay out. It's not a #47 bulb, it's three blue LED's in series with a 1500ohm resistor on a little PCB. I checked the volts in, and got +21v on the hot wire even when the light was out, and ground checks out OK.

I can take out the pilot light board assembly and light it with the analog power supply on my Tek bench, so it's OK. I have what passes for diagrams, they only go to 'board level' so they are no help at all.

Anyone else see this one, what did you do?
Ken

[Enzo]

Try this: resolder the parts on the little board. perhaps mounting it in the amp flexes it a little so something loses contact.

If I follow the description, you have +21VDC, then a resistor, then three LEDs all in series to ground. If you have +21 at the top, I assume with respect to the chassis, then what is at the other end of the resistor? At each leg of each LED. And is there any voltage at the ground end of this board? SHouldn't be, but if there is, then ground ain't ground.

Is the 1500 ohm resistor really 1500 ohms more or less? Does the +21 source come directly from the PS? Or is there a possibility some resistance has inserted itself?

[ken]

I found the fault, it was a bad trace on the light board. I was checking voltages from trace to trace on the board, and when I moved the board a certain way the LED's lit. I ended up removing the conformal coat over the traces and tinning
all the traces on the board. H&K is just like Carvin... use the cheapest copper laminate available to make PCB's. At least they don't brag about it like Carvin does in their catalogs.

Ken

[georage]

This was 2006, but i had the same problem on a zenamp head. The traces are amazingly delicate and get destroyed very easily. My problem was burned out leds. the led resistor ran very hot at about 90 degrees c. I replaced the led resistor with a 680 ohm at 2w and replaced the leds with red and it ran cool. the traces all came loose given any opportunity and had to be repaired. peace