Standby LED will not luminate

[TomCarlos]

Team MEF...

I am working on a Peavey Triumph 60 and the Standby LED will not luminate. The switch is working, 503 volts feeding to the amp, but the dang LED is bonkers.

Looking at the diagram, I have 503 vdc at the top of the 22K, 1w stack. Between the 22K resistors, I have 492 volts. At the lower 22K and base of Q1, I have 0 volts (and the Standby switch is on). On side of the LED (CR17) is at 28.8 volts. The other side (at the Q1 collector) is 25.2 volts. I tested the LED with a meter and Q1 as well. Could it be that the devices show ok on the meter but do not work in the circuit?

Q1 is a Peavey 70403919. It is a TO-92. I believe the equivalents are the MPS-8097, 2N6520, MPS-A18, 2N6539.

(By the way, I have been working on this amp for a week. Lots of soldering, tube socket replacements, misc parts. The amp is working once again and I eliminated the intermittent popping - I think).

Ideas on what to try next?

[Enzo]

My schematic calls those 220k resistors in stead of 22k. What are your actual resistors in the amp?

Look at the drawing. You have zero volts to ground at the base of Q1. If Q1 has a B-E short, that might explain it, but then you should have half of 503v between the two resistors. The two resistors plus the resistor from base to ground form a voltage divider, stepping the screen voltage down to useful for the transistor. But you have zero on the base, but also no drop across the top resistor. That says to me the lower resistor is likely open. And please verify if they are 22k or 220k. 22k used as divider here puts maaybe 11v on that base, whereas 220k puts about 1v there.

SO is the transistor shorted? Is the resistor open? This really is not a complex circuit.

The transistor is just a plain old vanilla general purpose NPN, original is 2N3391. A 25v half amp part. All it does is turn on the LED, so it never conducts more than 10-20ma. And unless something blows up, it never sees much voltage either. SO most any small NPN will likely work fine there.

The circuit is clever enough, the presence of screen voltage turns on the LED.

[TomCarlos]

Thanks Enzo.... Yes, those are 220K resistors. The schematic sure looks like 22K to me. But the board layout shows 220K. R64 was open. The LED is working once again.

Ok, now to reassemble the boards, connectors, and see if this amp will play nicely. I hate the way the boards are stacked !!!

[g1]

attaching schematic with correct values for those resistors

[TomCarlos]

Thanks G1... !!!

And the board diagram is much better compared to the one I found.

Ok, one last weirdo thing on this amp. Unless you have the footswitch, you cannot change between the channels. I believe "Crunch" is the default. And unless you use the footswitch, there is an audible hum. So I guess the footswitch is also providing a ground connection back to the amp.

[Enzo]

Looking at the drawing I would have said Ultra was the default, but yes, there are no channel switches on the panel. Gotta have the FS to get normal and crunch.

One tip: On the Triumphs, NEVER try to insert the FX plug with the amp running. You will almost always rotate the plug until it fits, and the pins stick out enough they touch the wrong things on the way by, and blow the 1A fuse every damned time. Don't ask how I know. But I have had many a repair bill from guys who did exactly that. Plug the FS in and THEN turn the amp on.

The FS (or lack of one) should not cause hum. There is no signal path through the FS, it is all DC control voltages for relays.

[TomCarlos]

That is definitely good info on the FX FS to know!! I will pass this along to my friend. He'll be happy to get this amp back and working. No local shop wanted to touch it, and now I see why.

Again, much appreciated.

[Enzo]

Didn't want to touch it? Hell, I'd take a shop full of these in a heartbeat.