Ampeg VL-1002 Effects Loop Hum

[IAmNotTheCosmos]

My Ampeg VL-1002 hums pretty loud all the time, but it hums VERY loud when I engage the effects loop. It's playable but annoying when it's just the normal hum, but the effects loop hum is loud enough that if I want to play at bedroom volumes and hear my guitar over the noise, I have to engage the amp's attenuator, turn it to zero, and then turn up the master volume. Then there's no hum whatsoever, but I know that's just a band-aid.

I've tried using a patch cable between the send and return, but it still hums. I've cleaned the jacks, replaced the NE5532 on the loop pcb, and, out of desperation, reflowed all of the solder joints on the loop pcb. Also, I ran a cable from the effects send to my computer to play through headphones -- no hum. So I know it's after the effects return somewhere...

It's been a few years since I replaced the power tubes...I play pretty much exclusively at bedroom volume, so they don't get beaten up too bad, but a tube change is probably in order. But that would only cut down on the base level hum, not the added hum that I get from engaging the effects loop, right?

Does anyone have a clue what could be wrong? Thanks for any tips!

PS - The amp was modified by Voodoo Amps by a previous owner, so I don't have the exact schematic for the modified version. The mod was done 10 years ago and the filter caps were changed out then too. For what it's worth, the filter caps aren't visibly bulging or anything as far as I can tell...

[IAmNotTheCosmos]

I tried new power tubes --the noise floor is a touch lower in general, but engaging the effects loop with a patch cable between the jacks still adds a bunch of white noise. I tried cleaning the multipin connectors that go to the board with deoxit, but that didn't help either. I also noticed that I lose a little bass with the loop engaged as well. Any ideas?

[Enzo]

The power tubes have no idea whether you are using the loop or not, so I suspect they are not the problem, or it would make noise ALL the time.

Plug a cord or an unwired plug into the FX return jack. Now at the free end of the cord or at the terminals inside the plug, measure for DC voltage across the plug - tip to sleeve in other words. Get 15v? You should see zero volts DC. If you get 15v, one of those diodes is shorted.

You sent a cord from SEND to some other amp and it is clean, but plug the guitar into the return jack and dial its volume control to zero. Still hum? And does the volume on the guitar affect the hum?

[IAmNotTheCosmos]

Thanks a ton for the reply Enzo.

I should say that the problem is more of a hiss than a hum now that I've spent more time with it. The amp as a whole hums a bit, but the loop just adds a wash of higher pitched white noise hiss. The loop pad adds a little hum, but it doesn't seem problematic.

I measured across the plug coming from the RETURN jack and got 0v. I tried plugging my guitar into the RETURN and the hiss is there regardless of the guitar's volume. I can say for sure that the hiss is there when the loop is on whether I have my pedalboard plugged in, a guitar straight into RETURN, a patch between the jacks, or nothing at all plugged in to them. The only thing that silences the hiss is if I'm running the amp with the built-in attenuator on (which is how I run it when I run the SEND to another amp/computer). Then the whole amp has almost no background noise and the loop behaves perfectly.

[Enzo]

Plugging in a zeroed guitar to the return jack was done to isolate the noise. Since teh noise continues, that means the noise source is after that jack.

[IAmNotTheCosmos]

Hmm yeah. But what is in the signal path after the RETURN that would only be in the signal path with the loop engaged? Making sense of this schematic is honestly a bit beyond my level of schematic-reading comprehension, but it looks to me like the loop engage button just routes the signal through the SEND and then RETURN jacks, and that everything else is always in the signal path. Is that right?

I'm also just having trouble figuring out what happens when the attenuator is engaged, since engaging it allows me to use the loop noise-free. I imagine the signal must be routed differently after the RETURN when I use the attenuator, since we know that the noise comes in after the return jack but that it can be circumvented by using the attenuator.

I also noticed a high pitch squeal that comes in when the loop is engaged (along with the white noise/hiss). It's almost out of my hearing range, but it's just barely perceptible. Not sure if the cause of these things is the same or not :/