[Persiflage]

So I've spent a number of hours trying to figure out the problem with my META42, but I've come up with nothing; I don't even know what the problem might be.


I bought a used META off someone a while back, but the on/off switch was damaged in shipping. I jury-rigged a quick fix while at college and lived with it. After I got home at the end of the semester, I took out the old switch and put in a new one.

Problem is, I must have hit something else while changing the switch. I can now hear a soft hum at all times. When I touch the case, the hum gets louder. If I touch the outside metal part (ground?) on either of the RCA jacks, the hum goes away. The amp still plays music correctly, no matter what I am touching.

It's in one of those nice aluminum Hammond cases. When I take it apart, and the main chassis and the back plate (where the RCA jacks, switch, and power-in are) are no longer touching, I can touch the back plate without causing the hiss.

If I touch the outside casing (the plastic part) of the wires leading from the PCB to the pot the hiss increases for the particular channel who's wire I touched.



I figure that I bumped something, and that two things that should be touching aren't, or that two things that shouldn't be touching are. I've tried playing with it to figure out what's wrong, but I haven't had any success yet. I have no idea what to look for, I'm mostly just stumbling around. I was wondering if anyone more knowledable than I might be able to give me some hints or a little advice. Any help would be appreciated.

Thanks,
Alex

 

[JMT]

It sounds like you have a ground loop that is causing the hum. Using the Hammond enclosures, all of the panel components have to be isolated from the panel. Was your amp one that I built? If so, I would be happy to repair it, no charge.

 

[Persiflage]

Yeah, I was guessing that it was something like that. I think I'll go in and look at how the volume pot is mounted, if it might be touching the case -- I'm not sure what else it might be. I'll poke around at the other components as well.

Thanks for the info. It's not one of the ones that you built, though.

 

[tangent]

Take the amp completely apart, so you end up with just two panels with stuff strung between them. If nothing is attached to the U part of the case, remove that as well; otherwise, just keep the end panels from touching the U part. Now, listen to the amp.

If the hum is worse, you've got some source of interference nearby that's confusing the issue. Fix this problem before proceeding.

If the hum has gone away, JMT's probably right, because now the panels are isolated from one another. You just need to isolate all of the components that touch virtual ground on one of the panels to fix it. This is the pot, and the I/O jacks in most amps. The power switch should have nothing to do with it.

If the hum is the same, either JMT's right, or there's a wiring problem on one of the panels. Touch the panels individually to see which one causes a change in the hum. Check solder joints, etc. Pay special attention to those components you touched when doing the original repair.

 

[Persiflage]

Earlier in the evening, I took apart the amp, removing both panels. The hum stayed the same, increasing in volume when I touched either the middle U piece, or the front panel.

I then unscrewed the headphone jack from the front panel. It's one of the big, locking Neutrik ones. I played with it for a while, to see if that would fix the problem (removing the pot would be a lot more difficult). There is a screw on one side of the jack (the part inside the case), surrouned by a red plastic circle. I found that if I touched that screw to the front panel, the humming problem went away almost completely.

Might that be something I could fix, say, by connecting a wire between that screw and the case? Or could it be a symptom of a different problem?

 

[tangent]

If touching the U part changes the volume of the hum, something from the amp circuit is still connected to it. Find out what, and why.

The screw on the Neutrik jack connects to ground inside the jack, so connecting that to the case connects the front panel to virtual ground in the amp; this is almost certainly the wrong thing. The reason the red thing is there is to keep ground in the jack from connecting to the chassis of the jack and thence to the chassis of the amp.

It sounds like your pot is floating, or connected to virtual ground. What you should do is insulate the pot from the front panel somehow and then run a wire from the PG pad on the amp board (down at the front corner near the pot) up to the screw on the rear of the pot. You can loosen the screw enough to wrap a wire around its shaft and then tighten it back down. This connects the pot chassis to virtual ground without also connecting the front panel to virtual ground.

 

[Persiflage]

Thanks a bunch for your help, guys. I grouded and isolated the volume pot, and everything got better. Still an ever so slight hum from touching the middle section.

I think I might have figured out what's causing it. In order to fit the utterly gigantic Black Beauty pot into the case, the maker had to turn the PCB sideways, and then cut off a bit from the front (the side near the pot solder point thingies). As I understand it (from reading your site, Tangent), a strip of ground runs along the board, very close to the outside. So by cutting off a small piece of the board, the ground strip is now exposed in two places along the sides of the PCB.

Now the PCB is stuck in one of the case's ridges, wedged between the two long, parallel sides of the case. There is a very thin strip of transparent plastic between the PCB and the case, on both sides. Would it be possible that some small bit of electricity is being transmitted through that plastic, between the exposed ground and the case? I don't see any other place where metal is touching the U-piece.

 

[tangent]

I suppose it could be. If you have an ohmmeter with sharp probes, poke hard into the case (to get through the aluminum anodization layer) and touch the other probe to one of the pads on the virtual ground bus. If you get anything lower than infinite resistance through this path, it's grounding out.

Cutting the ground bus should be okay. I've gotten away with it without trouble.

The pot shaft is now insulated from the case?

 

[Persiflage]

Quote:

The pot shaft is now insulated from the case?

Err, actually, the pot is outside of the case. I'm not entirely sure how I'm going to insulate the pot shaft from the case when I put it back together.

 

[tangent]

You must have the thing still torn apart then. In that case, the last of the noise may go away when you close it all up.

One way to insulate the pot shaft from the case is to use shoulder washers. It can be somewhat difficult to find the ideal washer for the situation, so you may have to grind a close size down a bit or drill one out, or.... A cheesier way to do it is to use electrical tape, but this can be ugly and you risk cutting through the tape when cinching the pot down.

If you don't insulate the pot shaft from the case, you must instead insulate the DC input jack from the case. If one or the other isn't done, the amp will fail completely.