Noise after recapping tube amp

[Boogie]

After recapping the PSU of my 1982 Mesa Boogie SOB, there's some noise. It's not a low hum like the caps aren't filtering. More like a broad spectrum noise with an extremely pronounced loud popping sound when you first touch the guitar strings (i.e touch the signal ground with your hand). When firmly holding the ground with my hand the noise goes away. The popping is far more than I've ever seen, tapping rapidly and lightly on the strings between the nut and tuning pegs (light enough there is no acoustic noise and very negligible signal) sounds like a drum solo. Even sliding my fingers along a wound string (they are elixirs) makes a loud staticy electrical noise (as the ground connection to my finger is varied) (i dont mean a string signal noise, i mean in addition to that and louder than it) which I've never heard before on any guitar or amp.

I read this online GuitarNuts.com - Troubleshooting Noise

"Soft popping when touching strings or other metal parts on the guitar ---> Your body is probably discharging a capacitor. If you are using an isolation capacitor between the signal ground and your bridge ground you may need to place a 220k resistor across it (this usually isn't necessary though). Also, ensure that you didn't accidentally get the capacitor between the jack and signal ground. "

I'm not using an isolation capacitor on the guitar and it happens with multiple guitars, and not on my other amps, its definitely the amp.

My guess its its a grounding problem with something not being grounded. Perhaps I did a dodgey solder joint somewhere? I'll go back over everything and check. There's a ground wire soldered on the PSU board ground that goes to a lug on the chassis, which i unsoldered (for access to the parts) and then resoldered, perhaps that's dodgey?

The other possibility is, I used "shoe glue" to glue the caps in place, some of it dripped onto the PCB and perhaps a bit onto the leads of a resistor under the caps in the PSU - could that be conductive perhaps? Or capacitative or something? I read online that shoe glue was ideal for gluing caps since it was electrically ok and it doesnt dry permanently hard (like epoxy etc) so its easy to remove if desired later on.

If the amp is wired using the chassis as ground, is it a good idea to make up a ground bus from actual wires, say between the PSU and preamp board, and the cathodes of the power tubes, rather than letting just the chassis make the contact? If i make a wire ground, should I disconnect the chassis grounds (to avoid ground loops) or is it ok to leave them there and have both? I read somewhere the ideally only grounds to the chassis should be the input jack and the mains plug earth connection?? Is it that much of an issue? The amp was fine with regard to noise before my recap job.

Or is the likely answer something completely diffferent that I havent thought of?

[52 Bill]

After recapping the PSU of my 1982 Mesa Boogie SOB, there's some noise.

The amp was fine with regard to noise before my recap job.

I think that you are over thinking this. Go back and check what you did. Look for broken ground connections or pc traces.

Adding wires should not be necessary if it was working well before you worked on it.

[g1]

And just in case, you mentioned other guitars, but did you also try other cables?

[Boogie]

Thanks, yeah am probably over thinking. There can't be that much wrong with it. I was using a 40 watt iron and trying maybe too hard to not overheat anything so maybe I cold/dry soldered a joint. Im guessing that's the most likely problem. I'll just go over it again especially the joints to ground. Some of the areas of solder were quite large and I was thinking at the time maybe I should get out the 80 watt iron but I wanted to err on the side of underheating, in case of frying the tracks on the board which would be much worse than a too cold joint.... 80 watts seemed like too much for a PCB even though its a big strong one with fat tracks. Would 40 watts be a good choice? I've also got a 25 watt one, they are just cheap irons not temperature controlled or anything.

I just thought of this (i had forgot) but there's a resistor that I noticed had moved up a bit in its position (away from the board) after I had already put the board back, the joint was on the edge and I kind of looked at it from the side and it looked like it was still connected, but I did think I wonder if that is ok - until now I'd completely forgot about that one! haha -- So that could be it.

I didn't try other cables but I've used the same cable (before and after) in another amp and it was fine.

From a theoretical point of view, just to increase my knowlege of amp electronics, what type of failure would result in that kind of loud popping when a body part touches the chassis ground?

[Enzo]

Well start at the start. Unplug the amp and measure resistance from the ground prong on the power cord plug to the chassis. DO you have zero ohms? Or is there an open?

And the wall outlet, are you sure the ground hole in it is actually grounded? Use an outlet tester.

[Boogie]

Thanks, I was thinking I'd test the ohms between everything that's meant to be a ground and everything else that's meant to be. A page I read said 1, maybe 2 ohms or under is ok and he once saw as high as 65 ohms which was obviously a problem.

I hadn't thought to test the wall outlet though it may be a good idea since the wiring in this room is kind of old. The popping is no problem with other amps though nor with this one before I did the recap.

Is there any truth to the rumor that new HV caps need to be run for several hours to 24 hours to form them? I was assuming there wasn't a need for this from the bulk of what I read.

[Jecarroll7]

Its easy to put a filter cap in backward or place it in the wrong position, you'll occasionally get some interesting symptoms including an extremely loud pop accompanied with shrapnel if it charges too long.

If you didn't take photos of the filter caps before taking the amp apart, compare the polarity of the installed caps to the schematics.

Great advice above, start at the beginning and double check everything.

[Boogie]

I'm pretty sure I got them the right way around but I'll quadruple check. Hopefully tonight I'll get time to work on it. I did take photos before, thought I almost did get one round the wrong way as three were in parallel and then the two biggest ones were in series, so their + and - ends were pointing in the other direction on the board. The originals were Sprague blue ones with the + sign almost right in the middle of the cap, only slightly closer to one end than the other (which seems dumb) but I checked them multiple times against the schematic so I'd be surprised if they are wrong. It's been powered on maybe 2-3 hours since the new caps went in and it seems stable , other than the popping noise when touching ground.

[Boogie]

Correction the biggest ones were actually Mallory - I'll post a pic

[Boogie]

[Boogie]

Problem solved It must have been a bad joint.

I opened it up and measured every ground path I could think of, from the ground side wires on every cap I added to chassis, from each PCB to chassis, from the earth of a guitar cable plugged into the input to chassis and to the ground pin of the mains plug and to the grounds of the power tubes and everything was as close to 0 ohms as my meter could detect.

Then I flipped over the power supply PCB and resoldered every joint that I'd touched before with the iron super-well, making extra sure the entire joint was melted all the way through (especially where there were multiple component leads on the one joint), putting in heaps of extra solder so the flux would clean everything out (so much I had to remove some extra solder on 2 of the joints). Finally scraped away a bit of gunk between two of the tracks.

Put it back together and its all good now