Troubleshooting Selmer T'n'B Mk III

[EFK]

I've checked and replaced the tone and coupling caps more than twice each; they're definitely not leaking. I was a little suspicious of a brand-new silver mica off the second plate of V3 but I jumpered it with a ceramic and it made no difference at all. Amp makes some static-sounding noises very occasionally when the chassis is open and I wave my hand around it, but I just thought of a great solution for the knob issue: remove the aluminum knobs and stick some plastic knobs on there. I'll be insulated from the pots! I thought about insulating the pots from the chassis, wouldn't be too hard with some fiber washers and some heat shrink, but frankly the pot grounds are still attached to the chassis and while there is no measurable (at least not on my meter) DC on the chassis or grounds, I still suspect both of being involved somehow. Plastic knobs are very cheap and easy!

[Steve Conner]

Definitely something fishy there. Either the pots have metal shafts, or they have plastic shafts.

If they have metal shafts, these should be grounded to the chassis, so noise from your body should not get in.

If they have plastic shafts, then these don't conduct electricity, so again noise should not get in through them.

So it sounds like you have metal shaft pots that aren't properly grounded. Check for continuity with an ohm meter between the aluminium knobs and the chassis.

[52 Bill]

...I was a little suspicious of a brand-new silver mica off the second plate of V3 but I jumpered it with a ceramic and it made no difference at all.

Paralleling a cap across another will not test for a leaky cap, as the leaky one will still be leaky. It will test for an open one though.

[EFK]

52 Bill - good catch. I'll remember that for the future. I went ahead and replaced it anyway. No difference!

Steve it is weird isn't it? It's only the volume pots! Although, all the pots act like mics if I tap on them, only the volume pot crackles and squeals when I touch it and turn it. Tested all the pots for continuity with chassis and all chassis grounds - all perfect with no resistance.

What I feel, but don't really know how to explain it, is that the either the choke or one of the transformers is "leaky" somehow. Leaking what I don;t know because I don't understand the theories behind it all, but I feel that somehow something is 'radiating' static into the pots. The vol. pots are attached to the grid wires off the second half of each 12ax7 (V1 and V3) and although those wires are shielded, maybe they have something to do with it. If I touch a grounded meter to the grid connection on either volume pot while the amp is powered on there is one hell of a squeal. Don't know if that means anything. There are some mV of DC, like 6-7 mV (on the <200mV scale) showing up on the 470K mix resistors and the line between them but damn if I can figure out where it is coming from. Something is leaking somewhere but I can't find it!

[Steve Conner]

Well, we're really scraping the barrel now!

All I can think of is: RF oscillations, or conductive eyelet board.

If you have a long run of wire going to a 12AX7 grid, this could cause RF oscillations. And these could get rectified by the grid-cathode diode, which would explain the DC. (It's not much DC though: 7mV across 1M ohm is 7 nanoamps, which I believe is well within the leakage spec for a 12AX7's grid.)

Anyway, so if anyone has changed the layout, maybe adding a long run of shielded cable that connects directly to a grid, then try adding a 68k stopper resistor in series right at the grid.

Squealing when you prod at grid terminals isn't that out of the ordinary, I guess. The meter leads are long unshielded wires that will act as antennas, and will pick up anything going, in this case the amp's own output voltage, causing feedback.

Last but not least: You don't happen to live next door to a radio station?

[EFK]

Steve it does indeed have long runs to the grids on V1 and V3 - completely across the chassis. I shielded all those and it helped, but not completely. At this point, since the lead dress in there was something of a birds' nest and everything inside is filthy and covered in gunk, I think the best thing to do is to just tear it all down and most importantly CLEAN it and rebuild it. I tried isolating the volume pots from the chassis and that also helped to some extent. I think I have a situation where the transformers are putting out EMI or something (I may sound stupid there but I'm working on it) and between the brass plate and a filthy, greasy chassis (and dirty, greasy board too), I'm getting unwanted ground loops. With this brass plate in there, which runs almost the full length of the chassis, I can't control where the pots or signal grounds are referencing. Getting those vol. pots off that brass plate did more than just about anything I've tried. Also thinking about some things I saw on one of the BillM pages about copper flux band on PT, or wrapping the ei leaves with a steel band, or little sheet metal shields etc.

No radio stations nearby - I'm a full mile from the nearest neighbor.

How do you feel about shielded wire vs. twisted pair? For example, the two wires off the board which run to the respective treble pots (and which are quite long, especially to treble channel): twisted, or shielded, or no difference?

[Wakculloch]

Hum can be induced from the power transformer in these amps because of the drop through design, if it is bolted hard down to the steel chassis, this will behave as a lamination of the transformer.
This can be easily cured with washers to lift the transformer and provide an air gap of 2 or3 mm.
Just another thought, and definately something to do if your embarking on a full stripdown.