Mojotone CTS pots?

[Randall]

I bought a handful of the nice brass shaft CTS pots in various values for a build, and I find that both 1 Meg volume and both 250K treble pots are scratchy in about the same place from around 6 - 9. Maybe the bass ones are scratchy also, but just not audible. Anyone experiencing this as well?

[J M Fahey]

Don't have access to them, but in general I suspect you have some DC across them.

Mind you, usual path for DC is leaky previous coupling caps, so check for that, but also sometimes the following grid is passing a small current which can cause some unexpected scratching.

That's why I don't trust a pot wiper to be the *only* path to ground for the following grid but always add an "unnecessary" 1M to 3M3 resistor to ground.

As a side note, some scratch should be unavoidable, because wiper contact can't be *perfect* everywhere , but modern pots use double or triple wiper contacts in parallel so at least *one* (and usually two) are making good contact at all times.

But if the Pot you mention cloned an "antique", up to and including a single contact wiper .... well, it would explain it.

[Sowhat]

I imagine that the 1M is an audio taper, are the 250K's audio taper too? Audio tapers can have a blip or scratch around that rotation because the resist is actually two different linear pieces joined together just about at that point in the rotation, if so just run them in a bunch (rotate back and forth) and that should smooth out that joint eventually. Squirt a little Deoxit in there to help the smoothing process along.

[Randall]

I've done the DeOxit red and blue and worked them to no effect. It's on both channels, and I can fine tune a little spot that I can even scratch like a DJ to some extent. The grid wires are pretty sensitive also. Seems strange that it would be both sides, but I'll look in that area.

I measured 20 - 30mV across those pots. Would this be enough voltage to cause a problem?

[Mick Bailey]

That amount of DC will do it. Just remember that any noise is being amplified.

[pdf64]

Maybe ultrasonic oscillation? The scratchiness can be due to the dc operating point shifting when it starts (due to sufficient phase shift/gain as the controls are adjusted).
The fact it doesn't occur when the bass control is adjusted is another indicator of this cause.
Try tacking in a cap (eg 470pF) to ground from the plate of the following stage to see if that stops it.
May just be lead dress with room for improvement.
Pete

[J M Fahey]

I measured 20 - 30mV across those pots. Would this be enough voltage to cause a problem?

Definitely.
And as noticed above, it gets amplified.

Find where it comes from.

Of course, the slight discontinuity where the 2 tracks join increases the problem.

I had pots custom made at a small factory which dwindled down to a Father/Son team, and being curious I watched the process, even helping assemble my own pots.

The typical semicircular pot track was actually born as a long phenolic strip where the carbon resist paint was applied with a roller, then cooked in an oven and finally punched into final shape.
Log pots used 2 rollers, each with a different resistivity formulation.
The overlap (which is what is bothering you) was clearly visible with a loupe.

The beauty of having direct access to the actual factory is that I could order weird pot values and tapers.
The same log strip cut upside down gave reverse log, and I had a batch made of "S" taper, meaning 0>5 Log , 5>10 antilog which is what you actually need for graphic equalizers.

Of course, the Factory went bankrupt thanks to the wonders of "globalization", he couldn't compete with the Chinese.

[Mick Bailey]

Omeg in the UK still do custom order pots.

[J M Fahey]

Omeg in the UK still do custom order pots.

Good to know.
Not for me, I'm too far away, but post the link here, it will be most useful for people needing non-standard stuff.
I guess they will have a minimum order, anyway, but for a small builder or a Luthier into active electronics who wants to build a few preamps or a pedal builder it may prove acceptable.

Just last week I was putting some order into my messy stockroom and found 2 boxes (100 each) of these custom made pots, one in 250K Log but with the hard to find "30% taper" , (which is what Fender actually used in Blackface and early Silverface) and 1M Reverse Log which I had ordered for PA Mixer input gain controls and MXR Dist+ dist controls.

I'll put them to some use, of course.

[Mick Bailey]

Potentiometer switch manufacturer, Omeg

They're used in a lot of British audio equipment. I Believe the minimum order is 10, but I found a reference to someone who ordered a single custom pot (I guess the price would be the same either way).

[Randall]

Thanks pdf64, you were right. Mostly. I found I had close to 300mv across the vol pot with both treble and bass pots wide open. Pretty sure it was HF oscillation. Close examination of HV wiring had me notice the preamp plate supply wire was coming out of the same filter cap as the others, and then traveling by itslef to the right side of the chassis to the connect point. I drilled a new hole for it under the doghouse and dropped it right next to the connection point on the board. That reduced my dc quite a bit. Then I re-ran and twisted my bass and treble wires and made sure they were hugging the chassis tightly. That got me down to between 3 - 5 vdc on both channels. I don't think there is anything more I can do. I don't believe any caps are leaking more than is expected, they do leak a little, correct? The pots are less scratchy, but they do still make some noise in the same place. I'm not positive, but I think I got some defects. CS at Mojo said it's probably dc leakage as well.

[Mick Bailey]

Caps shouldn't leak DC to degree that would be meaningful in an amp build, especially film, ceramic or mica caps. A rather difficult fault to detect with a new build is DC leakage through the circuit board material itself. Are you using fibre or phenolic? They can absorb moisture or have enough surface contamination to pass DC from your B+ supply. It looks like a leaky cap when this happens. You can sometimes measure DC from an unconnected terminal to ground. Sometimes several volts.

Now that makes pots really scratchy.

[Randall]

Mick I'd never thought of the board leaking dc. But in fact I am using fibre board, and I do measure anywhere from 100 mV to close to 4 V, depending where I probe. My bench is in my basement, where it is somewhat humid. Could this be the cause, and if so what can be done about a partially conducting board?

[J M Fahey]

Mick I'd never thought of the board leaking dc. But in fact I am using fibre board, and I do measure anywhere from 100 mV to close to 4 V, depending where I probe.

Ouch!!

My bench is in my basement, where it is somewhat humid. Could this be the cause,

Yes, definitely.

and if so what can be done about a partially conducting board?

Fiber is a very irregular material, all absorb water but some much more than others.

If yours came from a bad batch, sorry, but there's not much you can do.

Long time ago I had that exact same problem, even tried drying them overnight in a >100ºC oven, then submerged in molten paraffin (mineral wax in UK) .... 3 months to one year later problem was back.

Only permanent solution was to buy electrical grade "Pertinax/Paxolin" (phenolic impregnated boards) and redo boards.
Which really does not take long.
Drilling and eyeletting any Fender or Marshall PTP board should be made in less than an hour, if you already have a 1:1 template or an original to clone.
You might use even better quality unclad fiberglass board.

Even kitchen table grade Formica is better than fiberboard.

[Gtr_tech]

Ouch!!



Even kitchen table grade Formica is better than fiberboard.

There's a builder local to me who uses scrap formica from a office furniture mfgr near here. Its free, and he says it works great.