Checking cap foil lead polarity w/o oscilloscope

[chas.wahl]

I have no oscilloscope, and don't really care to have one; but I'd like to find a more definitive way to determine which lead is connected to the outer end of film-foil (and metallized foil) "non-polar" caps. I've tried the blunt instrument (cable) approach using alligator clips connected to a bare jack that's inserted over the instrument-end plug of a cable inserted into an amp; and I can't tell a difference convincingly between a cap going one way and then the other -- partly because when I disconnect the cap, there's a ton of loud hum (don't want to infuriate my neighbors), which obliterates my memory of the previous test when compared with the next one with cap reversed. I'd like a relatively instantaneous switch between one cap orientation and the other, and silence between them, with volume on the amp left untouched.

It seems to me: I need a reliable method of shorting out tip and sleeve of the instrument cable continuously in the interval between cap oriented one way, and cap oriented the other way. And that the answer might be a double pole "make before break" (aka "shorting") switch that will swap the connection of tip and sleeve to cap. I can't seem to find such an animal. What I do have on hand: a double-pole on-on-on slide switch, like this one:

Trouble is, it isn't "make before break" -- switch is open between positions. The thought occurred: get a momentary-contact push-button or toggle that I could actuate, to short tip and sleeve while flipping the other switch to reverse polarity (without unclipping the cap from the 'gator clips). Does that sound reasonable? Or does anyone know of a DPDT shorting switch that is reasonably priced?

 

[Phrygian77]

Do you have any kind of headphone capable amp? I use my Two Notes CAB M. The nice thing about that is I don't even really need the headphones since it has a dB meter.

 

[chas.wahl]

Nope, no headphone-capable amp. Besides, I expect the loud hum with an open input jack would be just as big a problem, in terms of my hearing and being able to distinguish louder from less loud (punctuated by loud hum) than with a normal amp; no?

 

[Blrfl]

You're on the right track; all that's missing is a tool to do the switching so you don't have to fiddle with it manually.

Get a small project box, a DPDT switch, a couple of binding posts (one red, one black) and a 1/4" jack.

Wire the Jack's tip (red) and sleeve (black) to the switch as shown on the left side of this picture:

Wire the lines from the right side of the picture to the binding posts (match the colors).

Using a meter, buzz out which position of the switch connects the black binding post to the sleeve on the jack. That's the "normal" position. The other position is "reversed." Mark the case accordingly.

Attach a cap to the binding posts and connect a cable from the jack to your amp. Do the blunt approach, but just flip the switch instead of fiddling with alligator clips.

 

[chas.wahl]

That's a standard DPDT switch (slide or toggle), but unless it's a shorting type, which I cannot find, there will be a LOUD hum or pop when I flip the switch, because there will be a small interval during which the amp jack is open. I don't mind pushing a button switch to make the transition.

 

[Blrfl]

Well, then, add a momentary, SPST pushbutton that shorts the tip and sleeve on the jack and hold that while you switch.

You must have the amp cranked waaaaaaay up.

 

[dan40]

I tried an experiment a few months ago when I was building an amp. I had some Sozo caps with the foil end marked so I set my meter to the lowest AC voltage scale, clipped my leads onto each end of the cap and squeezed it between my fingers. When I had the cap clipped in the correct way, I would see a small (mv) voltage reading. When I hooked up the cap the wrong way, the voltage would jump slightly higher. I tried this method with a handful of caps that already had the outer foil end marked and the results were always the same. I need to test this a bit more but it may be an easier way to find the outer foil if my results are correct.

 

[Peegoo]

I've thought of building one like that for a while. In the meantime, you can greatly increase the difference in noise level (making it easier to hear) by doing the following:

1. Curl the fingers on one hand around your amp's power cable.

2. Gently pinch the cap between thumb and finger of the other hand.

Try that and see if it helps.

 

[pinchegil]

This and my first amp “70s backstage 30” cheesy but it works

 

[King Fan]

partly because when I disconnect the cap, there's a ton of loud hum

My trick for that is start with test amp at volume 0, turn up until 'hum is obvious', like 2 or 3 (on the exact mark), then turn down while swapping cap ends, and turn up to the same exact mark. The hum doesn't have to be painful at all.

I may have to repeat one or two cycles to be sure, and if it's too hard to tell, I use one of the 'power cord' tricks; I like the one by @Peegoo , but have also had decent luck with simply setting the cap across the active power cord while testing.

A switch would be cool, though, I admit.

 

[chas.wahl]

Well, then, add a momentary, SPST pushbutton that shorts the tip and sleeve on the jack and hold that while you switch.

That's my next step, I think.

You must have the amp cranked waaaaaaay up.

Not really. I live in a NYC apartment, and am sensitive to noise myself, so try to be considerate to my neighbors.

I've thought of building one like that for a while. In the meantime, you can greatly increase the difference in noise level (making it easier to hear) by doing the following:

1. Curl the fingers on one hand around your amp's power cable.

2. Gently pinch the cap between thumb and finger of the other hand.

I noticed your advice in this direction, in The Professor's Big Bottle Single Stroke thread -- and had been planning to try that out. Thanks.

 

[Peegoo]

@chas.wahl

I've thought about this a little, and I think this might be a good alternative to your 'break-before-make' switch dilemma. Instead of switching to reverse the polarity, it sweeps smoothly.

Note: this uses a dual pot with a single control shaft--not a stacked pot with concentric control shafts.

Also: mount this in a metal box, ground it to the plug's shield, and use shielded coaxial cable on all leads that run externally. I think. I should build one and see if it works.

 

[tele_savales]

I've been using my scope. Since it's digital, I'll set it to display peak to peak volts so I don't have to strain my eyeballs to see the difference in the trace, which can be really hard to discern. Pretty straightforward and I can do a pile of caps at once when I'm not doing anything else.

 

[chas.wahl]

Hmm, very ingenious, as usual @Peegoo -- but with the greatest respect -- it seems to me that the problem of "not being shorted during the polarity swap" would not be solved. In the middle of rotation, for instance, each lead to the plug would see resistance equal to a quarter of the resistance of the pots (two resistances half the value of the pots, in parallel) to each end of the cap; so not really shorting the input. Since I already have the hardware necessary for a switched polarity reversal, don't have a dual pot, and if I don't have a momentary SPST, I could simply hold a wire in my hand and short a jack connected to an instrument cable -- I'll try that method first.

Do you think that use of an instrument cable masks the hum created by the cap, compared to using a plug into the input and short wires?

 

[Peegoo]

@chas.wahl

I don't see why it would need to be directly shorted. With the knob in the center, there would be a resistive short that would kill any pops through the amp. The lower the rating of these pots, the better.

I think use of an instrument cable would have no effect on this because the exposed (unshielded) conductors are identical to one that directly plugs into the amp, and the cap is fully exposed in both scenarios.

Well I AM depressed. I thought I had some stacked pots here, but all I have are concentrics. Maybe I'll drill and pin the shafts so they operate as a single unit. I'll slap this together later today and report on the results.

 

[Beebe]

Here's an answer I got from @El Marin when I asked this.

Post in thread 'Outer Foil Side on Emerson Cap?' https://www.tdpri.com/threads/outer-foil-side-on-emerson-cap.1074319/post-10733701

 

[chas.wahl]

OK, so I found a momentary NO SPST (doorbell switch at hardware store) and plastic project box. Here is my tentative schematic:

If I remember correctly, when the hookup is done so that the cap hums a lesser amount, then the lead connected to the sleeve of the jack (and amp input cable) is the one that's connected to the cap's foil shield -- is that right?

I plan to set it up with the pushbutton momentary switch on the bottom of the box, so that pressing the whole box down with my hand shorts tip to sleeve, no matter where the slide switch toggle is parked. The middle position of the slide switch also shorts the tip and sleeve, so that can be used as the "park" gear while caps are being exchanged. I guess that a tip switched jack is not required, but I typically buy the 12A or L12A anyway.

 

[Kevin Wolfe]

After doing all this, which way do you mark the cap and which way do you solder it in? I’ve never really been able to hear a difference.

 

[King Fan]

After doing all this, which way do you mark the cap and which way do you solder it in? I’ve never really been able to hear a difference.

Ah, I thought I'd posted a link to the Aiken article. Maybe it's in that other thread.

https://www.aikenamps.com/index.php/where-to-connect-the-outside-foil-on-capacitors

The which-end-where thing can get tricky; he has a good but technical discussion. Rob helps us out by showing the open foil end on many of his layouts.

Aiken also mentions types of caps that don't have an open foil (they'll be hard to tell for sure!) He's talking scope, but I've had good results by ear using just the volume knob as in my post above. (The quiet end to sleeve/ground is the outer foil end). A switch might be handy for those hard-to-tell caps -- Mallories have sometimes puzzled me. OTOH, I hope they're the ones that matter less as to noise in the amp.

 

[chas.wahl]

Most, if not all, of the YouTube and forum lore seems to support marking the lead determined to be connected to the foil (which I've confirmed is the one that's connected to the sleeve of the instrument cable plug when the noise or hum is less than with the cap connected the other way 'round). And that marked end should go toward the lower-impedance-to-ground side of it's location in the circuit. This is generally toward the obvious signal input side, but not always. Take a look at this post about it, where @robrob says the middle tone cap of a TMB, for instance, should be connected differently. And here's his Modern 5F6-A Bassman Layout, where he's marked the foil side of all the non-polarized caps with a "o".

Hearing a difference is apparently (I don't have much experience with such testing, hence this thread) more difficult with smaller-value caps than it is with larger-value ones. And my guess is that the smaller the cap value, probably the less it matters which way the foil end is connected.