Checking cap foil lead polarity w/o oscilloscope

tele_savales

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I think the fact that they paid careful attention to the foil end in amps, radios, and test equipment that's been road and bench worthy for 70 years or more, vs how things are built now vis-a-vis planned obsolescence, non-servicibility, etc kind of speaks for itself.
 

chas.wahl

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Look at the stripes, even on non-polar caps- they're consistently on the end that's more negative- doesn't need to be grounded, just more negative and that makes the - end the cathode. Look at the resistors, too- the first bands are all on the same end.
"The stripes" are not provided on lots of the (high-quality, nevertheless) metallized polyester and polyester film/foil caps that are routinely used by DIY builders: CDE 225P "orange drops", CDE "Mallory" 150s, CDE WMF and WMC -- and the derivatives by Vishay, Illinois, etc. Only the vintage molded and other caps, and some of the boutique (Sozo, Jupiter) caps seem to have them; but as Ronnie Rayguns said: "Trust, but Verify".
I don't remember where I saw it- might have been a page from BillM, but I made a harness with a 1/4" plug on one end and two alligator clips on the other, with the Red clip attached to the tip of the plug. This plug is inserted into an input jack on any guitar amp and the outer foil is found by touching the outside of the cap- if it hums, reverse it. If the hum stops, mark the end that connects to the outer foil and move on to the next cap. It mattered in my Bassman.
That's exactly where I started, and I think I said so. In practice, it doesn't work like you say at all -- the cap will hum when connected in either direction with respect to the clips, just maybe a bit less (or more) in one direction than the other, and determining which hum is lesser is often difficult. I ended by being dissatisfied about how long an interval in time there was between test in one direction and the other, plus, when the circuit is opened (clips removed but not shorted together) there's a God-awful louder hum from the amp (which is why input jacks are "shorted" type, to short the input when there's no jack plug inserted). As explained above, the device I made solves both the temporal and big unwanted noise problems, for me anyway. YMMV.
 
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King Fan

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Look at the stripes, even on non-polar caps....

To clarify, we're only talking about non-polarized caps, and only testing those without stripes. Also, 'more negative' gets complicated. I don't know if you had a chance to read the Aiken paper mentioned above, but he points out the better outer foil orientation may be toward ground or B+, depending. You're right, I do this testing by ear, with just a ¼" plug and alligators, and I simply turn down the amp while flipping the cap.
 
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ievans

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I was curious about the Vishay/Roederstein polyester film caps I just used in my Micro JCM800. They do have a thin black line and I oriented them according to the marking, but I couldn't find any info about whether the line actually marked the foil polarity.
IMG_4137.jpg
 

King Fan

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Looks good. I'm gonna just take a WAG that the only reason (and the long-time standard) for that stripe is to mark the outer foil.
 

sds1

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Looks good. I'm gonna just take a WAG that the only reason (and the long-time standard) for that stripe is to mark the outer foil.
My WAG is the stripe is just there for looks, I say this based on the fact that the datasheet is very detailed and includes description of all the markings on the casing, and no mention of foils. The line would be more suspicious to me if the factory/date code marking wasn't sideways like it is.

 

King Fan

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Battling WAGs. But you cheated, using evidence, even if it's evidence by omission, so you may be right. :)

Time to send a batch to Chas.Wahl Test Labs Ltd. for analysis…
 

58Bassman

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"The stripes" are not provided on lots of the (high-quality, nevertheless) metallized polyester and polyester film/foil caps that are routinely used by DIY builders: CDE 225P "orange drops", CDE "Mallory" 150s, CDE WMF and WMC -- and the derivatives by Vishay, Illinois, etc. Only the vintage molded and other caps, and some of the boutique (Sozo, Jupiter) caps seem to have them; but as Ronnie Rayguns said: "Trust, but Verify".

That's exactly where I started, and I think I said so. In practice, it doesn't work like you say at all -- the cap will hum when connected in either direction with respect to the clips, just maybe a bit less (or more) in one direction than the other, and determining which hum is lesser is often difficult. I ended by being dissatisfied about how long an interval in time there was between test in one direction and the other, plus, when the circuit is opened (clips removed but not shorted together) there's a God-awful louder hum from the amp (which is why input jacks are "shorted" type, to short the input when there's no jack plug inserted). As explained above, the device I made solves both the temporal and big unwanted noise problems, for me anyway. YMMV.
I know they're missing, but they should have been marked- that's a big ask, though. Since I'm not doing production, it's not hard to determine this.

Assuming you're referring to the buzz and hum with a Tele, there were some things that helped which were left undone on my Offset Squire. The cover of the neck pickup wasn't grounded, the wires were just laid in the routed areas-I twisted the wires in my Baja Tele and it helped, so I did it with this one, too. It was better. I also had noticed comments online about the Squire jacks being loose enough to allow the plug to fall out, so I bent the contact for the tip, to make a better connection and that reduced the noise, too. Unless I'm close to my TV and amp, the noise is pretty bad but as soon as I'm about 10' away, it's not bad at all- far better than many that I have heard online/YouTube and live. I ordered some Copper foil tape and that will be here tomorrow evening, so I'll shield the pickguard and see how that affects the noise level.

As long as the methods work, it's hard to say they're wrong. Well, aside from the online warriors who just have to dump on everything they see.

I don't remember what I was working on when I read about this- might have been the BF Vibro Champ but I don't think I unsoldered any caps to see if they made more noise in the other orientation, I just wanted to get the caps in and use it but I did check them ahead of installation and the amp was very quiet.
 

chas.wahl

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Time to send a batch to Chas.Wahl Test Labs Ltd. for analysis…
Hey, I'm just trying to do the best I can with caveman electronic tools -- my WAG is that if you really want to know which lead connects to outer corner of foil, use the oscilloscope method.
 
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chas.wahl

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OK, thanks a lot! I didn't realize that a Tele 3-way switch was "make before break", but it makes sense, so that the huge hum/pop is avoided by leaving at least one pickup always in the circuit. I didn't have one free, anyway, but it is obviously more automatic to use something like that. I would imagine that a Les Paul 3-way toggle switch (though twice as expensive) would do the same thing? And it would be a bit easier to fit the LP switch (esp. the right-angle variety) into a small enclosure.

Next time . . .
 
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Peegoo

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I would imagine that a Les Paul 3-way toggle switch (though twice as expensive) would do the same thing?

Off the top of my head, a typical LP-type switch will not do it because it's a DPST (usually wired as an SPDT--"on-on-on"). There's no way to flip the phase.

An even cheaper way of doing this might be to use a DP3T switch of the type used in the Fender Mustang.

Here's a little animation I did years ago to show how switches are named based on how they work.

GIF-Animated-Switch-GIF.gif
 

King Fan

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If you have a lonely Tele 3-way switch in your parts bin, you can slap a tester together in about an hour. This is 'make before break' so it should be nice and quiet. I haven't yet made one in a box, but I did proto it with a switch and seven clip leads.

That is so darn cool! Nice one, @Peegoo . It almost makes me want to build another amp so I can build one of those testers so I can test a bunch of caps. Almost. :)

To move from the sublime to the ridiculously dull, can anyone suggest a good source/type of alligator lead? I have the multi-color cheap-os with stiff but not grippy jaws and thin but kinky wires...
 
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Peegoo

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can anyone suggest a good source/type of alligator lead?

The reason the jaws are not really grippy is the vinyl insulating boots on the clips: they prevent the clip from closing nice and firm because they're a little too snug on the handles, and they maintain a sort of "to open, pinch here" grip on the clip. You can prove this by backing the boot off the clip and up the wire a little bit. Attach the clip to a component lead and chances are good it will hold firmly.

What I've done to fix this problem is goofy as hell, but it works: get a big pot of water boiling on the stove, and have a glass of room-temp water on the counter. Hold the middle of the jumper lead (both clips hanging down), and dunk the clips into the boiling water for 15 seconds. This softens the vinyl boot and allows the clips to expand them a bit. Lift 'em out and dunk 'em into the glass of water to cool them and 'set' the vinyl. When you've done all your jumpers like this, shake out the water from the clips and lay 'em in a pile on a warm-air register for an hour to get 'em good and dry.

There are other ways to do this, I'd guess. You could put them into a pillowcase, tie a knot in it, and toss it into a clothes dryer for about 10 minutes or so. Anything to heat up the vinyl boot and get it to be a bit more compliant.

A hot-air gun might work, but these are very very difficult to use when trying to gradually heat something evenly throughout without melting it. Proceed carefully if you try this method.
 

Peegoo

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It almost makes me want to build another amp so I can build one of those testers so I can test a bunch of caps.

If you have a stash of caps, go ahead and test 'em all and mark them now. That way when the next project comes up, they're ready to go.

GET MOVING, SOLDIER! 😉
 

chas.wahl

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Off the top of my head, a typical LP-type switch will not do it because it's a DPST (usually wired as an SPDT--"on-on-on"). There's no way to flip the phase.
Uh, I don't understand how a DPST switch could be wired as a SPDT -- to me ST or DT means how many positions (throws) a switch has, which can't typically be changed. In any event, I shouldn't have spoken without investigating how a Gibson switch works -- It seems pretty obvious looking at Gibson pickup wiring diagrams that there's typically a single ground, and all the switch is doing is grounding one pickup, the other, or (middle position) neither, with no opportunity to switch polarity. But then I go to Switchcraft's site, and see versions with 5 or 6 conductors. Plus, I can't really find any schematics for these switches; so lacking anything other than one of these to try out with my DMM (which I haven't done yet), I should leave that out.
An even cheaper way of doing this might be to use a DP3T switch of the type used in the Fender Mustang.
I'll have to try to find information about one of these, or get a real live one.
Here's a little animation I did years ago to show how switches are named based on how they work.

GIF-Animated-Switch-GIF.gif
"Double throw" confuses me -- DT toggle switches can be either 2 position or 3 position, like SPDT and DPDT you have illustrated. It would be simpler if the terminology left "throw" out of it and simply indicated how many positions, then specified whether each was on, off, (on), (off). Like slide switches are described.
To move from the sublime to the ridiculously dull, can anyone suggest a good source/type of alligator lead? I have the multi-color cheap-os with stiff but not grippy jaws and thin but kinky wires...
The problem with the ones I used (bought as a bag of 10 on eBay, which is about as good a guarantee of quality and satisfaction with the product as buying from Amazon) is that the sheath insulator is a bit tight, so that the clamping force and bite are reduced. Mueller sells ones that look better, tighter around the wire end, but more capacious where the clip "handle" needs to rotate freely.

Screenshot 2023-02-19 at 15.32.08.jpg


Pomona seems to be another mfgr who makes quality products of this sort. I typically buy the stuff à la carte, clips only and wire them up myself; but I'm sure one or both of those companies sell clips on a lead, if that's what you prefer.
 

Blrfl

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"Double throw" confuses me -- DT toggle switches can be either 2 position or 3 position, like SPDT and DPDT you have illustrated. It would be simpler if the terminology left "throw" out of it and simply indicated how many positions, then specified whether each was on, off, (on), (off). Like slide switches are described.

The number of throws indicates how many positions the switch has where it makes a connection. Using only the number of positions would make a SPDT switch with an off position in the middle hard to differentiate from a single-deck, 3-position rotary switch (SP3T).

For toggle switches, the usual nomenclature is on-on for two positions, on-off-on for three and parentheses around the momentary positions, e.g., (on)-off-(on).
 
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