The PHILCO Phorum
Zzzzap! Ow! - Printable Version

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Zzzzap! Ow! - Jayce - 09-25-2014

Well, this just goes to show us all that even a restored radio can still have its dangers in the oddest of spots. Basically, I have decided to get all my US Apex radios going and first off was my buddy reviving my Mantola 27 that croaked on me a few months back. Turned out the original resistor in there had somehow shorted against the chassis and blew the new electrolytic that had been put in. After some work, it performs even better than before and is once again here in my bedroom. Anyways, I was tuning the radio earlier today before work and hand my left hand resting on the opposite side. Now mind you, these use a metal thumb wheel for tuning, so there is no insulation between you and the tuning capacitor! There are three decorative, metal rosettes around the speaker that are also the heads of the speaker mounting studs. I let my left thumb go against one of those while tuning and.. YIKES! I pulled both hands away immediately! Took me a moment to realize what I had done, basically created a circuit between the tuner and speaker! Also reminded me to always be careful with these earlier sets as there is not as much isolation in them as later, transformered sets. There is reason why materials such as wood, bakelite, and plastic were popular for knobs and it wasn't just for looks!

RE: Zzzzap! Ow! - Arran - 09-25-2014

 I had a Victor 118 with that problem once, it means that part of the speaker field coil is shorting to the basket, which means 200+ volts of B+ between you and the chassis. It really has nothing to do with the inherent design of the set so much as an insulation breakdown in the speaker, it's not supposed to do that. Knowing the way these early AC sets were normally designed the chassis was likely slightly positive with respect to ground, the true negative would be at the center tap of the power transformer, so you weren't at full potential but still more then enough to give you a good jolt, or kill if you had a bad heart.
Regards
Arran

RE: Zzzzap! Ow! - Jayce - 09-25-2014

Hmmm, any way to repair this short of totally replacing the speaker or something to just make sure I avoid doing in the future?

RE: Zzzzap! Ow! - morzh - 09-25-2014

Be careful there, Jayce.
Personally, I always work with one hand. Even when tuning a fully assembled radio.

When rarely I have to use both (and it is a concious decision) I use a trick I was taught when starting at a factory: quickly stroke one finger against the second point of contact, very fast. If you get a jolt, it won't kill you as it lasts milliseconds, but you will know if it is hot.

Of course the best way is using a meter or not doing that at all.

RE: Zzzzap! Ow! - Jayce - 09-25-2014

This was just a freak accident for me. Usually I don't drape my other hand across table radios when tuning them. Now, it's not unusual for me to lean against a console when tuning it of course. The radio sounds and acts fine, so I am just going to leave things alone for now and just watch where my hands are at.

RE: Zzzzap! Ow! - 462ron - 09-25-2014

I remember when I was a teenager and just started getting interested in electronics, a friend and I came across an old console, can't remember the brand, and started operating on it in his basement. I was young and was just starting to learn so didn't know what I was doing. Anyway, my friend had his left hand lying across the speaker on the bench and with the other hand grabbed the metal shaft of the volume control. When he did, he got hit with full B+ and it picked him up and threw him against the wall. Luckily he survived but it cured him from having any further interest in electronics. I however continued my interest to this day but will never forget how young ignorance of radios could have turned tragic.

Ron

RE: Zzzzap! Ow! - morzh - 09-25-2014

My first tube amp (Soviet miniatures, ECC83, EL84 etc equivalents), I made it using fiberglass laminate PCB I etched myself, assembled it, plugged it in, started working on it and got zapped. 250VDC.

Somehow (and prior to it I was zapped by AC, 220V, numerous times) I felt DC was much more painful.

RE: Zzzzap! Ow! - Arran - 09-25-2014

Jayce;
  Often you can repair the short between field coil and the basket, it may be as simple as a break in the insulation of one of the leads going to the field coil or the primary of the output transformer (if it's mounted on the speaker). This is likely what it is since the field coils are usually wound on a cardboard bobbin like a transformer. Also may of those earlier electrodynamic speakers are bolted together so you can remove the bracket holding the pole piece, field coil, and hum bucker from the basket. On some, like the Philco speakers, they are mostly welded together, but you can get at the filed coil by pressing out the pole piece, this is something I have not tried but others on the phorum have. Barring repairing the short circuit itself you could try isolating the speaker from the mounting bolts through rubber grommets, or by mounting the speaker on a wooden spacer, and then mount the spacer to the cabinet bolts. 
Regards
Arran

RE: Zzzzap! Ow! - Jayce - 09-25-2014

I took a look at the speaker and think I found the problem. My friend suggested that it might be a stray strand of wire under the little protective cover over the speaker connections. I took a look and there was a small one on the speaker held on by two nuts. Took it off and although it had some insulation, sure enough the two heavy wires going to the field were literally smashed against the edge of the cover and the speaker frame and it looked original! I checked the wires and there is definitely a small, rubbed through spot on one that appeared to be touching the speaker frame. I carefully bent both wires up and away from the frame, and decided to leave the little cover off. I will have to meter it tomorrow, but I am fairly sure that was the issue. Utah really needed some better quality control! Now, would these be AC or DC?

RE: Zzzzap! Ow! - Arran - 09-26-2014

 It would be DC for certain, field coils always are regardless of whether they are used as a power supply filter choke or no. But you don't really need a volt meter to test it, you can check with an ohm meter to see if there is continuity between either end of the field coil and the speaker basket, it will read with a very low or no resistance on one end, and the resistance of the field coil on the other, just make sure the set is left off for a few hours first to discharge the filter caps. It's possible that it may have left the factory like that, but repair or replace the wires, if possible, and put that cover back on, it's there to stop unknowing persons from touching the wires.
Regards
Arran

RE: Zzzzap! Ow! - Joe Rossi - 09-27-2014

My dad taught me not to touch the metal or the components on a radio while it is plugged in, but that IF you need to do it, use your right hand and the back of your finger.  The electricity will naturally make your finger curl, and you could wind up holding what you touched.