500B Sparks

Was brushing out dust in my 500B... I touch the transformer and metal can capacitor at the same time with the metal part of the brush that holds the bristles together and heard and saw an arc. I don't think I did any harm but was wondering if this is normal, doesn't feel right o me... I know... I shouldn't be cleaning it with it on...

No that's not normal, and neither is you cleaning with it on and plugged in. Which transformer, as it's important!!!!!!

According to the Service manual, there are 4 CAN CAPS. 2 with insulators at the bottom and 2 without. The ungrounded 2 with insulators are next to the POWER TRANSFORMER. The grounded 2 are near the output transformers. Either case is not good.

Need some more info as to which cap and which transformer. Front, back, toward edge, toward center. Power or Output transformer. Is the unit running now?? Hum???

The chassis insulators look like gaskets going into the chassis at the base.

Are any cardboard can cap covers removed from the two with chassis insulators? Or, were you pressing down hard with the brush so the metal on the brush that holds the bristles in could touch one of those caps below the cardboard cap and a tranny at the same time?

That being said, it's not likely that this event is destructive. It may just highlight why and where more caution is called for.

These two... It sounds find...No hum or anything

Take your meter and read the voltage on the casing of that can. DC VOLT's. Red to can, black to chassis. That's one of the "isolated" voltage doubler can's. There is voltage onthe outer skin. That's why the original was isolated by the "gasket" at the bottom and the cardboard covering. Probably C88 if it's the single section can. 200uf/250V. Or C84 which is a 2 section 40uf/500V and 200uf/250V. In either case you touch that can and the one behind it, you're gonna get one hell of a shock. you're real lucky you didn't get knocked on your keester.

Find someplace that has 1-3/4" heatshrink tubing. Get enough to do 4 caps. Then cut some cardboard circles to fit the top of the cap(s). Glue to the top of the bare cap. Then cut a length of Heatshrink so it will overlap the top and cardboard and heat the tubing. Make sure it covers all of it. Do the plastic covered cap behind it also as that one has voltage on it too on the outer case. (better safe than sorry).

Even tho there is no hum, I'd play it safe and check all the voltages off that cap at the terminals. If they pretty much match the schematic it should be ok.

Larry

So it normal to touch the can and transformer and have it arc. Terry Dewick went through the unit about 6 months ago.

The two caps that are on the isolators aren't grounded. They are actually isolated from the chassis and above ground potential, they are series'ed to raise the High voltage (B+). the positive side of C84B will have something like 200V on it and the negative side of C88 the same. Positive side of C88 will have close to 395V on it. So the manufacturer puts a non conductive shield on the cap(s) (the cardboard covers) so you won't get shocked, or the kids get zapped. Have you done anything to it after Terry DeWick "went thru it" 6 months ago????

Larry

Yes I put a new volume switch in and lamps..So back to my question is it possible to touch the can and transformer at the same time and have it arc? If yes would this be normal to happen?

Yes. The can has voltage on it, and the transformer is grounded. Connecting the two would make it spark. Don't do that, its not good for it. Also, being as the can has voltage on it, I'd very much suggest not having your hands in there while its running or you might be lit up in an unpleasant way.


Would I be mistaken in believing these cans would have had the cardboard sleeve on them originally? If so, I'd be putting some heat shrink or something on them to help reduce the liklihood of being electrocuted. I don't like equipment that is inclined to kill me.

I agree no more cleaning while it's on... Here is a pic of the original before Terry restored it and a after pic.

Looks beautiful.
Shrink wrap can be your friend.
Lesson learned. Time to groove on.

Yup...listening to it right now. Thanks everyone.