The AM Forum

Gentlemen,
     I have an old SB-614 which was given to me. It was non-functioning, keeps blowing it's fuse in the power transformer primary circuit. I traced the problem to the negative 1400 volt supply. Specifically, C122, a 1600 volt 0.1 uF tubular capacitor, acts like a dead short. I pulled it from the circuit board, and it reads as a dead short on multiple different VOMs around the shack here.
     The negative 1400 volt supply uses a power transformer with a 600 volt AC reading across the secondary and mine is still good (not blown yet!) The circuit uses a voltage doubler with C122 across both secondary leads from the transformer, and another identical capacitor from the bottom of C122 to ground. The silicon diodes, D106 shunts the top secondary winding  to ground, and D105 is in series with the top secondary lead, then passing through a 470 k 1 watt resistor (which also was bad, and now replaced.) The output and negative 1400 volts has another identical 1600 volt 0.1 uF cap (C121) to ground.
     My question is concerning a suitable replacement for all 3 of these caps. Interestingly, the originals have a banded end, suggesting that these are polarized. Does anyone know what I could use as replacements? I did order from Mouser some 1000 volt Cornell-Dubilier Polyester Film caps, 0.22 uF, and I plan to put two in series to get me a 2000 volt rating with a 0.11 uF capacity. My only concern is that these caps aren't banded, that is, there is no polarity. Will these caps work in this circuit?
     Any ideas would be greatly appreciated.
                                                        Marty, KK4RF

The caps you ordered will work fine. The originals were banded to mark the outside foil which is to be grounded for the shielding effect. No shielding is necessary in this application.

Terry,
     Thanks for the reassuring words. You know a lot more about it than I do. I'm going to let everyone know how this repair job turns out once I get the parts back from Mouser. Thanks again.---Marty---

Check all the hugh value resistors around the CRT circuit. My SB 620 had a number of them go high out of spec. someone changed the caps in the ps before me.

Excellent idea. I have not yet checked those resistors for value. I'm often amazed how much resistors can change in value over the years. Thanks again for the idea.
    ---Marty, KK4RF---

Especially carbon comp resistors.  Most of Heath stuff use these and they're notorious for changing value.

Gentlemen,
     Got the new polyester film caps yesterday from Mouser and just got done soldering them in. The scope works and I do get a tracing on the CRT. Now I have to read the manual and learn how to actually use the scope.  Thanks again to all who replied. ---Marty, KK4RF---