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I have an old tube radio. There is a ceramic cap with:"RMC / 0.1 / 1400V / Z5U " written from top to bottom, respectively. Now, this cap has broken at it's base and can't be resoldered and so must be replaced. I have another ceramic disk cap, almost the same radius (across the flat aspect) with: " RMC / 0.5 / 20% +- / Z5U " written on it in the same fashion.

I'm wondering what the voltage rating of this possible replacement cap is and whether or not it would make a suitable replacement for the whacked one. I've been looking ALL OVER the Net for info on ceramics and their voltage ratings when not shown on the cap. I found nothing so any help would be appreciated. I read something that I find highly suspicious so I won't poison the mental pool here with that particular suggestion but I am quite interested in what the EE community has to say.

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My hunch is that the part is 100nF, and your replacement is 500nF.

I wouldn't risk using a cap without a voltage rating indication on it. Also, you're better off replacing that Z5U dielectric with one that's more temperature stable, like NP0.

I'd source a 100nF, 2kV NP0 capacitor as a replacement.

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  • \$\begingroup\$ ..whoops..that was the conclusion that I've since come to as well. I, by nature, try to use what I have on hand and then make the trip to the electronics store (20mi)! Thanks again, \$\endgroup\$ – M. Burton May 1 '13 at 4:24
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Your cap isn't a good match being 5x the capacitance. I presume this is a snubber across the output transformer, that's generally the only place they'd have such a high voltage. Although they tend to be on the order of 1 - 10nF, not 100...

Since it's Z5U dielectric, I assume it's newer than the radio and not original. Find the schematic and see what part should actually be there. The value could be wrong, or the voltage could be much more than needed, if the person used whatever was handy. At least tell us where in the circuit it is, we should be able to make an educated guess as to what the value should be.

NP0 isn't necessary unless it is in a critical circuit, but Z5U is pretty junk. X7R is much better at not much more cost. (xxU and xxV caps lose something like 80% of their capacitance when running near rated voltage! The things should be illegal to rate at 0V)

edit: if it's an american power transformer-less style, an 'AA5', it might be the cap from the line to chassis to reduce noise. It would generally be 50nF or less though, there. I suppose you should use an X or Y rated cap there to meet modern safety, and read about the danger of hot chassis radios before you get zapped!

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  • \$\begingroup\$ As for the voltage rating, depends in part if it's a receiver only. An 800v supply wouldn't be uncommon in a tube-type ham transceiver, and caps are usually chosen with a fair margin on the working voltage. \$\endgroup\$ – Chris Stratton May 1 '13 at 1:50
  • \$\begingroup\$ @ Marko = Thanks & your're right. I've yet to receive a schematic and I should have let everyone know that I didn't believe it to be a line to ground or isolating but a filter right before the first multi-cap. (..I'll get better at asking for help. Promise!) And thanks for the info about ceram-types, I'm not up on the variations generally. Rarely need to replace them, but VERY good to know. I'm new to this site, just found it today, and I LOVE it already. Thanks again. *** @ Chris S: It's a 16 tube AM/FM (AC) 1961 Knight KN300. And thank YOU for your input also. I really appreciate it. MB \$\endgroup\$ – M. Burton May 1 '13 at 4:36
  • \$\begingroup\$ I guess it's off to the Electronics store! (Lots of stuff to get) \$\endgroup\$ – M. Burton May 1 '13 at 4:37

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