Picture tube capacitor (21FDP4)

[ISawItOnTV]

I need to replace one of these flat, disc-type capacitors on another Philco 21FDP4 replacement picture tube (6.3v) . What is the purpose of this cap, and where can I get a replacement (or modern equivalent)?

[Eric H]

It appears to be item K7 on the Sam schematic which is actually a capacitor and a resistor in one package.

The cap is 5000Pf but I'm not sure about the resistor, if it's .2 Meg than that would b 200,000k.

Sams lists a couple different aftermarket part numbers, someone might have on of them laying around. otherwise you'll have to make one from individual parts.

This cap would probably work:http://www.mouser.com/ProductDetail/...p7xmDnliurA%3d

[zeno]

The other Q is how many volts to use for cap. Do 2 KV
to play it safe. Damned if I know what its for without the
rest of the schematic but it looks like isolation for safty.

73 Zeno

[electroking]

Maybe it is broken and that's wise you need to replace it, but leaving the original
part in place might be your best bet. I trust if an ohmmeter check reveals a resistance value within spec, the part can be assumed to be good. There is
hardly any way a ceramic capcitor like this would open.

From the schematic I assume the resistor is a loose tolerance unit of 600 Kohms
+/- 400 Kohms.

[ISawItOnTV]

Thanks everyone, that helps a bunch. I didn't realize it was the K7 cap/resistor network on the schematic. Now I can go forward with getting a replacement. According to Sams, its purpose is 'Picture Tube Isolation'.

electroking: The one pictured is probably fine. I have another 21FDP4 pic/tube that needs the replacement part (part of the cap casing is broken off).

Thanks!

edit: Would the value range of .2-1M equate to 200,000 ohms-to-1,000,000 ohms of resistance? I measured both K7 networks I have (including the damaged one), and got readings of 840,000 ohms on one, and 845,000 ohms on the other. If I'm correct, I'll get a resistor in the 600K-800K range. Not sure about the wattage though (1-2 watts?). The Mouser cap Eric posted should be fine.

[ISawItOnTV]

Originally Posted by Eric H

It appears to be item K7 on the Sam schematic which is actually a capacitor and a resistor in one package.

The cap is 5000Pf but I'm not sure about the resistor, if it's .2 Meg than that would b 200,000k.

Sams lists a couple different aftermarket part numbers, someone might have on of them laying around. otherwise you'll have to make one from individual parts.

This cap would probably work:http://www.mouser.com/ProductDetail/...p7xmDnliurA%3d

Would a 1 watt metal oxide resistor (680k value) be high enough wattage to go with the 5000pF 3KV capacitor you posted above?

[marty59]

Looking at the schematic, a 1/2 watt would be fine. Most hot chassis isolation resistors that I've seen generally were 1/2 watter's. There's not any current (except maybe from your body to ground) but mainly used as a safety component to minimize any floating potential.

A polarized plug or noting which blade is the hot would be good too.

[ISawItOnTV]

Thanks for the info Marty. A new power cord is on my replacement list, so I'll make it polarized.