Yaesu FT101 series molded cap that shorts

[KneeBiter]

Was told by a friend that is a radio tech that in the Yaesu ft101 series there is a molded plastic cap that goes from pin 7of the driver tube to pin 5 of the 6js6 final closest to the driver and it is a week spot and they short out for no reason even if not transmitting as long as the heater switch is on and takes out a bunch of parts including the transformer. Said to replace it with a dipped mica. Just a heads up for anyone with a 101.

[BBB]

Thanks, I'll check to see whats in there.

[KneeBiter]

I will post pics later of where and what.

[KneeBiter]

if you have this. DO NOT TURN ON YOUR HEATER SWITCH UNTIL YOU REPLACE IT WITH A DIPPED MICA.

[BBB]

Mine is labeled Yaesu with some indiscernible printing on it and looks original so I'll replace it before I fire it up.

Any idea on the capacitance value, voltage rating, ect?

Thanks for the heads up.

[BBB]

I think I see it on the schematic as C13, 80pf, 1KWV.

So That would be a 80pf cap rated at 1000 volts right?

[BBB]

Havin' a hard time finding that cap at that voltage rating. Anyone know where we can get 'em?

[BBB]

Found some of these for sale but the don't look like any cap I've ever seen. More like a power diode. Maybe better rated at 2KV?:

80 pF, 2 kV, RFT (DDR), HIGH V0LTAGE, CERAMIC CAPACITOR

Found some at Digi-Key but they are 82pf at 1000 volts diped mica. Would that be close enough? Looks like they're in Canada eh.

www.digikey.ca/scripts/dksearch/dksus.dll

[KneeBiter]

.001 dipped mica 3kv it what to use.

[2600]

The original component tends to wear out when the radio is used on AM a lot.

SSB transmit places less stress on this part.

It's not just the years it's the miles, too.

The original capacitance value is 82pf. Changing it to 1000 pf (.001 uf) will disturb the alignment for no good reason.

A part with a higher voltage rating than the original is a good idea. A silver-mica capacitor or molded mica like the original is closest to the designer's original choice. If you want to redesign it, that's okay. But if you just want to repair it with a minimum of time spent compensating for your substituted part, the original capacitance value and dielectric type is the best bet.

Years ago when we ran out of the 82pf 1kv silver-mica cap, we used a 150pf 500 volt and 180 pf 500 Volt silver-mica caps in series to replace this one part. This trick only works if you have both of those on hand. This doubles the voltage rating while keeping the capacitance value about the same. Capacitors in series have a lower capacitance value than either of the capacitors has by itself. The 150 + 180 combination works out very close to the original 82 pf.

You will probably find that the neutralization adjustment is affected by replacing this capacitor.

I really ought to post a sticky for that one, checking/setting the neutralization in tube ham transceivers.

Simple way requires a dummy load and wattmeter. If you don't have those, maybe this project can wait until you do.

If the old part still looks okay, and the radio is behaving itself, you're fixing something that's not broke yet.

That's the motto of the shade-tree mechanic: "Keep fixing it until it breaks".

73

[KneeBiter]

Thanks for your reply. I am just going by what I was told by a local tech that is a friend. He said that these caps tend to short not open. And if they short it will take out the transformer and other stuff as well. Said that they can short for no reason with the radio just sitting at idle with the heater switch on. Do you agree with this ??

[KneeBiter]

This is why I love this forum. Looking at the schematic I think the .001 is for the 160 meter switch position problem. My buddy must be a little rusty with the 101s. Lucky that I posted here and got the correct info. So use an 82pf 1kv or better for the spot I show above. Thank you 2600

[BBB]

Great info here.

I too read that a differnt cap failure will take out a fuse when switched to 160 meters. If any thing it promted me to look deeper into this fine radio.