Its certainly at the extreme for that cap.
The HT-57 comes in 7.5 and 15KV ratings. They are identical caps simply
marked different to reflect different safety margins.
Ive seen many LK-800's melt the solder or crystalize it at the switch
contacts. Suck it all out and reflow new. Same at the relay and SO-239. Use
an ohmeter to verify the path. Ive also replaced several open vacuum relays
in those amps.
Carl
KM1H
----- Original Message -----
From: <kk2ed@comcast.net>
To: <amps@contesting.com>
Sent: Friday, July 25, 2008 10:13 AM
Subject: [Amps] need 1000pf blocking cap
> Good Morning;
>
> I'm helping a friend repair an Amp Supply LK800 3x3CX800A7 amp which
> appears to have a bad plate blocking/coupling doorknop capacitor. It's an
> HEC HT-50 style 1000pf @ 5KV , X5V temp rating.
>
> The symptoms are as follows: Varying drive power varies plate and grid
> current, yet the load and plate tuning have no effect on readings, and
> there is no RF output at the antenna connection. The vacuum relay and all
> connections back to the blocking cap check good. I'm guessing the cap
> failed to an open condition.
>
> I have a box full of brand new HEC HT-50 style 5kv caps, from 50pf to
> 250pf, but nothing as large as 1000pf. There isn't enough room between the
> plate choke and tuning network, otherwise I'd fabricate four of the 250pf
> units together to reach 1000pf.
>
> Anyone here on the list have a 1000pf 5kv or greater cap for sale, or
> trade for my smaller value ones, before I place an order?
>
> One more question - he is pushing the hell out of this amp, to the point
> where he is easily reaching 2x plate dissipation. The amp had a larger
> Dahl transformer installed (3KV plate voltage), and plate current is
> around 2A+ when he pushes it hard, so I'm wondering if the cap blew from
> being stressed passed its limits.... According to the chart in the
> latest RF Parts catalog, the 5kv cap is rated at 5.6a @ 30MHz, but I
> recall reading in the past that the RF current is actually much higher
> than the calculated DC current, no?
>
> Should I just upgrade the cap to an HT-57 15kv 1000pf cap, which shows in
> the catalog as having a higher current rating?
>
> Also, he has the original stock LK800 transformer for sale if anyone is
> interested. If I recall correctly, after the diode and cap board, it was
> generating approx. 2700v no load, and 2400v loaded. If interested, contact
> me and I'll forward his contact info.
>
> Eric
> KE2D
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>
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