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Re: [Amps] need 1000pf blocking cap

To: <kk2ed@comcast.net>, <amps@contesting.com>
Subject: Re: [Amps] need 1000pf blocking cap
From: "Carl" <km1h@jeremy.mv.com>
Date: Fri, 25 Jul 2008 10:25:40 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>
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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