On Jul 19, 2005, at 1:53 AM, Adrian Rees (M1LCR) wrote:
> Hi all
> We have an LK 500 Amplifier (the pair of 3-500Z's model) that started
> to
> spark and arc quite badly last weekend when we tested it ready for
> IOTA.
>
> We've had a look at the amp, and it appears that the varnish on the
> anode
> choke has come off, and there are signs of tracking.
>
> Given that it'll need re-winding, what you boys suggest I coat the
> choke in,
> to stop it from unravelling ?
Hello, Adrian -- I use clear, gloss. polyurethane varnish that I apply
with my finger. A suitable type of Cu wire for HV RFC service is sold
by electric-motor rewinding shops. It is sold by weight. Bring your
own spool. Details on using such is covered in 'Amplifiers', Part 4 on
my Web site.
>
> Any other comments ?
The arcing you describe in the LK-500 amplifier is typically caused by
the combination of the anode's self-resonance c. 100MHz and the 0.3pF
feedback-C in the 3-500Zs. The result is intermittent regeneration c.
100MHz. Since the HF tank is a low-pass configuration, VHF energy can
not pass through, so it runs amuck and arcs the Tune-C and / or the
bandswitch. Decreasing the Q of the VHF suppressors will decrease the
amplification at VHF -- which may stop the oscillation problem. The
tradeoff is an additional power loss of c. 2% at 29MHz.
>
> Adrian (M1LCR)
>
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>
>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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