----- Original Message -----
From: "Rob Santello" <K6DQ@drumon.com>
Sent: Friday, February 18, 2011 6:57 PM
Subject: [Amps] Amp Supply LK-500ZB Upgrades?
> Hello List,
> I just acquired an Amp Supply LK-500ZB which looks to be in pretty
> good condition. I took some pictures and put them on a web page at:
> I have several questions regarding modifications and potential
> 1. The fan works at both speeds but is pretty loud in the Hi speed mode.
> Other than trying to isolate it from the chassis with some rubber
> grommets (or is there a better way to shock mount the fan), is there a
> more modern replacement fan that runs quieter and has the same or
> greater air flow? Are there any other cooling modifications that would
> be desirable to perform?
An EBM Papst Model 4600X fan is much quieter and all you hear is air noise
which still seems to annoy some.
> 2. Even though it might not be necessary, I want to beef up the power
> supply board. I plan on replacing the caps with 470uf / 450v
> replacements and replacing the diodes with 6A versions.
With the step start 470 is OK but you gain little or nothing; the circuit is
a FWB and not a doubler. I use 270uF from Ameriton. You gain absolutely no
value added with 6A10's in that amp which uses 1N5408's Save them for a full
> currently installed 1 meg ohm resister that are in parallel with the
> diodes necessary? Can or should they be removed.
They make no difference in or out.as the diodes were all from one source and
usually even the same batch.
I'm also replacing the
> cap bleeder resistors with Metal Oxide Film type resistors.
> 3. It appears the 10 meter mod is only half way installed. The input
> circuit appears to be there but there is no coil tap on the output tank
> coil. According to the manual on the BAMA web site, the tap should be on
> loop #6 as counted from the inductor end of the coil. That would put it
> 2 turns from the existing 15 meter tap and 3 loops from the end of the
> coil. Is this the correct location?
Yes, you may also have to move the switch detent. If you have to do that you
might as well add a 17M tap, just use the same 15M input by jumpering the
If you look at the input trimmer
> caps on the input board for the 10 meter position in my pictures, it
> looks kind of funky, Is this a normal install or does this need to be
If it works leave it alone.
> 4. Are the stock RF suppressors ok or would installing the "Richard
> Measures, AG6K - Low VHF-Q parasitic suppressor retrofit-kit" be a worth
> while investment for this amp?
Same as above and keep away from the voodoo science junk. My 500ZC
suppressors havent been touched since 1986.
Whenever I get an amp in for service the first thing I do is remove add on
junk. Then work on whatever problems there are.
> I have access to some parts from a LK-550 amp and have already installed
> the step-start circuit. Also considering installing the QSK board from
> that amp as well.
> Any other things that I should address or modify?
The RF lead to the top right of the front wafer can arc thru the nylon
washer beneath the ceramic spacer. Coat with your favorite HV goop and add a
Teflon sleeve over the wire and contact terminal.
They were a better than average amp in their price range when new and still
today. Quality components were used where they matter.
> Thanks for any suggestions!
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