[Amps] LK-500 ZB problem

[Carl]

>I bought a used Amp supply LK-500ZB.  Met the ham on his way to Dayton so I
> wasn't able to check it out beforehand.   So of course it has a problem.
>
>
> This amp has the QSK board with a qsk switch on the back.  All the
> schematics I have seen have 5 relays on the qsk board  while this one only
> has 3 relays.

My late 500ZC has 4, 3 DIP and the vacuum, earlier ones had 5 and I cant 
recollect seeing a 3.


> There is no continuity from the input to the output when powered off.
> Therefore no receive signal passes trough. Also the transmitter goes to
> high swr.  I know at a minimum that the  vacuum relay is bad.
>
> I have the following questions:
>
> Are the vacuum relays high failure ?

Nope, Ive replaced mine once and later found out it was OK after I ran a bit 
of real RF thru the RX contacts, a common fault/fix.  It likely has more 
heavy contesting and DXing hours on it than most others.


> Anything that typically causes them to fail.

Wrong antenna, an open/short, or contact resistance since they werent meant 
to switch microvolt signals and didnt use a contact wiping motion.


> Any other thoughts as to what might be bad ?

The electrolytics on the QSK board plus the DIP relay at the board edge. Ive 
repaired a few that took out a diode and resistor on the same board.

> Does anybody have a schematic of the qsk board with only 3 relays ?

Here is the 4 relay.
http://bama.edebris.com/manuals/ampsuply/lk450-550>

Best place to buy a replacement vacuum relay ?

Lots on Fleabay, Max Gain when he has them, etc.

> Any other mods or thoughts while I am working on it ?

Replace the ancient filter caps plus relay PS cap on the same board, and 
change plate choke to the big Ameritron who also has the best price for the 
HV electrolytics. I use Mouser for other parts.
Change fixed padder cap values as needed for your 160 and 80M antennas.

Carl
KM1H


> Thanks in advance.
>
> -- 
> Mike Stokes
> KK9V
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