[Amps] LK-500 ZB

[Jim Thomson]

Date: Tue, 14 Apr 2015 10:40:18 -0400
From: "Carl" <km1h at jeremy.qozzy.com>
To: "Jim W7RY" <w7ry at centurytel.net>, "mike stokes"
<mike9v at gmail.com>, "Amps" <amps at contesting.com>
Subject: Re: [Amps] LK-500 ZB

A shorted tube wouldnt be drawing positive plate current.

Sounds like a gas arc took out the zener and the amp is now zero biased. 
Look for a glow in the tubes when in standby mode, maybe all they need is a 
good regettering due to a long period of  no use.
I wouldnt advise using it until the problem(s) are fixed.

The PS board is fine and MFJ has 270uF replacements in their catalog as used 
in their Ameritron amps. Id also replace any equalizing resistors that are 
out of tolerance.

Carl
KM1H

##  if the zener is replaced with a string of 6A10s, its very unlikely  you will ever
short them out under fault conditions, esp if a 50 ohm glitch resistor is used in series
with the B+ lead.   If a HV fuse is place in series with the B+, and prior to the glitch,  
the entire...event.... will be over even quicker. 

##  I have had a few zeners short out over the years, pita. Zeners wont handle a SURGE, like
a 6A10 will, (400A surge rating).  Zeners are fixed voltage, no way to adjust the bias voltage. 
If you really want to go crazy, parallel 2-4 6A10s for each bias diode... to increase the fault rating
through the roof.   usually this is not required.   However the place to parallel  2-3  x 6A10s, is 
between the  chassis.....and negative terminal (cold end) of the HV caps. 

Jim  VE7RF