[Johnson] Desk Kilowatt meter
Rodger Singley
wq9nsc at live.com
Fri Mar 4 13:14:02 EST 2022
Bob,
Mine has never had an issue but I would suspect the movement itself may be hanging up. I would temporarily patch a different meter to rule out issues with the wiring/shunts/switch to see whether the problem is in the movement itself before you dig into it.
I restored my Desk KW around 16 years ago and since that time it has lost the grid compartment cooling fan motor and one crimped connection in the wiring developed a bad connection which wasn’t too bad since it was close to 60 years old at that point. I did add an external screen current meter that sits in a small case on the Desk KW, I really don’t like running a tetrode amp without screen metering.
Hope you figure it out without too much stress. I hate intermittents!
Rodger WQ9E
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From: Robert Nickels<mailto:ranickels at gmail.com>
Sent: Friday, March 4, 2022 12:07 PM
To: JOHNSON LIST<mailto:johnson at mailman.qth.net>
Subject: [Johnson] Desk Kilowatt meter
Has anyone experienced intermittent operation of the PA Grid/Modulator
current meter in the Desk Kilowatt?
I first became convinced that it was a flaky toggle switch so I tore it
all apart and carefully replaced it, with an identical NOS Arrow Hart
switch I had on hand. I checked it out using an adjustable current
source with the grid temporarily grounded since there's no way to do so
once it's all buttoned-up, and the meter worked perfectly and the
accuracy was good.
But after proceeding with checkout of the amplifier I still am seeing
very intermittent meter operation. I've rigged up a temporary current
measurement (by measuring the voltage drop across the appropriate bias
resistor) so I know when the proper drive is being applied, but
sometimes the grid meter shows zero, sometimes 20ma, and sometimes a
value in between, often > 5ma. Since the modulator plate current is
also intermittent this means the problem lies in the new switch or
wiring (both of which I think are unlikely) or the meter itself. One
difference is orientation, it was facing horizontally when I had it on
the bench but of course is vertical in actual use.
The amplifier works fine in all cases with proper plate current, power
output, and modulation, so this is strictly a metering issue.
I'd like it to be right, and if I go through the process of tearing it
down again I sure would like to find and fix whatever is going on, and
if that involves removing the meter from its housing it would be good to
know if others have been down that path.
Thanks and 73,
Bob W9RAN
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