[Johnson] Desk Kilowatt meter

[Steve H]

Since you seem to have confirmed the meter is good with a seperate DC test,
and replaced the switch, is it possible RF could be getting into the 
movement
via degraded RF decoupling caps or series coils?

My Thunderbolt has series L-C to ground networks placed strategicly on 
indicator lamps,
the caps are all 0.005uF ceramic discs. My copy of the Desk KW is kinda 
blurry but I dont see this kind of
filtering on M300 which I think is the plate current meter?

If indeed there is this kind of RF decoupling in there in some form, may 
want to check the solder and or ground connections
on those points.

Also if yours has the same neutralizing cap the thunderbolt uses I would 
inspect that for dust or buildup between the
electrodes. Mine was filthy and caused random parasitic spikes that took out 
some of the decoupling caps near the line plug.

The Thunderbolt has some similaraties in design and uses some identical 
parts I think.

--
Steve Hearns [ KA2PTE ]

----- Original Message ----- 
From: "Robert Nickels" <ranickels at gmail.com>
To: "JOHNSON LIST" <johnson at mailman.qth.net>
Sent: Friday, March 04, 2022 12:56 PM
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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