[Johnson] Desk Kilowatt meter

[hamfish at comcast.net]

Bob,

Some where around 2007 one of those Desk KW's followed me home. The previous owner had it for 10 years and never go it on the air! All because of the flaky reading of the M100. A real pain in the backside to trouble shoot. Those meters are buried! After getting RF cubical out of the steel frame, all the perforated metal covers off, and the meter shield off..........................the solder on switch S103 looked like rabbit droppings. On mine R109 is a nichrome wire loop. Nichrome is difficult to solder. 

The things tried by previous owner blew out at least one 810 and most likely a 4-250A. His comments all pointed to the mod iron. 

Two weeks to trouble shoot/adjust by me, 1/2 hour to clean up the rabbit droppings. Ran fine until 2020, switch S104 failed. 

Check those nichrome solder connections.

Regards,
Craig

-----Original Message-----
From: johnson-bounces at mailman.qth.net <johnson-bounces at mailman.qth.net> On Behalf Of Robert Nickels
Sent: Friday, March 4, 2022 9:57 AM
To: JOHNSON LIST <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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