[R-390] The second 390A

[David Wise]

That 10M is at DC, but there's significant RF at the test point, and the stray capacitance of the usual kind of DMM probe will short it right out.  Heed Larry's suggestion to splice a 100K resistor into the probe tip to decouple the probe from the test point.  This is a useful trick anytime you're looking for DC amid RF.  The resistor will reduce the reading slightly, but you can fudge it out with the voltage divider equations.

These test points show DC because the mixer grid and cathode act as diode detectors.  (Acting in shunt with the signal.)  What you read on the meter is the mathematical average value of the half-wave-rectified RF.  Obviously if you short out the RF there won't be much DC left.

HTH,
Dave Wise
SWL in Hillsboro, Oregon
________________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of dog <agfa at hughes.net>
Sent: Thursday, July 18, 2019 6:50 AM
To: r-390 at mailman.qth.net
Subject: Re: [R-390] The second 390A

I've been using a Fluke 189 or an old HP427A FETVM, both 10MΩ or more
input. I'll have to do some further inspections and also get the SA/sig
gen going. A bit of signal tracing back from the IF input should be easy
enough to do. Not sure how much I'll get done today, but maybe I can get
the first one looked at about the high voltage on E209 also.