In retrospect, my concern over the internal resistance of the meter is a non-issue for measuring resistance. I was definitely not thinking clearly when I wrote that.
Thanks again, Barry - N4BUQ > When measuring resistance, VOMs usually make a circuit of battery-meter(with > shunt)-Rcal-Rx, so the current through the DUT depends on meter range and Rx > itself. Some models can pack quite a bit of current on the R-times-1 range, > enough to destroy some semiconductors and panel meters. DMMs usually push a > fixed current (which rarely exceeds 1mA) through Rx, and measure the voltage > resulting. The only semiconductors in the R-390(*) design are the Diversity > AGC > diode and the bridge rectifier powering the antenna relay. There are no > thermistors or other intentionally nonlinear resistances. In most cases, your > choice of resistance measuring equipment is moot. > Dave Wise > SWL (inactive) > From: [email protected] <[email protected]> on behalf > of > Barry <[email protected]> > Sent: Monday, September 2, 2024 9:57 AM > To: Bob Camp <[email protected]> > Cc: R-390 Mailing List <[email protected]> > Subject: Re: [R-390] Measuring Resistance Values > Hi Bob, > Actually I was checking both. It was the resistance values which were > extremely > high and I was trying to make sure it wasn’t because of the meter I was using > and I suppose it wasn’t. > When I get it reassembled, I can check voltages with something more > appropriate. > Thanks, > Barry > > On Sep 2, 2024, at 11:33 AM, Bob Camp <[email protected]> wrote: > > Hi > > Ok, I’m guessing we have a typo. >> We’re measuring voltage at the test points rather than resistances. Let’s >> see, > > I’ve probably only made about 10 typo’s in this message so far … :) :) >> Some of the old VTVM’s had pretty crazy input impedances. It would a bit >> unusual >> to find one that was above 10 Meg *and* something you would use in a radio >> with > > hundreds of volts running around in it. >> Something like a Simpson 260 would be a much lower input impedance device >> when >> set up for looking at test point voltages. Simple answer in this case, grab > > that Simpson and see what it reads. > > Bob > >> On Sep 2, 2024, at 10:31 AM, Barry <[email protected]> wrote: >>> Did Collins specify what input impedance a meter should be to properly read >>> the >>> resistance values in the manuals? I did see where a TS-352 was mentioned but > >> that's selectable between 20,000 ohms/volt and 1,000 ohms/volt. >>> I started checking the RF Deck in my R-390 and, starting with V201, I noted >>> a >>> few values that are significantly high. I was using a modern DMM with 10M >>> input >>> impedance so not sure if that could be the problem but I doubt it would >>> account > >> for that much difference but who knows. >>> The voltages were off a bit as well but not nearly as much as the resistance >>> values. I suspect the radio will still work with wacky voltages but the R > >> values really have me thinking I have work to do. > >> Thanks, > >> Barry - N4BUQ > >> ______________________________________________________________ > >> R-390 mailing list >>> Home: [ http://mailman.qth.net/mailman/listinfo/r-390 | > >> http://mailman.qth.net/mailman/listinfo/r-390 ] > >> Help: [ http://mailman.qth.net/mmfaq.htm | > >> http://mailman.qth.net/mmfaq.htm ] > >> Post: [ mailto:[email protected] | mailto:[email protected] ] > >> This list hosted by: [ http://www.qsl.net/ | http://www.qsl.net ] > >> Please help support this email list: [ http://www.qsl.net/donate.html | > http://www.qsl.net/donate.html ] > ______________________________________________________________ > R-390 mailing list > Home: [ http://mailman.qth.net/mailman/listinfo/r-390 | > http://mailman.qth.net/mailman/listinfo/r-390 ] > Help: [ http://mailman.qth.net/mmfaq.htm | http://mailman.qth.net/mmfaq.htm ] > Post: [ mailto:[email protected] | mailto:[email protected] ] > This list hosted by: [ http://www.qsl.net/ | http://www.qsl.net ] > Please help support this email list: [ http://www.qsl.net/donate.html | > http://www.qsl.net/donate.html ] ______________________________________________________________ R-390 mailing list Home: http://mailman.qth.net/mailman/listinfo/r-390 Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
