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 >> 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 > ______________________________________________________________ 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 ______________________________________________________________ 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
