[R-390] Measuring Resistance Values

[Barry]

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: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of
> Barry <n4buq at knology.net>
> Sent: Monday, September 2, 2024 9:57 AM
> To: Bob Camp <kb8tq at n1k.org>
> Cc: R-390 Mailing List <r-390 at mailman.qth.net>
> 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 <kb8tq at n1k.org> 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 <n4buq at knology.net> 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
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