[R-390] Measuring Resistance Values

[David Wise]

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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