My 410B has a built in 22 megohm isolation resistor built into the
DC probe. My GR VTVM does not but I put a one meg resistor in series
with it. It has a 10 meg input resistance so the extra megohm does not
affect the voltage.
I am attaching an S-20R handbook to this post. Its my scan and
fairly decent, at least readable. The same scan is on BAMA along with
some others. It does not have a voltage or resistance chart. I hope it
is of some help to you.
I bought a supposedly rebuilt S-20R several years ago at a swap
meet. Worked but I found the rebuilder had left out the noise limiter
and some other stuff I don't remember. In any case, I did a thorough
rebuild using polypropylene caps and replaced several resistors. The set
does not have the original electro-dynamic speaker but has a good
quality PM speaker. The PM speaker can be used by replacing the speaker
field coil with a 1500 ohm resistor as is done in the S-40A. It is a
surprisingly good receiver. I have another on the back burner to be
restored.
FWIW I also have a Tek DVM. Same input resistance for DC, I
sometimes use an isolation resistor at the measuring point but the 410B
is more convenient.
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On 3/4/2025 5:38 PM, don Root wrote:
Richard ?do your DVM¡¯s have an isolation probe for capacitance
decoupling back to the DOT?
Resistance of DVMs can be? very high, but ?test wires have capacitance
that will ?mess with tuned circuits as the LO.? Halli sometimes
indicated which pins can be measured with only a VTVM , but the S-20R on
Bama has no chart and the schematic I see is mostly not readable.
**_
--
don??? va3drl
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
