[Drake] T-4XB rRealistic resistance measurements.

Sat, 26 Oct 2002 18:36:07 -0600

Are you measuring with the mode switch in the function switch in CW and the
Transceive switch in "Spot?"   If so, pin 1 of V3 is grounded through the 330K
and 150K resistors and S2 (in spot position.)   The only thing in parallel is a
68K resistor going to pin 2 of V4, but this should have no effect.    Of course,
with 330 K in series with 150K that would certainly add up to 480K that you are
getting, and would be correct.

However, the instructions say set the function switch to Tune for resistance
measurements.  That puts R9, a 1Meg resistor, off the plate of V1B, in parallel
with the 330/150 K resistors.   This is going to change the parallel resistance
of that circuit, dropping it down to approximately (I didn't bother to figure it
out!) the 330Kohms specified.  (Actually I did run a quick check rounded off on
the calculator and it comes out to 324K, which is pretty close to what Drake
suggests.)

Also, that same line goes to pin 3 of the power supply plug, which is the AGC
jack in the power supply.

So it sounds to me like the resistance measurement you are getting is correct,
but it should drop to some value less than the 480K of the two series resistors,
when the function switch is in Tune, with that 1 meg resistor in parallel.   All
measurements are recorded with the power supply connected but unplugged from the
AC line.

If you switch the function switch to Spot, you should get the 480K you are
getting.

Hope that helps.

73
Ed W5HTW

"Dan W. Dooley" wrote:

> Has anyone ever made accurate resistance and voltage readings on a T-4XB?
> Trying to troubleshoot some problems here and it is very difficult to do so
> when readings obtained do not and can not match what the manual says they
> should be.
>
> Case in point, the manual says the resistance reading from pin 1 of V3 to
> ground (in the chart on page 33) should be 330 K.  I get about 480 K.  Big
> difference!  The problem is, that in the actual circuit - and the schematic,
> is a 330 K resistor and in series with that is a 150 K resistor.  Nothing of
> resistance in parallel with them.  There is no way I'm going to see 330 K in
> that path as the book suggests.
>
> So if I can't believe that reading, what else can I not believe?  Can't even
> believe the voltage readings.  Sure makes troubleshooting difficult.
>
> Dan W. Dooley  WB5TKA  Bedford, Texas
>    e-mail to: [email protected]
>    Web address: http://www.qsl.net/wb9tka
> May Goddes love blest ye alle
>
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