[R-390] Resistance Measurements and Trouble Shooting

[Jim Brannigan]

A few things to keep in mind.  The charts in the manual are representative
of all the radios and not unique to your particular radio....The type of
meter in use will affect the voltage readings.  A high impedance VTVM will
give more accurate readings than a VOM and some DMM's have low impedances.
Also, bear in mind that many of the components are +/- 20% of the stated
value.

Ideally, you would want to record the voltage and resistance measurements
when the radio is operating properly.  This would provide a unique reference
against a future problem.  Since many boatanchors are received in less than
"pristine" condition we have to make a lot of assumptions.  First, I check
the power supply voltages.  If the manual says that the plate supply should
be 300VDC and you measure 290VDC, than expect all the plate voltages to be
down 10%.

I create an Excel spreadsheet, in the same format as the voltage and
resistance tables.  The values (or adjusted values, as above) are entered
into the appropriate cells.  The measured values are entered below them.  In
adjacent cells I create a formula to "Test" the entered values.  I use "Plus
error" or "Minus error" messages as appropriate to indicate a value out of
my defined parameters.  This gives me a picture of the whole table and is
useful for spotting trouble areas.

Jim


> Here is a question from someone who is trying to learn this trouble
shooting
> business.
>
> After circuit resistance measurements have been made, at what point do you
> say that there is a problem and start checking individual components?  Is
it
> ( 5%, ( 10%, (20% of the value stated in the manual, or ?  I realize that
> the circuit under consideration makes a difference; some obviously have to
> have closer tolerances than others.
>
> The item under consideration is a Hallicrafters HT-32B transmitter.  I
don't
> think resistances in say the audio circuit would have to be as tight as
the
> balanced modulator, for instance.  Or am I wrong?
>
> What is a good place to start?
>
> David C. Hallam
> KC2JD