From: Eric Webner 
Subject: Re: [drakelist] R4C xtal bank bad

"calibrate" the metering circuit at 100 mA. These meters only cost 
about $1.25 and are NOT highly accurate, linear meters. So we make a 
"reference" point for the _important_ meter reading and use it as an 
"indicator" only for the far less important absolute plate current 
reading at full power.

73, Garey - K4OAH
Atlanta


		
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Garey,
 
I feel that I've got a good handle on this radio, and thank you
for your support. Once I get a 3.3 ohm resistor in there, however, and
assuming the meter series resistor no longer sets up an accurate 100 ma
reading, is there a way to calculate the proper resistance, or is it just
easier to wire in a pot temporarily to figure out the correct
value?
 
73,
 
Eric KA8FANGarey Barrell
<k4oah{at}mindspring.com> wrote:
Eric
Webner wrote:> Garey,>
> Several weeks ago when I discovered the play in one of
the bandswitch > wafers, I set the wafer where it should
be and used adhesive on the > inner wafer and shaft. It
appears to work very well, and with no > binding. I
wouldn't recommend that for all the wafers, but just one >
seemed ok.> > Regarding my low output, I
may be ok. First of all, I was not loading > it to full
power, since one of the errata sheets that Drake put out >
said to load it for only 340 mA in order to avoid any spurious output.
> After reading your latest message, I loaded 80 and 40
meters to full > power, obtaining 135-140 watts measured
on my Bird wattmeter. Loading > it to full power ran the
plate current to 500 mA!A plate current reading of 500
mA tells !
you that
 your 3.3 ohm meter shunt has gone up in value. Probably to about
5.6 ohms! :-)> > I also took
your suggestion and checked a few components. R44, the 680
> ohm resistor, is good. The cathode resistors are good
and very well > balanced. The screen resistors checked out
as well. However, the > resistor across the meter measured
5.6 ohms. I disconnected one end > from the circuit board
and the resistor snapped in half. I couldn't > come up
with another one, so I hope to try a 2.2 ohm unit (measures
> around 2.6) so I can get on the air this
weekend.That will "work", but you really need
to replace it with a 3.3 ohm unit. For "emergency"
purposes, you can measure the 2.6 as accurately as you can. then
adjust the bias voltage for 100 mA of current through that
resistor by measuring the voltage drop across said resistor as
accurately as possible. Then note the reading that gives you on
the p!
late
 meter. If it reads 50 mA (or whatever) that becomes your "bias
adjust" setting for the finals. Then just Tune and Load for
maximum output, using the plate meter to adjust for minimum plate
current.Then, when you get a "real" 3.3 ohm
resistor in there, again accurately measure 100 mA through that
resistor and pick a "select at test" resistor to make
your meter read 100 mA. I wouldn't worry too much about the
"340 mA limit", since the meter isn't that accurate anyway.
The only way to determine the "cleanest" operating
point is with a spectrum analyzer, and it just isn't that
critical. If you're driving a 5 KW amplifier then you need to
worry about it.> > Do you
recommend that I vary the other resistor in order to achieve
> accuracy on the meter, or can I play around with values
on the shunt > resistor (the 3.3 ohm
resistor)?> See
above.> One other 
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