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

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.
 
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)?
 
One other observation: my low power output could be attributed
to this meter shunt resistor being off, as that would cause me to back off
on the loading quite a bit. This is also why I didn't want to load it to
full power (500 ma???) for fear of melting a couple sweep tubes. It also
makes me wonder about my bias. Is there a voltage one should shoot for, or
is getting the bias (idling) current still the preferable approach? Do I
need to realign everything if I need to reset the bias?
 
OK, enough questions for now. Thanks for all the good
responses!
 
73,
 
Eric KA8FAN
Garey Barrell
<k4oah{at}mindspring.com> wrote:
Eric
-There isn't a "good" way to correct the
wafer "play" once it occurs, short of wafer
replacement. I have seen some with very thin Teflon? tape wrapped
around the shaft that seemed to work. I saw one where someone had
cut small pieces of brass shim stock, carved out the hole, and
epoxied them to the wafer. Inexpensive, but highly labor
intensive! :-)The Load control should be in
the 2.5 - 4.5 range for a non-reactive 50 ohm load. The Plate
control should be in the 3 - 8 range. As long as a
"dip" can be achieved, it doesn't really
matter.Your power output is very low. If you load for
maximum output, you should get 130 plus watts out on 80 - 20 in
Tune position, 150 -160 watts in CW. Power that low is most
likely from weak finals. With "new finals", you
probably have an alignment problem. If you have done t!
he
 alignment, check the 680 ohm resistor at R44, (located under T7,
just at the end of the final feedthrough panel under the
chassis,) especially if you did the alignment with a metal
screwdriver. Shorting the alignment caps to the chassis, hard not
to do with a metal screwdriver in those little holes, will damage
this resistor. With regard to the plate current, this is most
often a result of a meter shunt resistor that has gone up in
value. This is a 3.3 ohm resistor on a small board under the
chassis near the relay. The 'best" way to correct this is to
replace it with a 1W (the original is 1/2W) carbon film resistor.
At the same time, check the 15 ohm 1W cathode resistors for the
final tubes. They should be close to 15 ohms, (+/- 5%) and closer
to each other, (2%). Best replacements are Ohmite OY ceramic
composition, although the carbon film types seem to work fine. If
you haven't already, check the screen resistors for the!
 finals
 as well.After you replace the 3.3 ohm resistor, it is
best to measure it's actual resistance and the voltage drop
across it with an accurate DMM to determine the
"actual" plate current, then use this information to
verify that the Plate meter is measuring accurately at 100 mA.
The meter is not great, and not highly linear. The 100 mA point
is most important since that is what you use to set the bias for
the final. If it is NOT accurately reading at 100 mA, you will
have to reselect the resistor marked (select at test) which is
mounted on the slide switch linked to the meter switch between
the Plate and Load controls. It's usually around 200 ohms, but
can be anything from 47 to 360 ohms.The 50-123 knob
will work, but you'll be amazed at how it's "difference"
sticks out. Of course it will bother YOU much more than 
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