[Boatanchors] AR-88 Question

[Richard Knoppow]

     The diagram for adding an S meter to an AR-88, CR-88, 
etc., shows the meter paralleled with a 100 ohm 
potentiometer.  I was lucky enough to acquire an original 
RCA meter for my AR-88F. What I find is that the current to 
the meter is such that a 100 ohm pot is impossible. Now, I 
should explain that in the AR-88F two of the IF stages, 1 
and 2 have their cathodes connected to a 5K pot to trim the 
gain for use in a diversity assembly.  The meter, when added 
to these receivers, goes in the line from the combined 
cathodes.  Of course the current is double that in a 
non-diversity type receiver. However, that is not the source 
of the trouble. I estimate the current in a standard 
receiver to be on the order of 12 ma, this about the normal 
cathode current of a 6SG7. The _measured_ maximum current in 
the leads to the trimmer pot in my receiver is just about 24 
ma, which is about right for two tubes running at maximum 
gain.  The meter itself is a 5ma full-scale meter with a 
measured internal resistance of about 6 ohms. I've confirmed 
that its a 5ma meter. The full scale voltage is around 0.3 
volts.  Now, a little exercise with Ohm's law tells us that 
the parallel resistor for this meter with a _single_ tube 
should be about 3 to 4 ohms and for two tubes about 1.5 to 2 
ohms. I find I can zero the meter with a resistance of about 
1.5 ohms so the math is correct.
     What I suspect is that the value of the pot supplied 
for the meter is actually 10 ohms, not 100 ohms. A ten ohm 
pot would do OK where one wanted a value of around 3 ohms.
     I wonder if someone on the list with an AR-88 or other 
member of the family with a meter would please determine the 
value of the pot. It could be done by disconnecting one side 
of the meter and measuring the resistance across the leads. 
If I am right the 100 ohm value is a mis-print which was 
never caught and carried over to many versions of the 
handbook.
    OTOH, if I have missed something please tell me.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com