[Boatanchors] AR-88 Question

[Richard Knoppow]

----- Original Message ----- 
From: "mac" <w7qho at aol.com>
To: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
Cc: <boatanchors at mailman.qth.net>
Sent: Sunday, December 23, 2012 10:51 PM
Subject: Re: [Boatanchors] AR-88 Question


> Richard,
>
> The manual for my RDQ/AR-88D says that "if and when" 
> meters become  available they can be connected up to a 
> pair of wires already included  in the cable wiring.  The 
> meter is described as having a 5 ma movement  with zero 
> deflection to the right.  No further information is 
> provided.  Examination of the schematic and the parts list 
> shows a 100  ohm  resistor in the cathode of the 1st IF 
> tube but no wiring running  off and no meter.  Also, no 
> cathode connection over to the second IF  tube.
>
> My AR-88 came with a Hamerlund meter home-brewed into 
> place.  Never  have dug out just how it's hooked up but it 
> zeros to the left and  works OK.
>
> Dennis D.  W7QHO
> Glendale, CA
>
  I have collected a fair amount of literature on the
AR-88 series, thanks largely to folks on this list but also
to a fellow on a typewriter list.  A couple of the books
show the meter connected.  Also, there is a version of the
RCA instruction sheet for the meter kit.  Not many sets had
factory installed meters and none of the particular model I
have, the F, because it was meant to be used in a diversity
reception system where signal strength was monitored by a
separate piece of equipment.  The wiring for the meter is
present in the F but is used to connect a gain trimming pot
connected to the cathodes of the first two IF stages.
     I was able to obtain an original RCA meter.  The
measured constants are:
Full Scale current...5 milliamps
Full Scale voltage...30 millivolts
Internal resistance..6 Ohms

     The diagram for the meter in the early AR-88 books, the
CR-88 book, and in the RCA meter kit instructions show the
meter connected from the 100 ohm cathode resistor in the
first IF stage to ground with a 100 ohm resistor connected
across it to adjust the zero.
     I think this wiring is in error.  If you calculate the
division of current and the current needed to operate the
meter such a connection requires a resistance of around 3
ohms across the meter. After puzzling about this for a time
I figured out that probably the 100 ohm pot is supposed to
go from the ground end of the 100 ohm cathode resistor to
ground and the meter from its wiper to ground. The 6SG7
draws about 12 milliampers of cathode current when biased
for full amplification (calculated from data in the RCA tube
handbook and verified by measurement).  So, if the pot is
supposed to be wired as I think the drop across it would be
something over one volt. Obtaining 30 millivolts from that
would still put it near one end but would work. Also, since
the meter resistance is only 6 ohms the drop through the pot
is greater than calculating for an open circuit at the
wiper.  On the F type the ground end of the cathode
resistors for the first and second IF tubes are connected
together to go to the gain trim pot so the current is about
double that in a standard set. My experiment used a 25 ohm
pot in place of the 100 ohm meter pot and that seems to work
well. I have read somewhere that some found the original
meter connection to produce a "scotch" meter and used this
arrangement to fix that problem. On my set the meter seems
to read about right but I have not checked it with a
calibrated signal generator yet since I am still doing other
restoration work.
    A note, the IF trim pot can be continued in use by just
wiring it ahead of the meter.
    I was hoping others with sets with factory meters could
confirm my suspicion that the diagrams are wrong.


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