[Boatanchors] AR-88 Question

[Richard Knoppow]

----- Original Message ----- 
From: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
To: <boatanchors at mailman.qth.net>
Sent: Sunday, December 23, 2012 12:50 AM
Subject: [Boatanchors] AR-88 Question


     I think I found the answer. The diagrams for the meter
circuit are wrong. They all show the 100 ohm zero set as a
rheostat, that is a variable resistor placed across the
meter. This will NOT work. I have proven that to myself in
practice and by calculating the current and voltage applied
to the meter.  What I think is correct, and what works in
actual practice, is to use the variable resistor as a
potentiometer. One end goes to the ground end of the cathode
resistor of the first IF, the other end to ground. The meter
is then connected between the wiper and ground.  This works!
I tried this on my AR-88F which has both first and second IF
cathodes connected to a common point which normally goes to
a 5K rheostat to ground as a gain trimmer.  With the gain
trimmer disconnected the circuit works just as it does in
the receivers where only the first IF stage goes to the
meter.  The meter can be connected in series with the gain
trim pot if one wishes to keep it working.  Where the
receiver is used alone the gain trimmer does nothing useful.
It is necessary where two or more receivers are used for
diversity reception where its important to equalize the gain
of all receivers.





>     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