[Boatanchors] AR-88 Question

[mac]

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

***********************

On Dec 23, 2012, at 8:05 PM, Richard Knoppow wrote:

>
> ----- 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
>
>
> ______________________________________________________________
> Boatanchors mailing list
> Home: http://mailman.qth.net/mailman/listinfo/boatanchors
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Boatanchors at mailman.qth.net
>
> List Administrator: Duane Fischer, W8DBF
> ** For Assistance: dfischer at usol.com **
>
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html