[R-390] R 390A range of RF gain

[Roger Ruszkowski]

Nelson,
 
The behavior you describe is about right.
 
Pretty much the RF gain is run at full gain and you back off
the audio as needed.
 
 Again the design guys back when were trying to use stock parts already in the logistic support chain
to avoid the cost of adding new parts and increase the use of parts in stock to generate demand
and thus drive down the expense of production by increasing volume.

Many of the receiver parts are not exactly the optimum value, but they work.

If you want to spread your RF gain out, consider padding a 1 K resistor across the 5K pot.
The two resistors will be in parallel. The value will always be smaller than 1K .

The cathode would then run from 0 volts max gain to maybe + 5 volts minimum gain.
If that value does not give you complete cut off then up the padding resistor.

I read that back in the days, that small value (500 - 2000) ohm pots did not hold up well.
The chemical mix needed for the range of resistance 
and spread over the available surface area in the pot just did not hold up.

We see the poor adjusting performance of the IF gain pot as an example of how
non responsive the small value pots are / were. 

So your RF is good as is.
Placing a small value pot in the RF gain will likely lead to not so smooth RF gain change as the pot is adjusted.

Roger AI4NI
 
 
 
-----Original Message-----
From: ngietz <ngietz at mymts.net>
To: R390 <r-390 at mailman.qth.net>
Sent: Wed, Mar 11, 2015 11:59 am
Subject: [R-390] R 390A range of RF gain


Still working on this 1963 Teledyne radio, and since it’s my first 390A, I’m
not
totally familiar with it.  So... how much of a control range should I expect
on
the RF gain control?  I get nothing until I dial it up to about 6 on the
scale.  I
tried subbing the original (I believe) with a nice Allen Bradley 5K
pot and it
seems to operate about the same.  Is this typical?
    The pot swings
the cathode bias measured at the pot from 0V (max gain) to 
+21.5 (min gain). 
Does that sound right?
   Nelson