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 Post subject: Re: My R-390A refurbishment
PostPosted: Apr Mon 03, 2017 4:27 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1316
Location: Champaign IL 61822
I continued tinkering. I forgot that the AGC delay still exists. Its the 44 Megohm resistor.
Yes, that looks high, but with (see below) 2.8 Meg to ground on the AGC line that
is just fine. I used my 75 megohm multimeter probe and checked: it gets the Grid-cathode
voltage of the RF amplifier to within 0.25 volt of zero, and stays there for a small increase
in noise input. That's just fine for HF (below 35 MHz) use.

While the circuit I posted for the AGC worked just fine, I still found it not to
my taste. So I modified it as below. This removes the use of V506A as an AGC
amplifier entirely. I found that modern electrolytic caps work just fine on the AGC line.
So I rewired it to have switch-chosen values of 0.47, 2.7 (0.47 + 2.2), and 7.3 (0.47 + 6.8 )
uF. The latter cap is a high quality 50 volt electrolytic installed on the front panel.

The fast speed is good for AM or RTTY, the medium one for fast CW, and the slow one
for SSB and those sluggish Novice CW folks. It takes the capacitance in uF for a
-6 volt AGC signal (at V506A grid) to decay to -.25 volt, in seconds. Note that this is a
more or less linear decay, not exponential, because of that 44 Meg resistor.

It presumes that C535 is increased to about 68 pF. Optionally, but recommended,
decrease R526 from 27K to between 13K and 20K, or increase R527 to get the
same gain increase.

Also ... the limiter would be better with a linear 50K pot, but I've yet to
find a high quality period one with switch.

Attachment:
R390ModsBothBiasedWeb2.png
R390ModsBothBiasedWeb2.png [ 177.56 KiB | Viewed 85 times ]


Edit: change image to include additional bias on limiter so it cuts off completely.
2nd Edit: fix error in schematic


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