[Central-Electronics] 20-A On Bench Issues

[Jim Candela]

Hi Group,

     A few weeks ago I took my main station QRO modified 20A offline so 
I could address some accumulating issues. This rig has been doing my 
main duty since about 2004, so that is pretty good for any boatanchor rig!

     So today, I under went surgery to replace the 9 Mhz carrier null 
pots since they were dirty such that I just could not hold the carrier 
down for any length of time. These by design are 1K 2W carbon pots, 
single turn. What I had in there was 500 ohm pots with a 470 in series 
with one side. Since the old 1K pots were not centered this approach 
worked, and the 500 ohm pots were pretty much centered after 
installation in 2004. Over time those pots got noisier and noisier, and 
being sealed units, they were not easily cleaned with a shot of solvent. 
Remember the bottles of carbon tetrachloride we used to use to clean 
pots? :-)

    So in my junk box was a pair of Bourns KnobPots. These were ten turn 
1K pots with a built in turns counter. Here is a link to them:

http://www.bourns.com/bu/bu11_13knobpot.html

I am using the type pictured on top. The knobs are the same shade of 
black, and only stick out 1/8" more than the other knobs, so the 
finished job looks like it could be stock. These are wirewound, so do 
they work? Absolutely!!

Without redoing the 9 mhz phase shifter settings, on 80m, LSB with 50 
watt tone (carrier nulled), my Icom R-75 was seeing S9 for the LSB. 
Switch TX to USB I get < S zero. So if Icom has 6DB/S-Unit, that means 
9X6=54db SB rejection. Switch RCVR to USB, and I have S9 again. Then 
switch TX to LSB, the Icom drops to S1. That means 8X6=48 db SB 
rejection. Not too bad, and the settings seem pretty stable.

This 20A was converted to a pair of 7591's replacing the 6AG7's. Any 
replacement for the 6A&7's must be high gain with low interelectrode 
capacity. If the plate to grid capacity rises to much, the final MUST be 
neutralized. I did that too on another rig, but that is a different 
story. At the time this was good only on 160-80m, and finicky on 40-20 
at reduced power output. Still on 80m I could peak 65w PEP out on SSB. 
Perfect for driving a 8877 linear to legal limit.

So over the last week or so, I overcame the finicky nature on 40-20m, 
and raised the power level on 160-80 to 70W carrier, and 80W PEP SSB. On 
40-20 I get 60W carrier. Playing on 75M AM, I can run 20W carrier with 
headroom for 100% upward modulation. At 25w carrier the positive peak 
begins compression at about 75% upward modulation. I am still using the 
same pair of "Fisher" branded 7591's as before. Compare this to a stock 
20A, where on 75M AM it is good for no more than 3-4W carrier and even 
there the upward peaks compress limiting upward modulation to around 75% 
when the negative peaks hit 100%.

My next mission is to document what I have done. I had a notebook with 
all my notes from 2003 - 2004. I think it got put in the trash with the 
newspapers. I have some things documented, but maybe only half. So I 
need to reverse engineer what I did for my own needs, or maybe anyone 
else who might be interested.

Jim
WD5JKO