[Central-Electronics] 20-A On Bench Issues

[Jim Candela]

________________________________
From: k6jek <k6jek at comcast.net>
To: central-electronics at mailman.qth.net
Sent: Wednesday, October 19, 2011 10:29 AM
Subject: Re: [Central-Electronics] 20-A  On Bench Issues

Congratulations.

Those multi-turn Knobpots are an elegant solution but nothing that spiffy is bouncing around my junk box.

What frequency did you use for adjusting the opposite sideband suppression, 1225 as the manual suggests?
On Oct 15, 2011, at 10:33 AM, Jim Candela wrote:

Jon



Reply by Jim, WD5JKO:

  Well I was using 1 Khz. I don't have the 20B phase shifter, but I did some empirical diddling, and added an additional R-C to PS-1, and another resistor at one of the pots. As usual it goes out a little at 3 Khz, and then comes back in. So this extends the range up to 5 Khz. So from 150 hz to 5 khz a sine wave tone input makes a DC note all the way except I see a little ripple around 3 Khz.

  In fact last night I went through it all again since I changed my crystal from 8998 to 9006 Khz (CB xtal). I came up with a way to adjust the audio phase shifter that worked really slick. With mode switch in SB1, I basically sampled the RF to scope channel B, and the audio (from rear RCA jack - with RF bypass  1.2K and 2200 pf) to scope channel A. Put scope in X:Y mode, and run up the RF (using tone input - carrier nulled) to about 50% capability. Then adjust scope levels till you have a square box.
The top and bottom of the box will show curvature. Adjust the two audio pots until the curvature is minimized while switching to SB2 and back to SB1. Your done when you have a near perfect box, and is the same on both sidebands. Then run up the power, and then back down. The box size should change, but the shape should be a constant. DONE!

  Doing it the old way was easy on just SB1 or SB2. Getting them both correct was a mother that I seldom ever could do. The RCA phone jack audio output has a lot of RF (9Mhz) hence the need for the R-C low pass filter. 

Jim
WD5JKO