[Hallicrafters] SR SERIES RIT/CAL DISCUSSION

[Walt Cates]

I had another question today that has popped up several times in the last few months so I thought I would post my reply.

QUESTION: Why is my RIT CONTROL off center and different for each band. 

The CAL ADJ and the RIT CONTROL are used to fine tune the bias voltage on the vari-cap (also referred to as a veractor diode) in the VFO. With the RIT off both the TX and RX are fine tuned by the CAL ADJ pot. With the RIT on the RIT CONTROL fine tunes the RX and the CAL ADJ controls the TX. These two pots are in parallel in the vari-cap voltage divider network. On a perfectly aligned band when the CAL ADJ pot is in its electrical center position then the RIT CONTROL will also be in the center of its rotation.

 

If all the oscillators in the rig are dead on then the RIT CONTROL and the CAL ADJ will run in the electrical center of their rotation. But we have a problem with the heterodyne oscillator. Each band has its own xtal and there is no way to adjust individual xtals so the oscillator will run exactly on frequency. In the old days when xtals were cheap and readily available we would select xtals to get them as close to on freq. as possible. Now, I am lucky to have 1 or 2 spares. Spec says +/- 500HZ. 

 

What this means is that we can have as much as 1000HZ difference from band to band (if we are in spec). This is OK, because with the CAL ADJ we can bring the rig dead on as we go from band to band. However if we use the CAL ADJ pot to calibrate a band and we use the RIT the RIT CONTROL pot will need to be adjusted an equal amount off electrical center to match the new CAL ADJ setting. This is normal and it is common for the RIT to be a little off top dead center.

 

So here is what you do to insure you are on freq on a band using a SR-400. ( other models in the SR series use 100khz calibrator and a different switch to turn the calibrator on and off). With the RIT off turn on the CAL ADJ (pull out on the CAL knob). Tune the VFO dial to the nearest 25khz point to where you intend to operate. That is if you are going to operate at 7.240 (west coast swap net) adjust the dial to 7.225 or 7.250 exactly on the dial. Adjust the CAL ADJ for a zero beat ( I actually like to go for about 500hz tone). Turn on the RIT and adjust the RIT CONTROL for the same zero beat or tone. Now flip the RIT ON/OFF switch back and forth while adjusting the RIT CONTROL until there is no difference in the tone when switched. Ok now, assuming the VFO is in proper alignment, you are calibrated for that entire band and ready to operate.

 

Bottom line is, if your RIT CONTROL is top dead center on each band you have an exceptional rig and all your heterodyne xtals are dead on. If it isn't you have a normal rig.

 

73, Walt, WD0GOF


Best Regards, Walt Cates, WD0GOF, http://www.myhamshack.com/WD0GOF/<http://www.myhamshack.com/WD0GOF/>
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