[Collins] Regulating the 140 vdc supply line....correction 75S3 receiver

[Dr. Gerald N. Johnson]

On Wed, 2008-10-22 at 04:46 -0500, w9hak wrote:
> Has anyone gone through the trouble to regulate the +140 vdc line in the
> 75S3 receiver?  Shortly I will be in my receiver to replace the 56 volt
> zener for the variable bfo. Since I am in the neighborhood I thought it
> might be a good idea. Your thoughts please.
> 
> Smith Bradford
> W9HAK
> 
In the 32S-1, I found regulating the oscillator screen voltage gave
better frequency stability than regulating the plate voltage. I think
that would hold true in the 7SS-3 except the screens are already
regulated. At least in the S-3B/C the oscillator voltages are regulated.
Not the whole 140 volt bus. To allow for that the plate transformer
voltage has a higher output voltage. 290 volts end to end instead of 220
volts end to end in the S-1/2. I don't have data for the S-3/3A.

Years ago I found my S-3B would almost hold good enough stability for
300 baud packet all day, but not quite. Still for voice and CW it was
superb. I don't know that more regulation would make it better without
regulating the temperature of its environment too. The zener diode
regulator probably doesn't have the stability of a modern 3 terminal
regulator but there are few (maybe only one) that will work at 140
volts. Even then there will have to some voltage drop in the regulator
(it can only take off the peaks, it can't fill the dips) so if the whole
radio was regulated, the operating voltage would need to be a bit lower
and that may affect the performance, for sure the available audio
output.

The TL431C programmable zener diode can be extended with external
transistors to high voltage and has significantly better voltage
regulation compared to ordinary zener diodes but that's getting more
complex than there is room within the receiver. 

Another factor is that the heat of the regulator adds to the thermal
drift of the receiver.

I think that unless the receiver is wandering about aimlessly from
voltage variations that adding more regulation won't help it a whole
lot. It would be easier to use a Sola to regulate the line voltage
because of the need for the transformer change and the wiring change to
regulate only the oscillators as done in the S-3B/C.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer