[Collins] 30S-1 help

[Dr. Gerald N. Johnson]

On Sat, 2006-07-29 at 16:51 -0400, Phil LaMarche wrote:
> I am having a 30S-1 completely gone over by a local engineer ham.  All is
> good except the tuning to ZERO.  It originally slammed all the way left.
> Replaced a large number of components and now it is off ZERO but still won't
> tune to zero.  See note below.
>  
> Any ideas, Please?
>  
> Phil
>  

The tuning meter is comparing RF PA plate voltage to RF drive voltage.
There are two RF volt meters involving capacitive voltage dividers and
germanium diode (1N34) detectors. on CW, the ratios between the
detectors is set by C112, an 8 to 50 trimmer. Much like the troublesome
ceramic trimmer in S-line and other similar vintage Collins rigs if not
exactly the same part. The SSB adjustment is refined by R105, a variable
resistor.

If one is sure that the PA tube is not loosing gain at high power
(caused by limited cathode emission) one can adjust the capacitor, C112
for zero at full CW power output, then adjust the pot R105 on SSB. One
must be sure the diodes have similar characteristics such as forward
drop and forward resistance. It would have been simple to have replaced
the 1N34 by silicon schottky diodes but their higher threshold can be a
problem. I might try vintage 1N270 in place of the low generic quality
1N34 diodes. But work at matching. For this purpose matching their
rectification efficiency at 30 or 40 MHz would be a good match
procedure.

I'm slow to answer because I've been off to the Central States VHF
Society Conference in toasty Bloomington MN as well as a bit of vintage
trolley riding, surplus store searching, and antique engines and farm
equipment. Now I have to unload truck and trailer and go work on
spraying beans.


-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer