TECHNICAL SUPPLEMENT FOR THE SR-160 & SR-500 by WDØGOF August 2010
13
SR-160/500 DRIFT
CAL/RIT CIRCUIT CONSIDERATIONS
The SR-160 and the SR-500 have been plagued with drift problems as they have aged. The 160 had minor
drift problems from its inception. When the 500 came along an attempt was made to correct part of the problem
with the inclusion of a zener diode in the RIT/CAL circuitry. I have found two problem areas contributing to the
drift problem. The first is ageing of critical parts in the VFO and second is DC instability in the RIT circuitry.
SR-500 RIT
SR-160 RIT
The linier range for the 1N963A is from 0.1 ma to 2 ma. The original design of the zener ckt has the zener
drawing 2.2 ma. That puts the zener just above the knee of the flattest part of the curve. Under normal
circumstances this would still function properly. But this does explain why it takes longer for the 500 to stabilize
after turn on. If you operate in an uncontrolled environment such as field day in the fall, chances are, it’ll never
stabilize. If you stick with the 1N963A then R97 should be a 75K 1% or 2% resistor. If you go with a different
zener diode pick a current level between ½ and 2/3 up the flattest part of the curve. The zener c resistor
network current (0.7ma @ 12vzv) will be used to calculate the value of R97. [150V – V
z
/
(Iz + (Vz
/
15.2k)] or for
12 v zener ~ [ 138v/(I
z
+0.7ma) ]
There is nothing magical about 12v, that's just what they used on the SR-500. On the SR-160 the voltage at
the top of the resistor network is 19v. There are two things to be concerned about when changing the voltage at the
top of the network. First, the higher the voltage the wider the tuning range of the CAL and RIT controls. At first
that sounds like a good thing. However the wider the range the more unstable the vfo and the more difficult it is to
track the Rx and Tx. Note also any change in the voltage will require re-tracking and alignment of the VFO.
