TECHNICAL SUPPLEMENT FOR THE SR-160 & SR-500 by WDØGOF August 2010
14
VFO CONSIDERATIONS continued
Use metal or carbon film 1% or 2% for R97. There is not much you can do to improve the resistor network,
R98, R99 and R100. There is not much point in replacing R100 with a more stable type. The pots R98 and R99
with an equivalent resistance of 12.5K swamp out any effect caused by R100. The pots should be cleaned and lubed
with a suitable control lubricating/cleaning spray. If R98 and, or R99 have been replaced you should check to insure
they are of equal value and have the same taper. Differences in these two pots will effect the operation of the RIT.
Cleaning of the relay contacts and S3 is also important.
The addition of a zener and replacement of R97 is highly recommended for improving the operation of the
SR-160. The same process is used to determine the value of components on the 160 as used on the 500.
RIT OFFSET AND RANGE TEST
To test the range of the Cal/Rit circuit turn the RIT off. Set the CAL control to the center of its rotation. Set
the band switch to 80 meters and the frequency dial to 3.8Mhz. Connect a frequency counter to pin 1 of V2. Fine
tune the frequency dial for 9.000Mhz on the frequency counter (If the dial is off do not be concerned at this time,
tracking and alignment of the VFO later will correct for any error). Now rotate the CAL control and note the
frequency shift in both directions. It should shift + and – 4Khz or a total shift of 8Khz.
If the total shift is more than 8Khz then the voltage at the top of R98 and R99 is too high. If you do not get enough
shift the voltage is too low. Nominally the voltage should be from 9 to 11 volts. If the frequency shift is not equal in
both directions from the mechanical center of rotation then R98 is at fault.
Now return the CAL control to the mechanical center and readjust the main tuning for 9.000Mhz. Turn the
RIT control ON. Adjust RIT HI/LO control for 9.000Mhz. The index line on the HI/LO knob should be within 15
o
of top dead center. If not then you may have a problem with R99.
NOTE:
R98 and R99 are not linier taper pots. I do not know why Hallicrafters used audio taper pots in this
application. The result is that the shift is not centered. You will see about 3Mhz to the low side and 5Mhz shift to
the high side. (This will vary depending upon the level of voltage at the top of R98 and R99.)
To test the divider network, disconnect the wire to the wiper of each pot. Disconnect the wire to the top of
R99. Set both pots to the mechanical center of their rotations. Measure the resistance from the top of each pot to the
top of R100 to insure they are both 25k pots. Measure the resistance from the wiper of both pots to the top of R100,
you should read very close to 9K. If they are not within a couple hundred ohms of each other then that will explain
the offset between the CAL and the RIT controls. At this point you must determine if the offset is offensive enough
to warrant the trouble finding matched pots that will fit and the work involved in making the repair.
VFO CONSIDERATIONS
The VFO in the SR-160 and the SR-500 are identical.
IMPORTANT NOTE FOR ALIGNMENT OF THE VFO: The CAL control shifts the VFO for TX and RX
when the RIT is off. The CAL control shifts the TX only when the RIT is on. With the RIT on the RIT control
shifts the RX.
It is important
to insure that the CAL and RIT pots are in the center of their rotation during all
alignment and tracking operations. You can test proper alignment by setting both controls to center. Then tune in a
signal and switch the RIT on and off. There should be no change in the receiver tone.
