TECHNICAL SUPPLEMENT FOR THE SR-160 & SR-500 by WDØGOF August 2010
16
VFO CONTINUED
THE PROBLEM
Over the last 5 years every single SR-160 and SR-500 I have had in the shop has suffered from freq. drift. I
reviewed the repair files and eliminated the obvious random component failures. I found that a combination of at
least two of 4 specific components was involved in all restorations. These components are R113, R114, C129, and
C133. The exact nature of the failure mode is unknown. Whether it was crystallization of the carbon resistors
causing noise or dielectric breakdown generating drift really doesn’t matter at this point.
THE TEST
For this test I collected three rigs two SR-500’s and one SR-160 all of which demonstrated the drift
problems. I cleaned, lubed, aligned and cleared all other problems. When I pre-tested the three units I found roughly
the same for all three:
> After 30 minutes of warm up the drift was about 150 Hz per minute and random in direction.
> After another 30 minutes the drift trend was upward about 900 Hz per ½ Hr. with the short term drift of about 100
Hz per minute.
> After three hours it was +2200Hz and still going up, still had short term drift. All three rigs displayed similar
characteristics.
FIRST CORRECTION
I performed the RIT/CAL ckt changes on all three rigs. I went with 12 v zeners and 75K metal film 1%
resistors. Following the changes I re-ran the test. The short term random drift was gone. All three rigs were then
drifting in the upward direction from 10 to 30 HZ per minute and around 800 Hz to 1100HZ per hr.
SECOND CORRECTION
I replaced the 4 suspect components in all three rigs. CAUTION stay away from NOS parts. If you use NOS
parts you are getting old technology and outdated manufacturing processes and materials which were the root of the
problems. Better materials, technology and processes are available today therefore producing better products. Be
sure the C129 is an NPO cap and that C133 is N470.
Re-test proved that the continual upward drift had been eliminated. The specs on the radios states that after
30 minutes of warm-up no more than 300 Hz drift should occur. The spec is vague and no further time limits are
stated. I’m assuming that the intent is that once warm-up is achieved then the drift total +/- should not ever exceed
300 Hz.
One 500 and the 160 took 45 minutes to stabilize but from that point on the drift was from -20 Hz to +130
Hz for the 500. The 160 drift was from 0 Hz to +230 Hz. Both rigs remained within that range for 2 hrs at which
time I terminated the test. The other 500 stabilized in 15 minutes and the drift went from -110 Hz to +90 Hz.
SUMMATION
Due to the small population of the units tested this cannot be considered a definitive test with irrefutable
results. However due to the consistency of the results and the repairs made over the last 5 years I believe that it does
warrant a look here first approach when you encounter similar problems.
