{Collins} 651S-1 issue



My 651S-1 (S/N 2308, red LED display) has an odd tuning problem and I thought I'd see if this might by chance sound familiar to anyone.    The 10 Mhz, 1Mhz, 100Khz, and 10Khz tuning steps work as expected, but when you tune in 100Hz steps using the main tuning dial, there is an odd jump in frequency.   For example:

Display       Actual
8000.0     8000.0
8000.1     8000.1
8000.2     8000.2
8000.3     8000.3
8000.4     8040.0
8000.5     8040.1
8000.6     8040.2
8000.7     8040.3
8000.8     8080.0
8000.9     8080.1

As you continue to tune, the same sequence repeats - I've used 8 Mhz as an example but the same jumps in frequency occur through the tuning range.  Sensitivity and everything else seems normal, in fact you can tune to any desired frequency if you do the math to correct for these weird offsets.    There is clearly a pattern here but I haven't yet been able to correlate what is going on to a particular circuit, other than it seems that bits are being added.

Since the display shows the correct frequency steps up and down as the encoder changes, that indicates that the correct BCD data is being presented to the synthesizer and suggests a problem somewhere in the A10, A11, A12, or A13 boards.    I've done basic things like re-seating the boards and checking for damaged components but everything looks fine.   Since these boards are full of flat-pack ICs made by Motorola, Raytheon, and Sylvania the chances of doing board-level repair are somewhat daunting.

So I wonder if there might be any sources for spare boards either to borrow for substitution or to purchase?   Or, if I'm able to identify a particular IC, is there any hope of obtaining something like a Sylvania SM153 frequency divider IC?

Thanks and 73,
Bob W9RAN





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