Re: {Collins} 651S-1 issue

Well, here is what I think is happening, and I really know nothing about that receiver, BUT, thinking in the binary world (since I was a computer programmer for most of my working years), it looks like the (4) bit is missing.

What I mean is the first three bits in a computer are (reading right to left) are 1 followed by 2 and then 4.


001 = 1

010 = 2

011 = 3

100 = 4 - but you are missing the 4, - you see a 0

101 = 5 - you see a 1

110 = 6 - you see a 2

111 = 7 - you see a 3

If you go higher, 1001 would = 9 and then it all will repeat but you are seeing a 1.

So, somewhere you are missing the (4) bit and maybe above that for a while.

Again, I am "just" a computer programmer with a HUGE knowledge of electronics and radio but most of my work was programming most of my life.

I certainly hope this helps you to begin your trouble analysis.

Walt Cheatham - K7CCA

On 10/28/2017 7:04 PM, Robert Nickels via Collins wrote:
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,

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