Modified 2m Dymar Lynx 2000 - Advice Needed

[__i4cy__]

At a recent rally I acquired a Dymar Lynx 2000 (M2080A) transceiver already modified for standard 2m, 25kHz channel spacing amateur use. The rig seems to work fine, though I would like to reprogram the EPROMs to use some of the preset channels that are not currently used to other specific 2m amateur band frequencies. I believe the unused channels are 24-39, 60-69, 78-89, 98 and 99.

I have managed to reverse engineer the M2080A frequency synthesiser by referring to its schematic diagram, the chip datasheets and some PROM listings that I managed to find online. Having tried to calculate the channel 00 (zero-zero) RX frequency which should be 145.6MHz from the EPROM data, but I get a wrong answer.

Here's my working out

The SP8793 dual modulus prescaler is divide by 40/41 so:
M = 40

Since the reference crystal is 6.4MHz, and RA0, RA1 and RA2 are tied high. The MC145152 frequency synthesiser reference frequency ( Fr) is:
Fr = 6.4MHz / 2048 = 3.125kHz

Referring to the TMS2516 EPROM S3F listing files, channel 00 setting is stored at EPROM address 0300h, which outputs data bytes for:
EPROM-A = 86h
EPROM-B = D8h

Combining these two bytes (86D8h) as a 16bit word in binary bits gives:
1000011011011000

The lower 6 bits are connected to the A counter, and the upper 10 bits are connected to the N counter.
A = 011000 = 24
N =1000011011 = 539

Calculating the output frequency
Fo = Fr (MN + A)
Fo = 3.125 ((40 . 539) + 24) = 67.45 MHz (BUT SHOULD be 145.6MHz !!!)

Either there is an error in my calculations or the information I have is incorrect. Any help would be appreciated. Thanks.

[Jon_G4MDC]

67.45x2 +10.7=145.6?

I think you missed the IF frequency, why the need for x2 I don't know...

Maybe the ref is 6.25kHz not 3.125 - this would be a logical minimum channel spacing.

If RA0-2 are all high ref divider is 8192? RA2 high RA1,RA0 low = divide 1024 then it all works.

[__i4cy__]

Thanks Jon. It looks like you're most probably right, as you do simply get the number I was trying to calculate (145.6) by multiplying by 2 and adding the IF. Perhaps the crystal is oscillating at its second harmonic. Anyway at least I feel 99% confident that the EPROM data values are right.

73.

[Jon_G4MDC]

No problems. The EEProm will have the LO frequency as program data not final RX frequency and low side injection is common on VHF High band.

Why the x2 error is a small mystery. I think I would check the actual states on the pins RA2-RA0 with a multimeter. A reference frequency of only 3.125kHz on an FM set which needs modulation bandwidth almost that high would be nigh-on impossible.

[__i4cy__]

To conclude after reprogramming the EPROMs, I found that...

Programming 86h into EPROM A and D8h into EPROM B at address 300h will set the RX frequency to 145.6MHz on channel 00.

In the case of TX as the 10.7MHz IF is not included as part of the calculation. Programming 91h into EPROM A and 90h into EPROM B at address 700h will set the TX frequency to 145.6MHz on channel 00.