Thanks Neil.. my suspicion is as you suggested, as I've seen waveforms
present at the 2 inputs of the counter IC with signal generator inputs at
Input A & B, and that perhaps one of a couple of other ICs may be the
culprit.

In numerical order, just before the ICM7226 counter IC there a
U18 quad 2-input Exclusive-OR
U19 HD74LS74AP dual D-type positive edge triggered F-flops with reset and
clear

Of course, there are numerous other signals to the counter IC that are
essential for proper operation, where I believe a schematic would be most
helpful.
There is a Test Point pin TP1 beside U18, for example.

The only obvious non-factory soldering is around the input area of the Input
A circuit components, where it appears that some resistor leads were lifted
to check their values.
The soldering under the sockets for the 2 newer ICs (U18 and counter IC U20)
looks identical to the rest of the wave-soldered factory assembly
connections, although oddly enough those 2 ICs are the only ones with
sockets.

FWIW, I wouldn't claim that it's not possible for me to be fooled, but I've
seen a lot of soldering and hand soldering generally always has certain
characteristics that differ from factory connections, such as inconsistent
amounts of solder applied to some pads and several other traits.

--
Cheers,
WB
..............


"nesesu" wrote in message
...

My incination would be to closely examine the soldering, both rework
and original. If that looks good, then apply the same signal of about
1 kHz sine to both inputs and compare the signal at the A and B input
pins on the counter chip with a good 'scope. Vary the input level and
see that the processed signals remain identical. If the signals are
identical, then the two input channels are probably okay and the
problem lies in the the control circuitry or the counter chip. Since
you see operation when probing, that suggested the counter chip is
good, but thre is a fault in it's control due to some sort of poor
connection such as solder, a dirty switch as N. Cook suggests, or even
'lead rot' on one or more DIP packages.

Neil S.