On Jan 1, 8:01*am, "Wild_Bill" wrote:
This model is a factory assembled model, and wasn't available as a kit,
Dual counter A-B, single 8-digit LED display (7-segment LED green or red
Input A 5Hz-175MHz
Input B 5Hz-2MHz
The Input B doesn't show a count when a signal is applied.
The Input A section seems to operate as it should.
I've seen that the actual circuit section for Input B does have an output
signal which is connected to the main board at J103.
I can also see that the counter/display IC is getting good signals at the
correct pins.. input A and B, and control.
Some repairs were made by previous owner, it seems, such as replacement of
the original Frequency Counter/Timer display driver U20, with an ICM7226AIJL
(high reliability ceramic).
I first assumed that someone may have applied over-voltage(s) to the input
channels, and smoked the counter/display driver IC, but I've seen random
numbers displayed as I probed around the area of IC18 and the counter/driver
IC, which indicates that the unit can display readings for Input B (no
signal applied to Input A), but the displayed count is incorrect and
An Intersil ICM7226A/B application/datasheet was included with the counter
unit, and found to be installed in the unit.
The ICM7226 is capable of 2 inputs A & B, and up to 100MHz as a
multi-function (frequency/period) counter and 8 digit display driver.
The the counter/driver IC, and a circuit IC U18 CD74HCT86E (quad 2-input
Exclusive-OR), and possibly one other support IC may have been replaced.
There was evidence of what appears to be probe-skid-type, or tool scratches
on the bottom of the board in the area of the input circuitry section of
The majority of the ICs are Hitachi, and a couple are Motorola and RCA
A third BNC input connector located on the rear panel is labeled TOTALIZE..
Another aspect that I'm unsure of is one of the functions.. there is a
selection of FR as one of the unit's functions, and since there is already
one marked FREQ, I think FR represents something else.
The other functions a
TI (time interval?)
CHK (check display and indicators)
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.