Finally, to my big surprise, it's a ICM7216D that runs the show...
This version of the chip functions only as a frequency counter.
I tested it, and was astonished by its precision: less than 0.1% error at 10 MHz with a 2 V p-p signal.... Incredible.
3 crystals on board: two of 10 MHz, and a third I couldn't read completely the frequency, but it was somewhere in the 3 MHz range
Other chips:
- 74LS04
- 74S196N
- MC10116P
- An 8-pin DIP chip on the VHF input, with markings stripped off (WHY
?
), but its designation seems to end with "40". Probably a VHF amp and divider ?
- 7805 regulated linear PS
I tested in the 10 MHz range and got a stable and precise readout until almost 12 MHz (2 V p-p). Quite impressive !
In the 60 MHz range, i was able to get the readout until 60 MHz, since my siggen cannot go higher than that.
All in all, for a couple of bucks, I think it is was a bargain !
Now, on with the hacks !
First (and maybe only) thing I would like to do is to protect the VHF input against too high input voltage > 3 V (as mentionned on the case).
Since I do not have the schematics nor the specs, I do not know at what extense this input is already protected or not....
The 8-pin chip mentionned abode is mounted on a small PCB directly soldered on the rear of the VHF input BNC. I will try to reverse-engineer it, but it will not be easy because of that soldering.
To get a first lead, do anyone know of a VHF amp/divider (maybe 1:6 or 1:2 followed by 1:3), in 8-DIP, dating the nineties, and whose designation ends with "40" ?