Hi community
I recently started tinkering with old electronics to learn a bit about it. I've got a Hameg HM806-1 80MHz 2Ch scope off eBay, which I managed to get working, but I'm stuck with two things and decided to seek advice here:
1. the cal board
It's a HM205 CAL-02 PCB which is supposed to output 2V / .2V 1KHz / 1 MHz square wave. While I can get the amplitude right with adjusting the trim pot, the frequency is always wrong (too low, never 1 KHz or 1 MHz).
I tried replacing the 10uF capacitor which had a bit of a high ESR (I think?), cleaned contacts and sockets and tried swapping the ICs, but nothing helped.
What else could it be? I measured the resistors but I'm not sure about the results. They all seem ok except the low value ones, they're higher than what's on the schematic (R904 and R913 measured 57 Ohm instead of 51 Ohm, and R914 7,5 Ohm instead of 6,8 Ohm).
The VCC voltages are 2.65V for the 74HC132 and 2.85V for the 74HC00, which seems kind of low.
Attached is the schematic for reference.
2. calibration procedure
I can't find the calibration procedure for this scope. The only service document is here
https://www.sky-messtechnik.de/reparaturdienst-service/downloads (this company is now in charge of Hameg products, but they could only point me to this downloads page), but there is are no procedures, only schematics and a description of the pots. Without the procedure, I have no idea how to do it properly.
Thanks for your help