Fluke 845 AB Repair

[TheDefpom]

Part 1:


Part 2:
Coming soon...

[Dr. Frank]

I'm already very curious since months.
Hopefully, the oscillator transistors have survived w/o battery pack.
Frank

[TheDefpom]

Yes it all still works fine, Part 2 includes me replacing the missing battery pack.

[srb1954]

I'm already very curious since months.
Hopefully, the oscillator transistors have survived w/o battery pack.
Frank

For the 845AB battery version they seem to be relying on the battery pack to clamp the PSU voltage to the circuitry to safe levels. Without the battery pack that supply voltage could rise to higher than desirable levels.

For the 845A version, without the battery, there is a 10V Zener across the supply to clamp the voltage to safe levels. It might be advisable to add that Zener into circuit.

[srb1954]

You don't get an earth connection to the internal metal case as it is isolated from the main case and is connected to the guard terminal.

The 2 transistors on the extreme right side of the circuit diagram make up a synchronous rectifier to provide a transformer isolated DC O/P for an external recorder.

[Dr. Frank]

I'm already very curious since months.
Hopefully, the oscillator transistors have survived w/o battery pack.
Frank

For the 845AB battery version they seem to be relying on the battery pack to clamp the PSU voltage to the circuitry to safe levels. Without the battery pack that supply voltage could rise to higher than desirable levels.

For the 845A version, without the battery, there is a 10V Zener across the supply to clamp the voltage to safe levels. It might be advisable to add that Zener into circuit.

The battery pack acts like a zener diode, that's what I already have written @ YT on the first video, as well that w/o battery pack the oscillator transistors could be damaged by overvoltage.. the output voltage of the transformer is quite high.

It would definitely be a big mistake to add the 10V zener diode, CR 207 as indicated in the AB manual, as this would completely disturb the charging of the battery.

In my 845AR, I have replaced this crude zener stabilization (R202, CR207) by a proper LM317 stabilizer circuit.

Frank