Today my Fluke / Philips PM6681 arrived. Including a nice ring file with the owner's manual and the programmer's manual.
I've powered it on first before I turned it apart. The vent is a bit loud, perhaps it has some issues with the bearing.
Front side, feeded in 10MHz from my RTB 2004:
![](https://i.imgur.com/AnjtAlA.jpg)
Back side, lots of dust but no bugs:
![](https://i.imgur.com/fooQWzu.jpg)
There was this mysterious plastic thing on the frontside. Turns out: this is a cable holder!
It sits directly on the spare hole for the C-input (prescaler 3GHz for example)
![](https://i.imgur.com/1ULLWRv.jpg)
Case opened, top view.
![](https://i.imgur.com/DEr0ddO.jpg)
Replaced CR2032 battery:
![](https://i.imgur.com/MOU0Qva.jpg)
The old CR2032 had only 3V left, I've measured the new one with 3.3V
![](https://i.imgur.com/NR84B8B.jpg)
Mains input with mains rectifier. No RIFA madness in sight.
![](https://i.imgur.com/wAxqzRi.jpg)
The shielded inputs A and B and a high sophisticated delay line.
![](https://i.imgur.com/tiC9VfP.jpg)
This is a funny bodge, no idea if this was done by Fluke or the previous owner.
![](https://i.imgur.com/Vh2141P.jpg)
Some multipliers next to this bodged oscillator:
![](https://i.imgur.com/mAE4a2A.jpg)
This is the 10MHz oscillator:
![](https://i.imgur.com/g4mNUWh.jpg)
And next to it sits the 100MHz multiplier:
![](https://i.imgur.com/mCdcP3q.jpg)
This is the bottom side, upper part:
![](https://i.imgur.com/A6FRIAf.jpg)
lower part with PSU:
![](https://i.imgur.com/44D5LYL.jpg)
Lots of wiggly lines:
![](https://i.imgur.com/EEp3Eww.jpg)
Manuals:
Operators manual:
![](https://i.imgur.com/a1edJxa.jpg)
Programming manual:
![](https://i.imgur.com/UVww1lR.jpg)
Next step is cleaning this buddy and checking the electrolytics with my incoming DER EE 5000 (should arrive by end of the month)
And then I'll add a 3GHz prescaler and a 10MHz OCXO to improve stability.
Edit:
Added the read-outs of the two AM27C010-70 EEPROMs.
V. 1.05 - Rev. 14
Edit 2:
Somebody found the Calver81 software which is needed to calibrate this counter:
https://www.eevblog.com/forum/repair/philips-fluke-pm6680-cnt-81-frequency-counter-firmware-and-cal-constants/msg3570350/#msg3570350Hope you've enjoyed this little teardown.
Cheers!