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.
Since it's a standard 80mm fan you can replace it with any other standard fan of that size with a matching air throughput.
And there are plenty outside there -I chose a
noise silence optimized maglev one and mounted it with rubber anti vibration mounts.
Front side, feeded in 10MHz from my RTB 2004:
![](https://i.imgur.com/AnjtAlA.jpg)
[..]
Oh, quite good for that very basic (standard) quartz oscillator.
Now you have a (uncalibrated) thermometer.
![Wink ;)](https://www.eevblog.com/forum/Smileys/default/wink.gif)
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)
[..]
Couldn't imagine that. The previous owner was kind of pragmatic, wasn't he..?
![Laughing :-DD](https://www.eevblog.com/forum/Smileys/default/smiley_laughing.gif)
This is a funny bodge, no idea if this was done by Fluke or the previous owner.
![](https://i.imgur.com/Vh2141P.jpg)
That's interesting! I'm curious what signal is on that connector. Have you found out yet?
And next to it sits the 100MHz multiplier:
![](https://i.imgur.com/mCdcP3q.jpg)
[..]
Yep, that's the one that adds the bit of extra resolution to the PM6681 over the PM6680/PM6680B (and also the PM6685).
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.
Hope you've enjoyed this little teardown.
Thanks, I did, though the inside of my PM6681 is just a Torx screwdriver away. I'm sooo lazy..
![Cool 8)](https://www.eevblog.com/forum/Smileys/default/cool.gif)
Did you find a spare mask for the channel C Option inside your unit? I think one of mine has one inside, not sure.
Go for a GPSDO straight away, save the money of an OCXO upgrade. You have to warm the OCXO every time you want to use the counter (unless you don't care for the standby power). With one GPSDO + distribution 24/7 you can every instrument syntonized with from power-on.
![Evil >:D](https://www.eevblog.com/forum/Smileys/default/evil.gif)
Since you already ordere a replacement for the filter IEC inlet I won't say anything more but that I still have to order five for all my Philips/Fluke counters (2xPM6685, 1xPM6680, 2xPM6681).
![Scared :scared:](https://www.eevblog.com/forum/Smileys/default/scared.gif)