...and now for something completely different!
A new Test Equipment hit my bench: The Fluke PM6681 was delivered finally!
And for though I don't really need it and it wasn't advertised I'm lucky I got a unit with a high precision OCXO that has proved accuracy and stability by now. The instrument was also packed well in a double-layer cardboard package.
BUT: On arrival the instrument had a thermal issue: After powering it up it was working nearly normal only for completely showing rubbish after warming up for a while.
Even the "check" function that counts the internal 100MHz source with that as its own reference showed unreasonable values jumping around. On top of that vibrations made them dance even wilder.
What to do first? Contacting the seller to get (partial) refund? Or look inside and see if it's something serious?
I took the wrong decision. I got to my bench and cracked it open.
First thing I found out was that it's really a thermal issue: I took a hefty 120x120mm² fan and cooled the bare PCB. After a short while the instrument started to work nearly normal -means: stable and plausible- again! Removing the fan restarted the game.
The self test threw an error for ASIC No 2...
A quick look over the PCB (upside and bottom side) showed some earlier rework. Not bad but not really good: solder residue and thick solder points. The rest of the board had a (original) solder work that didn't really look good as it was oxidised.
I found the first cold solder joint after a few minutes. But resoldering it didn't heal the instruments behaviour. Oh no..
I studied the service manual and identified the ASICs and their surrounding circuits and after poking around confirming I was on the right way I began to rework all solder joints of the TH parts -I had seen that some of the parts legs were cut after soldering -bad decision, everyone knows now -but back when it was manufactured..?
I took my magnifying glass and began to check the whole board. A lot of solder points looked like they could be cold ones.
I reworked 2/3 of the questioned solder joints and the behaviour of the counter changed!
It was now stable unstable..
It was after 3 o'clock in the morning when I decided to catch some three ours of sleep before getting up for work again. Boy, I was frustrated at that point!
After work I immediately got home and went straight to my bench and finished reworking the rest of the TH solder joints.
I was sceptical if that would bring the counter back to normal life but I had a rest of hope.
So I plugged in mains power -fingers crossed- and observed the display..
It showed stable 100.000 000 0 MHz. I was relieved that it worked normal again and not stable unstable as before. But no thumbs up at that early point of time. I have waited for several ours now and the counter still works normal and stable.
It works even more stable than my PM6680 and PM6685!
I think I'll have to examine whether its due to the higher resolution of the PM 6681 or due to upcoming cold solder joints in the PM6680/6685 now..
But for the PM6681: Winner!
-in the first place. I gave away the chance of getting a refund and repair afterwards.
I'm curious if the seller will ask me for feedback and if he will reveal some knowledge of the counters faultiness
PM: I wrote "nearly normal" to describe the behaviour of the PM6681 counter: After reworking the TH solder joints of the analog part the counter worked even more stable and accurate than it did before in the first minutes warming up.
I'm away now to make up for the missing hours of sleep..