Hi Coromonadalix,
I have the old service manual, the “new” one on Ebay is essentially an update though, and I have no idea what it includes. I will ask the seller. However, the manual is still 60 Euro delivered and that is a Lot for me... Selling copies would be unethical (and maybe illegal).
Anyway the problem I believe is in the counter and not in the board. So maybe the "old" manual may suffice.
Yes the PM6666 operates fully, smoothly and without any problem. I believe that it is something that failed in a previously working complete product; from the PM6666 sn, the GPIB card was installed at the factory. So it cannot be an incompatibility.
Also, I ruled out the card itself, it works in the other counter I just purchased on Ebay, and the PM6666 freezes with both cards.
So it MUST be the data path interface on the PM6666, I followed the path from the connector (labeled BU105 on the service manual) and I have three lines of interest: one is a 5mhz reference, and the other two are SCL and SDA (yeah, I2C bus) which go to a series of IC, I believe the three that could have something to do with the problem could be IC 101, 113 and 114, all labeled I/O (and all PC8574).
Both lines go to the CPU obviously (MAB8461P, with 6K ROM and 128 bit RAM), and to a custom IC, labeled IC 104 -CNT (controller?) an obscure OQ0501, built by Philips. This chip takes the 10 Mhz from the main oscillator and I believe divides it, as it supplies the 5MHz to the CPU and to the I2C bus. The 5MHz also goes to the GPIB card, where it is used by both the onboard CPU (another MAB84xx) and the GPIB IC.
BTW, it makes me think that maybe it could be possible to build a simple I2C to USB bridge to control directly the PM6666 from USB bypassing GPIB, however this is way beyond my possibilities - and I believe it wouldn't work all the same, since the problem is in the main board anyway.
Now what to do?