Please help me with HP 5245L counter repair

[Jurek_p]

Hello

I am an electronics enthusiast and hobbyist, currently grappling with a technical challenge that is stretching the limits of my abilities.

Not long ago, I acquired an HP 5245L generator on eBay, driven by my fascination with vintage electronics and a fervor for restoration projects. I've since devoted countless hours to restoring this device. I have successfully replaced all the neon bulbs, whose leads were broken, and swapped out the Nixie tubes with new ones.

Despite these victories, I've encountered a persistent issue with one of the cards. This card, located in the 14th slot, isn't displaying all digits correctly—it specifically fails to display the numbers 2 and 8. Notably, when the tube should display the number 2, the voltages on all 10 pins of the photocell exceed 35V, and the whole tube lights up. Please note, this issue occurs ONLY when displaying the numbers 2 and 8—the tube displays all other numbers correctly. On the bright side, the counter itself is still functioning and accurately operates in self-test mode.

At this point, I suspect that the photocell might be damaged. I would like to ask your opinion on this, and whether you think it's possible to repair it?
Or could you perhaps guide me on where to find a suitable replacement?

Jurek

[Wallace Gasiewicz]

Wow! An ancient Photo coupler !!!Since you have done so much work on the unit, I suppose you have checked and the correct neon bulbs are lighting up for the "2" and "8"  ??If they are I guess you are correct in assuming the problem is in the photocell array.  But maybe you could isolate the photocells involved and replace them? Perhaps on a separate little board that you could slip in?

[pqass]

Curious Marc has a 5245L repair series of three videos that may be helpful to you.
At the point in the video below (8:30), he discusses his flip-flop issues and later how the BCD decoding works with neons and photoresistors.
His issue was a broken flip-flop that was very finicky about the choice of transistors.
See 16:02 for a shot of the photoresistors on the underside of the black enclosure. They don't look like they are individually replaceable.

https://youtu.be/mgi34ogzbvY?t=509

[factory]

I thought CuriousMarc decided the flip-flop was OK in the end and attaching the scope probe prevented it working. 

Looks quite an early one from the picture.

It's possible the photoconductor plate has corroded traces, I've had several with this fault, cure was a fitting a replacement from scrap boards, occasionally they appear for sale after the seller has removed the tubes. There are at least two different variants of the photoconductor plate, depending on the BCD code used.

The only other solution is to modify it, using the CMOS decoder & HV transistor circuit, HP used this in the last batches of these counters, you would lose the fade effect when the digit changes though.

David