KI found a clue in the circuit description, par. 4-195, " A lighted photo conductor has a resistance of about 20k ohms, & an unlighted element has a resistance of several megohms." Sounds like a CDS photo cell. Of course, these elements are all driven by neons, which are called "lamps" in the manual. They are saying the firing voltage of the neon lamps around 70v, which means the neons would have had to be matched for this circuit to work.?

? ? ? ? ? ? ? ? ? ?Regards Brad?

I'm starting to have a pretty good understanding of how the electronics supposed to work, at least most of it, but unfortunately I believe all 4 of the logic chips on mine are dead.
I suspect the damage, at least to this extent was done by me when I started messing with it and randomly connected 24V to any orifice I could find.

I put the mechanics back together and as far as I can tell that works just fine.



I ordered replacement logic chips, I'll replace them when they come in next week.

After zooming in quite a bit I see that the counter modules are powered by -15 and +20V rails. They use 2N404 Germanium transistors.

My HP5233 counter appears to use identical boards.It is basically worthless, other than as a collector's item but it has all six boards and the Nixies. It is only a 2MHz counter. I've never powered it up, it was part of a large lot of old test equipment that I bought over 20 years ago.I also have a 5223 (300KHz) counter from that same lot. I would need help to dig them out of storage.

Boards from the 5245L and 5248 use the same basic circuit, as well.

BTW, Nightfire is planning on closing their store fairly soon.

On Thu, Mar 16, 2023 at 8:18?PM Harvey White <madyn@...> wrote:

74 series logic is standard TTL, so 0 to 0.6 volts is a good logic zero, and anything above 2.4 volts (preferably 3.2 to back bias an internal diode) is a logic one.? IIRC, 7447 outputs are simply saturating transistors to ground, with the anode of the display switched on or off to enable the digit if scanning.

Harvey

On 3/16/2023 5:27 PM, Michael A. Terrell wrote:

What about the 7441/74LS41 BCD to decimal decoder designed to drive Nixies?

I have a 312B, which uses LED displays, with a much simpler counter design.which uses the 74LS75 Latch and the 74L47 BCD to seven segment decoders. The 312A manual that I have is so speckled tht I can't read it to determine the logic levels.

On Thu, Mar 16, 2023 at 12:57?PM Greg Muir via <big_sky_explorer=[email protected]> wrote:

Brad,

?

If I can recall the neon-photocell approach was very similar to that used in the HP 5245L counter series display boards.? If you can find an old board (base P/N 05212-6016) you might have some luck in finding similar photocell devices.? The suffix to the part number (“Series 6xx”) changed reflecting the board functionality.? There was also changes in the design of these boards where they either contained an enclosed module termed “Decoder” (One HP part number is 1820-1683”)? or a “Photoconductor Plate” where the photocells directly drive the Nixie tubes.

?

It appears that HP changed to the “Decoder” module in later model counters.? It is a black plastic enclosure on the board with a removable cover but I have never opened one up to see what is inside.? Given its construction I am assuming that it, too contains photocells.

?

Greg

What about the 7441/74LS41 BCD to decimal decoder designed to drive Nixies?

I have a 312B, which uses LED displays, with a much simpler counter design.which uses the 74LS75 Latch and the 74L47 BCD to seven segment decoders. The 312A manual that I have is so speckled tht I can't read it to determine the logic levels.

On Thu, Mar 16, 2023 at 12:57?PM Greg Muir via <big_sky_explorer=[email protected]> wrote:

Brad,

?

If I can recall the neon-photocell approach was very similar to that used in the HP 5245L counter series display boards.? If you can find an old board (base P/N 05212-6016) you might have some luck in finding similar photocell devices.? The suffix to the part number (“Series 6xx”) changed reflecting the board functionality.? There was also changes in the design of these boards where they either contained an enclosed module termed “Decoder” (One HP part number is 1820-1683”)? or a “Photoconductor Plate” where the photocells directly drive the Nixie tubes.

?

It appears that HP changed to the “Decoder” module in later model counters.? It is a black plastic enclosure on the board with a removable cover but I have never opened one up to see what is inside.? Given its construction I am assuming that it, too contains photocells.

?

Greg