HP5245L Freq. Counter Repair

[Paul B]

hi from memory over 35 years there is the high frequency and low frequency in the same counter
as the count repels through and only the lower digits are the high frequency counters
when you switch to store on the back and the frequency is not the same for a fixed frequency
the problem is usual the neon ageing and the trigger voltage has changed so the flip flop is not in correct state
i can enplane how to sort this if required Paul B

[Paul B]

If you want a better scan of a manual ARTEK do them for about $10

[factory]

Artek don't have the later manual, or the 1980 change sheets, I have dead tree editions of the two that Artek offer.

Some pictures of the failed decoder plate I had, the traces had corroded away & cleaning made the damage more obvious, a replacement from a donor board cured it. There are at least two types for the +1224 , +1248, -1248 BCD output options.




Different version.


David

[flanneltuba]

@Paul B: I tried a number of ways to improve the degraded performance of the original photo coupler devices myself, to no avail. They suffer from pure age related deterioration, is my guess. All things must pass...

Here for better or for worse, is a pic of my the diagram of my CD4514 + UNL2003 replacement concoction, which works quite well. It is effectively a take-off of the HP upgrade, but with fewer components.

"Please excuse the crudity of the model. I didn't have time to build it to scale or paint it." I can redraw this into a more conventional schematic drawing if needed, this is just my crude working sketch from my bread-board development work.

But...it works. Refer to the previously posted pics for where to tie in the ABCD counter bits, -150V, GND, and X-fer In to the original circuit. Let me know if you have questions. Happy to help.

- Scott


[Edit: Also, you will need to replace the common anode resistor for the nixie display with a 12KΩ resistor, and disconnect the R6, R7, R8, and R9 resistors to the neon bulbs. My pic above shows CR1-8 disconnected, but you can leave them in; they do no harm to stay.]

[Paul B]

Hi Scott
Thank you for the picture regarding the photo resist yes a lot of counters might fail with that ageing bit neon ageing can be sorted
I have recently given a few of these counters to a friend so will probably get involved in getting them going 
I think it would be worth making a slightly better drawing as a lot would like to see it passably on the HP site

Paul 

[flanneltuba]

Not having any proper schematic drawing software I did a quick search and opened the first free schematic software that looked reasonably uncomplicated, that being Scheme-it offered on the DigiKey site. I spent an hour or so and got a ½ way readable drawing sketched out. I took a quick screen shot of it, since the software seemed reluctant to allow me to actually save my final product, so it's surely doomed to disappear as soon as I close the browser page it's on.

So, well, for better or for worse, and surely a trained practitioner of such software could do far, far better, here's a screen capture of my HP nixie display driver replacement circuit. As far as I can tell this is essentially the same design as HP's later display drivers, but with ULN2003 darlington arrays instead of discrete transistors. If someone has an actual schematic of the HP circuit, I'd be interested how close I came to duplicating it from a grainy ebay snapshot of one of these later display cards. 

- Scott

[factory]

Here are a couple of the crappy scanned pages from the manual on the Keysight webpage, there are several more diagrams for different variants, all are in the change sheets at the end of the pdf.
https://www.keysight.com/gb/en/assets/9018-02735/user-manuals/9018-02735.pdf

David