Recently I decided to check out and, if necessary, repair several older HP units that used Nixie tubes for their numeric displays.
The first unit I checked was a 7-digit HP 5216A counter which had a number of problems, including the MS (Most Significant) digit's Nixie tube not illuminating.
While trouble-shooting this problem, I thought that one issue might be with the Nixie Decoder/Driver IC, a custom HP device whose part number is 1820-0092 (shown below in the HP 5216A display PCB).
This device is a 16-pin DIP IC that decodes a 4-bit Binary-Coded-Decimal (BCD) value into one of ten low-true (open-collector) outputs representing the digits 0-9. Binary values that are outside the range of 0-9 are not decoded and, in these cases, all outputs are left in their undriven state.
Note that the 4-bit BCD values that drive the decoder are low true. That is, a '0' is represented by an input value of 0b1111 (i.e. 0xF), not 0x0000. A '1' is 0b1110 (i.e. 0xE), not 0x0001, etc.
But where do I find a replacement 1820-0092? This part is no longer manufactured, and if I were to replace it I would need to come up with an equivalent design (unless I could find one on eBay, which I could not).
The TTL SN74141 IC is almost identical in function to the 1820-0092, but its inputs are high-true, not low-true, plus it has a different pin-out. Unfortunately, this IC is also difficult to find. However, there is a Russian equivalent IC that can usually be found on eBay or Amazon, the K155ID1, and labelled in Russian as: К155ИД1:
This IC can be used in conjunction with 4 inverters (converting the low-true 4-bit BCD inputs to high-true) to replicate the original function of the 1820-0092, per the schematic, below:
Although the schematic shows a 74141 IC as the decoder/driver, its pin-out is identical to the Russian K155ID1 device that I actually used.
A 74LS04 (hex inverter, from my junk box) is used to invert the four BCD inputs.
Below is my implementation on a small piece of perf-board. At the far left of this "adapter board," on the under-side of the perf-board, are 16 pins in a DIP pattern that will insert into a socket that I will mount on the HP 5216A's PCB at the location of the defective 1820-0092. Pin 1 of this 16-pin DIP "plug" is at the upper left.
The K155ID1 is the IC in the middle. Its pin 1 is at the lower right. And the SN74LS04 is at the right, with its pin 1 also at the lower right.
Below is the HP 5216A board with the faulty 1820-0092 removed and replaced with a 16-pin DIP socket:
And the following image shows the adapter board mounted in the socket:
Note that the adapter board extends over all or part of two other ICs. Also, the Nixie tubes are very close to the edge of the adapter board (a reason to keep it small).
That's it! It seems to work fine.
Standard Caveat:
As always, I might have made a mistake in my equations, assumptions, drawings, or interpretations. If you see anything you believe to be in error or if anything is confusing, please feel free to contact me or comment below.
And so I should add -- this information is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
No comments:
Post a Comment