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Thursday, 22 November 2018

A complete waste of time?


I decided it would be a good idea to buy this from eBay:

Its an old HP DVM and Frequency counter; with a glorious Nixie display.

Initial testing revealed some issues with the frequency counter part of the functionality which I traced to an IC that was basically a set of NOT gates, so I removed it and replaced it with a good old 7404:

So, now the frequency counter is working, but the digit in the image below shouldn't be a 4 but a 0:

It works when the digit is supposed to be displaying a 4, 5, 6, 7 or blank:

So the rules of BCD tell me that the line that represents "4" is always stuck LOW no matter what the actual signals tell it to do.

There's a bunch of logic in the system, but there's basically a 4-bit latch that seems to have appropriate signals coming into it for this digit, but the "4" line is just stuck low.

The part is labeled a HP  1820-0116, and I think it's one of these:

So I've ordered some - the "2" on the end of the display also seems to be a similar problem so more to investigate.

So, I removed the suspect chip and fitted a DIL socket in its place:

I then swapped the chip in the "option 001" which is also socketed and bingo:

Now, I will need to do the same to the "2" digit chip as that is also stuck with a line low. Having investigated further, the 7475 I mentioned above doesn't seem to be a direct replacement for the HP device - I suspect the logic is the same, but the pin out isn't. I think the HP device is like this:

So, to try one of these I will have to make an adapter board of some kind. But maybe.....

Is it normal to buy test gear, to fix the test gear you just bought on eBay? I could always find another one of these for "spares or repair" and get some bits from there.....

I think I've got a bad case of GAS. I'm told there is no hope or indeed cure.

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