More Counting with Nixies

Well,

As part of my TEAS which I described previously, I have acquired a further HP Counter/Timer as "spares" for the one I already have (yes, this is part of the condition it seems).

So, I started with the normal replacement of the electrolytic capacitors in the PSU and then did some debugging of the electronics itself. I found one of the 4 bit latches to be suspect so I removed it:

It didn't work out well for the poor thing, but removing DIL devices from through hole boards can be quite tricky at times. I did manage to remove some working parts from another board I have:

although removing the Nixie socket was extremely challenging, and I ended up prizing off the top plastic part and removing one pin at a time.

So, in the broken unit we now have:

A socketed replacement for U9 - this was the chip I suspected of being bad.

I have also inserted sockets for the "Option 001" which is the addition of the extra digit and then used parts from the spare board above to create this option. You also have to move a resistor that drives the Overflow electronics to that it does this one decade higher.

You have to admire the design and build quality of these units, they certainly don't come like this anymore:

The unit has had a good clean and everything now seems to be working:

The top unit is the newly repaired item, the one on the bottom is from last time.

Now, what next?

And for my next trick...

Well,

You will remember back here where I was talking about a complete waste of time?

So, I've persisted with the referb and finally I have a working unit. It's now permanently installed in the pile of test gear.

Apart from the repairs, I have replaced the electrolytic caps in the PSU:

And bodged together "Option 001" which is the extra digit:

Works rather well:

Not bad, is it?

A complete waste of time?

Well,

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.