Another Fluke oldie: 8110A

[david77]

This thing was sitting on ebay and nobody seemed interested so I took pity and grabbed it for myself.
Just kidding, I was actually looking for an old nixie Fluke for some time, the 8120A was always what I wanted but this'll do nicely for now. I used to have one but it was missing some unobtanium IC's  .

Here it is: The Fluke 8110A



It's a 4 1/2 digit 11900 count bench multimeter, DCV/ACV and ohms range. No amps range, but hey I've got enough other meters for that.
This meter was probably made in early 1972, so it's 40 years old and works perfectly. I had to do some tweaking to get it back on track. It was only out by about 50mV on the DC range, ohms was spot on.
I haven't been able to find a service manual but luckily all the trimpots are clearly labled.



The only thing wrong with it I could find are the DIP sockets that have this fluffy white stuff on them, obviously something felt it needed to escape the plastic. There was a post not long ago where someone posted about it, what was the conclusion there? I have to find the thread, not sure if I should replace the sockets.



The meter has options 01 Battery power and 02 Printer out. I can't figure out how the battery option is supposed to work as there's no room inside for any form of battery. The printer option is there, it's got a 36 pin Centronics socket on the back. I don't expect it will work with a standard parallel printer.

Now for some more old school electronics porn:



I wonder what's inside that grey block down on the right side?







And the obligatory comparison with another meter:

[retiredcaps]

Nice find and good teardown.  I only started electronics has a hobby about 2.5 years ago, but I do appreciate the older stuff.  I love seeing these types of posts.

When I look at something as simple as a resistor made today, the color bands are so narrow and cheaply painted on, it is hard to tell the different color schemes.  Not so with the older stuff.  Here you can clearly see the thick bands and quality of paint.

[lowimpedance]

quote: I wonder what's inside that grey block down on the right side?

Step up converter for the Nixie HV supply ??.
 Nice old meter, happy to see it has a good home.
lowZ

[david77]

Yes it probably is the step up converter - there isn't one anywhere else.uu

[nukie]

You are lucky those pots of that age are usually growing whiskers causing a lot weird problems! Nice pictures thank you.

[SeanB]

Just wash and scrub the sockets with IPA and then rinse the board clean afterwards then dry in a low heat. No need to remove the IC's and risk damaging them, just do a quick scrub upside down then clean off the residues and they will be clean. Tip is to hold it over a slightly angled mirror in a dish to collect the drips.

[david77]

Thanks for the suggestion Sean, I'll do that.

Today I found something wrong with it after all, when cold the zero in one of the Nixies would flicker brightly, making the display hard to read.
Now, without the manual I did some fault hunting in the display driver section. I had a hunch it would be a failing transistor.
The display is a multiplexed affair with a HV driver per tube and a 7441 digit driver. With a bit of ice spray and a hot soldering iron the flaky transistor was quickly found and it's neighbour for the next tube showed the same behaviour.
The two culprits were RCA 60994 transistors in TO-5 metal can cases. Luckily they still mostly work so I could figure out that they're NPN devices. Sadly I have not been able to locate a datasheet for them. Maybe anybody out there could help me out with one?

I suppose the faulty transistors weren't always able to switch off fast enough causing the zero to light up while the row signal jumped to the next digit already.

For now I replaced them with BF393's which seem to work quite fine in there. The transistors have to switch the anode HV for the Nixie's, I measured about 190VDC. The current will be very small <5mA I assume, as is common with Nixie tubes.
The BF393's work well in other Nixie tube equipment I've built, so I suppose I can leave them there.
However a bit of info on the RCA 60994 would be welcome anyway.

[SeanB]

It is a NTE 128 equivalent...... 80V 1A power device. For a Nixie the BF393 will be perfect, lower leakage and more likely to survive as it has a higher voltage rating. I would suggest you replace all of the digit drivers there as they probably all came from the same die so all have the same voltage rating, just these 2 failed first. The Vce rating is a minimum, often they will be higher and that is what Fluke designers relied on, but time has taken it's toll.

Datasheet for the NTE128 is http://www.nteinc.com/specs/100to199/pdf/nte128.pdf if you are interested.

[david77]

Thanks again.
Not a good design decision by Fluke to use 60994 there, but then again it's worked for 40 years  . Probably not the product life expectancy they calculated with back then.

[SeanB]

They probably tested the incoming transistors to select those with a Vce of 120V or more, then used them. Possible that the cans leaked over the years or had sodium contamination in them which took the 40 years to degrade the die inside. NASA has had problems over the years with 2N2222A's that crept into the supply chain that would fail after 15 years of use. Voyager and Pioneer used some, but luckily the design in of redundant systems that were truly redundant and which were designed at the outset to be fault tolerant has kept them functioning till now. The units on display at the Smithsonian are real flight ready equipment, only thing not there are the RTG units, as they were used on other probes over the years. Put power on and they probably would power up perfectly. Same with most of the unused Apollo hardware there. Even the used ones probably will work.