Electronics > Metrology

Transmille 8081 ADC

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I was the unfortunate winner of the broken Transmille Fisherprice 8081 that was listed on ebay some time ago.  :palm:
Long story short is that something unwise was done with the input that  vaporised part of the input circuitry and PCB.  The +15v regulator was burnt short circuit which fed -15V & +25V unreg to everything on the -15V & +15V rails. Toasty    :-BROKE


Spot the difference?

I eventually got the thing running and giving readings but wasn't happy enough with it to make any posts here about it.

Transmille sometimes respond to messages placed on their system regarding the 8081, but aside from a brief exchange of useful messages with Martin Cox they show little to no interest in support. They won't release any diagrams or service details and they ignore requests to purchase spare parts from them.

I've still got more work to do on it, but I've finally gotten around to do something I have had in mind for a while. Reverse engineering the ADC board.
Please bear in mind that I haven't reverse engineered a complete board for anything before, so there will be errors.

Here's my attempt.


I'm thinking about making another ADC PCB because the reference voltages run through a silly number of vias, quality of the original really isn't great and all of my hacking about has taken its toll.

I think I can delete IC16 by using the unused /2 in IC14 in the remake. Comments, fixes and suggestions for changes in the remake are welcome.

Edit: fixed unannotated diodes and duplicated resistor numbers.

Would be interesting to see high res photos of the boards (all, not just ADC) of the unit for education purposes. Not very popular unit out there.

I had one of these in a lab I used to work in, never used it much. I remember that it was still rather experimental when they had it, but the lab manager never knew how to use it. Of the times I touched it I just referred to it as a random number generator, the last 4 or 5 digits were always jumping about. This was before I got my VoltNut and started to hunt for better readings. I left the lab about only a year and a half.

I did have some stuff from Tansmille about it, I will see if I can find it.

The ADC looks quite similar to the mark-space ADC of the older Solartron 7081 and similar. The counter part went to the µCs, though still quite a bit discrete logic left.

Much of the parts look like standard parts. So they could be replaced if needed. Some of the OPs may have well survived a -15 and +25 V operation.

Running the reference through many vias should not be a issue - the voltage of 10 V is relatively high and thus a low µV range of largely constant thermal EMF there should be no real problem.

There are other points that can be more important than that:
It starts with having 3 separate crystalls that can and will interact in some way. Ideally they would rund in sync and things may be OK - but chances are they are not that equal and would synchonize to a different ratio, ausing trouble on the beat frequency. The creation of the +-10 V reference would be quite important. The buffering of the reference would also be a mayor issue. With long traces there can be RF range issues and some length may even be a good thing, though more a source of trouble.
Anyway with 2 µCs with internal flash memory it would be hard to duplicate the board without reusing these chips.

The ADC type has principle limitations. So it would likely not be low noise, no matter what. A principle limitiation is that is needs a realtively stable input signal and switching the input would be slow - it may need some 10-100 ms to recover, before giving a stable reading. So a fast auto zero cycle is not an option and ADC drift would thus be an issue. At best I would see it comparable to the Solartron - more like lower grade.

It could still be good INL for very long integration times, if everything else is done right.
With the PLL chip there is still a chance to get some interference, so good INL is by no way guaranteed.


--- Quote from: TiN on July 18, 2021, 04:44:10 am ---Would be interesting to see high res photos of the boards (all, not just ADC) of the unit for education purposes. Not very popular unit out there.

--- End quote ---

There isn't much more to see over the pictures your have on xdevs.
Is there anything in particular you want to see?


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