Fluke 8600A battery powered multimeter - convert to line only

[Hexley]

Well, it appears that no one has actually built/tested an adapter using a UVEPROM or EEPROM to replace the 7488. 

I just saw this thread and though you might be interested in my 16F628A-based PIC-to-PROM adapter which fixed an 8600A with a dead 7488 for me a few years back.

The 7488 is removed, and a ribbon cable jumper is run to a separate socket with the PIC in it. Kind of a kludge, but it works. I have attached the assembly code for the PIC, and a diagram of the interconnection.

Good luck.
Hexley

[Edit: removed the incorrect pinout file. See below for the corrected version.]

[GregDunn]

EXACTLY what I was looking for, thanks!  That meter got pushed down in the queue because of a new arrival which essentially worked out of the box - but it will be back on the work table soon...

[GregDunn]

OK, so I took a few minutes to look over the PDF file.  I see the adapter on the left, and the pinouts for the PIC and PROM at the top - but what is the diagram in the center?  It only has 4 wires connected, and they are not connected to the same pins as on the adapter.  If it's supposed to show wire routing for the pins which don't line up between the two packages, then there seems to be an error.

[Hexley]

Good catch, Greg. The list on the left has RA1 and RA5 swapped. The diagram in the middle is actually correct, as you can confirm by checking the pin assignments in the assembly code. I've attached a corrected version.

Some further notes:

The diagram in the middle is supposed to show how closely the 18-pin PIC could be shoe-horned into the 16-pin footprint of the 7488.

Some of the PIC pins line up directly with the corresponding pins of the 7488. That would be true of RA2-DO1, RA3-DO2, and so on. But four of the pins do not line up, as you noted. Those pins need jumpers. So, for example, pin DO4 of the 7488 needs to connect to a pin on the far side of the PIC, RA1. And DO5 connects down to RB2 towards the bottom of the PIC. And so on, for GND and for VDD.

The two bottom-most pins of the PIC are unused and left dangling.

My original plan was to mount the PIC on top of a 16-pin DIP header, connecting the pins from the PIC to the header either directly or via jumpers as shown. IIRC, there was not room in the 8600A to fit such an assembly. So I ended up using a 16-pin flat cable between a header plugged into the 7488 socket and a remote socket for the PIC, which was stashed around the forward edge of the power supply where there was plenty of room.

Hope this clarifies things a bit.

[GregDunn]

Oh, good - I was afraid I was missing something obvious.   

I had already come up with an idea of connecting the thing via a ribbon cable, because it is very tight in there between the AC converter and the input divider...  I'm sure I would have spotted which one was swapped when I actually started to make up the board! 

[GregDunn]

Moving back to this meter - I thought I'd install a line supply on the one which is working, get it out of the queue, and then use it to check out the one with the wonky range behavior, so I know which parts are good and which aren't.  I'd be happy at this point to have 3 working 8600A units while I am doing some other work.

I tested the built-in transformer which is part of the battery-operated unit and no matter which taps I used, I can't get it to provide a rectified, filtered output of less than 20VDC.  That's way too much for any regulator I want to use, and depending on the digital circuitry pulling the supply down to a reasonable value when loaded is not, to me, a good engineering solution.  I figured that if I'm going to replace the transformer, I'll just make my own supply instead of faffing around with the weird Fluke topology.  So I fished around and found a 12VCT transformer exactly the same size as the existing one.  Set up with a standard FWCT secondary filter, it provides a solid 10VDC which is perfect for a standard 7805/LM317 regulator.  I've tested it and it works, so the tough part was installing the transformer. 

As luck would have it, the one I found has standard mounting ears but also a set of flanges which hold the outer frame onto the device.  I bent the ears under so they'd keep the frame snug, and bent the little flanges down.  They almost perfectly matched the ones which the Fluke-supplied transformer had, that allowed it to be soldered to the board and grounded to the mains neutral.  (I'll show the process when I do the second 8600A; it happened so quickly I forgot to take pictures of the first one)  If you look, you can just see the pins disappearing into the original board mounting holes at the corners of the transformer.  So that was a fortuitous discovery and will allow me to fit everything in with no cutting or shaping.  I'm going to put the small filter/regulator board where the secondary parts would go in a standard 8600A and just hook the output into the 5V rail where it would normally connect through the ferrite bead and front panel switch - so it'll work just like the original except no batteries to mess with.

[GregDunn]

New power supply works great!  Mains neutral is grounded to the transformer as on normal line-powered units, and a stable 5.01VDC is supplied to the existing power supply converter board.  It's drawing just under 0.08A at the transformer primary, which is about 9W - well within spec.

The K1 relay has some issues with oxidation on either the contacts or connector pins (readings were a little jittery until I re-seated it), so before I close up the unit I'm going to take it out and do some gentle cleaning of all contact surfaces.  Then a double-check of the calibration against my reference 8800A.  This one is about ready to join its siblings on the test bench.

Edit - fixed typo.

[GregDunn]

OK, this may be the last post on this particular repair (and there was much rejoicing!) but I had to make note of one particular item of interest.

On another forum I saw some comments about replacing the 5V power supply of a battery-operated 8600A with a linear regulated supply, and there seemed to be a lot of bashing of the whole "replace it with a 7805 regulator" concept, particularly (speculative) increased temperature vs. the original battery-regulated supply with its series line capacitor and zener/SCR regulated secondary.  I wasn't concerned about pushing the current capability of the 7805, because when running the meter from my HP bench supply, it only drew about 450mA; the only thing which might be an issue was the power dissipation.  So the last thing I did before buttoning this one up was to measure the regulator temp vs. an original 8600A's 5V regulator.  Theoretically, mine ought to run warmer, because it's supplying current to the digital section as well as the ±15V converter, right?  I gave each meter an hour or so to warm up, so that it would be a fair comparison.

Max temp of the line-powered 8600A was about 137F on the regulator chip (using a peak-hold IR thermometer).  The modified unit peaked somewhere around 130F.  The transformer and rectifiers were not even warm to the touch.  So, my modified unit actually runs cooler than the original line-powered unit.  Evidently this won't be a concern.

[GregDunn]

OK, I lied - there were a couple of things I needed to follow up with.

@bd139: the fiberglass pencil was a great idea - it takes a lot of the junk off without damaging the traces or board surface.  I'll never be without one now!

Also, yes - the second 8600A that I bought as parts/broken also has the mains power connections miswired compared to the schematic.  Red and Black (so marked on the board) are the correct traces to use for powering a regular single-primary power transformer.  This is odd, but the fact that two meters from widely different sources have the same issue leads me to believe it's a widespread problem.  It still seems wrong, though, because the schematic is drawn the way I'd expect it to be, and the meter isn't.  Oh well, it'll be fixed when I add the new supply.