Project Time! #5

WordPress decided to finally block the classic editor for good – which makes me very unhappy. This block editor is a disaster, maybe useful if you are one of those my-phone-is-my-life hippies but I guess I’m too old for that. I use my phone for making calls, and not even of the video variety. So as I’m writing this 70%+ of my PC screen is a white, blank space with a small column of text somewhere in the middle, and I can barely see the font. If it wasn’t for FireFox 140% page size I’d be struggling to write anything at all. It’s “progress” like this that annoys me the most so don’t even get me started on Windows “evolution”.

It’s not my intention to turn this into old people rant, I just wanted to warn you that from now on things might look different, and not in a good way. And I dread the next ordering window, I used to be able to just copy-paste the forms directly into page code, I have no idea how to do that now.

Anyway, originally I wanted to make a whole post about choosing the right kind of J-FET transistors for the input switches in meters like Fluke 8600A – especially since the ones they’ve used are not made anymore (and that’s been true already in late ’90) and difficult to find. Usually your only source is another meter as a part donor, or maybe ebay if you don’t mind that a few of those transistors would cost more than what the whole meter (in working conditon!) is worth. And you’re very likely to get an expensive fake anyway. But a very technical post like that would be boring for most people, and with the latest WP changes I don’t even feel like doing it anymore. So I’m just going to mostly wrap this up with a few more photos.

The inputs – a very important part of the meter, you are going to plug and unplug the jacks pretty often, it’s a good idea to have a fuse for current measurement, and – in general – you might be applying high voltages to these so there better be proper clearances and insulation there. Well, if you are a Fluke fan then I’d suggest you never open one of these 8600A, ignorance is a bliss they say. I’ve opened a cheap phone charger once and promptly dumped it into a nearby recycle bin shortly after, even though it was still working – I would not dare to connect it to AC outlet in my house anymore. Anyway, I had to rewire the Fluke inputs because:

  • Clearances, what are those? Surely a thin layer of old plastic and/or half a milimeter of air gap between bare metal parts will handle 1200V AC just fine.
  • Let’s just use a thin, strained wire to connect ground returns to PCB, that will obviously not just snap off on it’s own after years of use.
  • Insulated fuse holder? Like, an actual fuse holder, not a cheap short metal tube on one end? What are you, a safety loving socialist?
  • This here single spring blade will hold the fuse in place just fine 95% of the time. And the fuse will not move to sides and make a poor contact when you turn the “holder” to lock it in, not at all.
  • If it works, it must be a Fluke!

This is how it looks now. Trust me it’s a major improvement over what I found:

FLUKE 8600A inputs

The range-selecting GAL with the capacitor update I mentioned before, and a peek at my first attempt to rebuild the power supply. The AC converter board is still out but everything DC almost worked by this time (BTW note the input fuse, the fabled Fluke quality and safety, it’s why the meter was so expensive):


Unfortunately my idea of using efficient DC-DC converter to create 5V for the logic didn’t quite work out. Even though 5V is not used in the A/D converter, there was enough noise present on the PCB to upset it. Ignore the extra wires going to the rectifier bridge, I was using that with external DC power supply because I didn’t want to have 230V running throught the board (right up to the ON/OFF switch in the front panel) while I was working on this.

FLUKE 8600A power

I could not get a zero reading in lowest DC ranges with this setup, so sadly it had to go. I ordered a custom 7,5V/1A transformer for this project and went with linear 5V regulator. But I was stubborn and decided to keep the +/- 18V DC-DC converters since those feed 15V regulators so there shouldn’t be much noise. And actually the battery-powered option from Fluke uses a similar setup, but there’s a bit more EM shielding. I can’t replicate that here though, no space for it – the battery option has an extra PCB (and a different transformer BTW) which would prevent me from installing the data output unit that I want to have.

Since I had a curious issue with the ohms converter not being able to calibrate – that is until I swapped one of the factory-selected precision resistors for a different value – I decided to create my own PCBs to replace the burned ones. I wanted to know if the carbonized PCBs are the cause (some leakage currents perhaps) and also I was not entirely sure connecting more than 100V input would be safe anymore, considering all the damage to the input divider. Plus it turned out one of the relays on the divider was on it’s way out anyway (yup, that unwrapped one). My PCBs are not 100% identical to the originals but work just as well, if not better – while reusing only the custom parts and semiconductors, the rest is new.


FLUKE 8600A Ohms

This is how it looks installed, with the updated PSU and new transformer:

FLUKE 8600A new PCBs

This transformer is a split-bobbin type but I decided to add a bit more mains RF rejection by running the input wires through a ferrite bead, as well as adding two 100nF SMD capacitors next to the bridge (on the other side of the PCB) to supress harmonics. Original transformer had electrostatic shield but this should be just as good. This one is 230V only but that isn’t an issue for me.

FLUKE 8600A trafo

I was able to calibrate the DC ranges, there aren’t any noise problems except the AC converter, but that one is another story – kinda works but has issues, so I will attempt another reproduction PCB at some point I guess.

And finally the data output unit, now upgraded with USB port so that it can be used with a PC directly, or remotely via LAN+RPi. The DOU output is fully isolated and thus so is the USB. I made it in such a way that USB module is only powered over the host cable, in order to prevent any backfeed between it and DOU.


A small modification to the case was required to make sure the USB port will come through and stick out just far enough for the plug to fully lock. It was either that or a massive edge connector plug with PCB and a lot of extra chips to process all the 5V signals present on the original printer port. I think this approach is better and more importantly, it works.


Sadly, the meter only reports what range it’s in, not what mode is selected. But still it’s pretty useful, I’ve implemented a few SCPI-like commands and this is what the USB communication looks like:

Fluke,8600A,2106127,1.2 (2021-03-13)
+04,953e+0 V DC
+04,954e+0 A DC
04,953e+0 V AC
04,953e+3 Ohms
+04,954e+0 V DC
+04,953e+0 V DC
+04,953e+0 V DC
+04,953e+0 V DC
+04,953e+0 V DC
+04,953e+0 V DC
+04,953e+0 V DC
+04,953e+0 V DC
+04,953e+0 V DC
+04,953e+0 V DC
+04,952e+0 V DC
+04,952e+0 V DC
+04,953e+0 V DC
+04,953e+0 V DC
+04,953e+0 V DC

To sum this up, I’ve fixed or replaced:

  • 50 ohm pot (blown; R8 on DIV board)
  • F2591 J-FET with selected BF245C (blown; Q14 on main board)
  • F2591 J-FET with J111 (suspicious; Q15 on main board)
  • CD4053 chip (failed; U16 on main board)
  • ROM chip with 16V8 GAL (failed; U9 on main board)
  • 2,7M and 4,7M resistors (drifted way above 5%; R74, R82 on main board; R17, R18 on DOU)
  • wire connections on input jacks (broken; on main board)
  • factory-selected resistor with 1% metal-film of higher value (something drifted; R90 on OHMS board)
  • new power supply section of my design (missing parts; on main board)
  • fuses for input and power supply (missing parts; on main board)
  • new PCBs (charred; DIV and OHMS boards)
  • reed relays along new PCB (suspicious; DIV board)
  • 470nF film capacitors (open/short; C14, C20 on AC board)
  • 150k resistor (drifted above 5%; R43 on AC board)
  • 15k resistors (drifted above 5%; R32, R33 on AC board)
  • 25nF ceramic capacitors (cracked; C6, C7 on AC board)
  • FD700 diode (missing; CR16 on AC board)
  • case screw (missing)

The case screw was actually a serious problem. It’s an imperial thread (and it comes in coarse/fine variants too!), kind of difficult to find in Europe.