Making a PM9565 DC-option module for my Philips / Fluke PM6306!

[Dekatron]

A while ago I bought a Philips / Fluke PM6306 which was supposed to contain the PM9565 DC-option, but it was unfortunately missing.

As those of you know that have been searching for a PM9565 DC-option unit it is more or less unobtainable these days unless you come by one from another unit.

I Googled some and found this forum and the thread from user "coromonadalix" where he embarked on this journey: https://www.eevblog.com/forum/testgear/phillips-fluke-pm6304-pm6306-pm9565-dc-measurement-option-board/ since I read that user "HighVoltage" owned a PM9565 DC-option unit I sent him some messages asking for better photos of the circuit board so that I could check to see if it would be possible to make a copy that would be as identical as possible to the original design. I got some very good photos back and embarked on this journey myself and now after some months I've finally been able to build the first circuit board as not only took it time to trace it but I have struggled to get hold of the components now that they are hard to find due to the Covid-19 pandemic.

There were some BOM-errors that I had to fix as well as making a 10K/0.1%/5ppm/0.4W resistor from four 0.2W resistors put in series and parallel. I was also given some of the Mini Melf resistors from an eBay seller that was kind enough to share from his large stock when I explained what I was going to use them for, I missed the auctions where he was selling these values I needed.

I put the circuit board in my PM6306 a few days ago and have made some initial measurements and they are all find. The service manual for the PM6304 says that the instrument needs to be re-calibrated after a PM9565 DC-option unit has been installed and that is the only nut to crack as there is no service manual for the PM6306 to be found anywhere and it differs from the PM6304 instrument. There has been a partial success in finding the commands used for entering calibration here: https://www.eevblog.com/forum/repair/i-have-found-fluke-pm6306-rcl-calibration-command/ but it seems no one has found the store and exit commands so that is the next nut to crack.

You can see my unit working here:

As soon as I have been able to calibrate the PM6306 with my PM9565 DC-option unit I'll release the files for the circuit board as well as the updated BOM so that anyone can build their own PM9565 DC-option, but without the calibration it is not usable for any proper measurements.

A big thank you for all help from "HighVoltage" & "coromonadalix" as this project would not have been possible without their help!

[HighVoltage]

All your hard work paid off, congratulations.

Have you noticed a change in calibration to your references?
I had removed my DC board before and used the instrument without it and it seemed to perform perfectly alright.

[Dekatron]

Thanks!

The only thing I have observed is that right after I had connected the module for the first time the calibration date reported by the calibration command and the calibration text changed to "Off" - meaning that the instrument is out of calibration. It seems like the firmware identified that the module had been connected or I have touched something while mounting the module - the calibration date was still there when I had replaced the battery so it definitely happened after that and it was there on several occasions afterwards when I have been trying to find how to use the calibration and also to identify the somewhat differing menus when the PM9565 DC-option is installed. However all of the calibration registers still contain the same calibration values as before so nothing has changed apart from the last calibration date being erased and the text "Off" is now displayed.

All measurements of my calibration components show correct measurements with the DC-option board installed up to 500KHz compared to previous measurements I made of these (General Radio 10nF capacitor, Hewlett Packard Q Standard inductor and a few different standard resistors - all within their tolerances and calibrated sometime during the last 5 years). There is just a small difference in DC measurements of resistance while using the PM9565 compared to the AC measurements but those measurements are within +/-0.1% so I guess that they are just fine but I will try to find out how the proper calibration can be performed for the PM6306 with my PM9565 module installed to ensure that it is still within calibration.

I haven't looked at the wave forms of the measurements signal but since I made sure that I incorporated all of the "guard" traces surrounding the signal traces, and also made sure that I placed all of the ground planes as identical as possible to those on the original circuit board, I think that all will be fine.

[HighVoltage]

Your PM6306A was shipped by the factory without the DC option, meaning the DC board was originally not installed. Maybe because of that the DC option was not calibrated in your unit.

My PM6306 was shipped with the DC option and therefore I can remove the DC board and install it again, without any Error in regards to CAL.

So, maybe you only need to calibrate the DC section?

Too bad we don't have the Service Guide.

[Dekatron]

Some further investigation show that with Program 8, described in the Service Manual for the PM6304, I get a "Caldt OFF" meaning that it is out of calibration, however when I finally got some access via the RS-232 interface I got my original calibration date returned from the CAL? command!

Looking at the circuit diagram I saw that there is a signal, DC_AVAIL, that is grounded on the PM9565 when installed, but which otherwise is pulled high and read via a 74LS245 transceiver on the CPU board, named PORT EXTENSION (D108). So there definitely is a way for the CPU to check what extensions has been installed and then change a flag for the calibration and show it as out of calibration when the module has been installed and the unit hasn't had a calibration performed after that instance if Program 8 is run.