Yet another Fluke 8050A LED display conversion

[gcwills]

I recently inherited a Fluke 8050A bench multimeter from a friend. However the LCD display had gone the same way as many of the displays in these meters and was very faded and the characters were bleeding.

Fortunately there are several online resources that describe replacing the LCD display with seven segment LED displays:

https://lous.home.xs4all.nl/fluke/Fluke8050Asite.html
http://mrmodemhead.com/blog/gallery/fluke-8050a-led-conversion/

However one such conversion particularly appealed to me. It used the more readily available common cathode displays and I had several of the 14mm displays in stock.
In addition, it eliminated the 50 or so series limiting resistors making wiring up all of the displays easier.

You can see Rich Williams 8050A mod at the bottom of this page: https://www.diyaudio.com/community/threads/fluke-8060a.367928/

Here is an image of my 8050A with the replaced display:



To come are some more images and details of my particular version of Rich's approach.....

[gcwills]

The steps I took to replace the LCD:

1. Disassembled the unit and removed the display board
2. Cut the pcb track to isolate the LCD backplane drive at J1 pin 19.
3. Connect backplane pins 2&6 of the 4054/6 to -5V (pins 7/8)
4. Mount the LED displays on a matrix board and glue the board to the display pcb.
5. Connect all LED common cathode pins to -5V
6. Connect all of the remaining pins to IC11-16 (details in the links above)
7. Glue a piece of translucent red perspex into the front panel of the 8050A
8. Reassembled the DMM!

I simplified the wiring as my 14mm LED displays were too large to fit 5 across in the space available. So I used my power sander to grind down one LED display. You can see the "abbreviated" display on the left in the image below. As a result there was no room for a minus sign so my meter dispensed with the minus, rel, HV, db indicators.

[lowimpedance]

Looks like there maybe some space above the digits to squeeze a row of LED'S to indicate dB, rel, HV and even minus. You could just put a small label external above the LED'S.

[gcwills]

Looks like there maybe some space above the digits to squeeze a row of LED'S to indicate dB, rel, HV and even minus. You could just put a small label external above the LED'S.

Thanks for the suggestion. Yes those LEDs could be added if required.

[gcwills]

A request!

I have spent some time attempting to add a minus sign to my modded 8050A with little success.
Here is the schematic from Rich Williams diyaudio thread:



Earlier 8050A mods that used common anode LED displays used an NPN transistor to decode the "+" and "-" outputs from IC11. In this schematic you can see that Rich used a PNP transistor to drive the common cathode display. However any of my attempts to get the PNP "minus" circuit working were unsuccessful.

Its a long shot but I thought if I put it out there someone may be able to assist?

***EDIT*** See my later post that corrects the wiring for the PNP transistor.

[MathWizard]

I have a Keithley 197, with an pretty small LCD w/ no backlight. And the Zebra strip is also making poor contact. I'd love to upgrade it to a big LED segment display, more like your Fluke's.

I need to load test the transformer, but I think I did before, or read that it was only good for about another 100mA, yeah that was some Kerry Wong, he did a conversion and blogged about it. But yeah all those LED segments are a bit too much.

But I wouldn't mind getting a little bigger transformer, even if I had to make some sort of support for it. Or add another little 1. The buttons on mine, and KW's, were a bit bad, so he made a whole new front panel. I might do the same, to fit all those big LED segments.

Either way, I wanted a bright and bigger overall screen, so even if I go with an OLED one, I want it to be 2x the size or more of the original.


I look forward to decoding some or all, of how the LCD drive works. And then programming an STM32 chip, or an ATmega169P, if it can do it.

[lowimpedance]

A request!

I have spent some time attempting to add a minus sign to my modded 8050A with little success.
Here is the schematic from Rich Williams diyaudio thread:



Earlier 8050A mods that used common anode LED displays used an NPN transistor to decode the "+" and "-" outputs from IC11. In this schematic you can see that Rich used a PNP transistor to drive the common cathode display. However any of my attempts to get the PNP "minus" circuit working were unsuccessful.

Its a long shot but I thought if I put it out there someone may be able to assist?

As your not using a segment in the common cathode display,  but a discreet LED (?), why not implement the mosfet version shown in the first link on your OP,  'lou's.home'. A 2n7000 from jaycar or altronics should work.
Just add a discreet LED  and series resistor.

[gcwills]

Thanks low impedance for your post. As I'm using common cathode displays I attempted  Rich's PNP with no success and also the 2n7000 arrangement with no results.
As the link that you refer to indicates, the methodology Fluke used to drive the + and - LCD display is unusual,,,,,

[lowimpedance]

You are using a separate LED ?. If so it can be used as a 'common anode', as in the arrangements in the first two links (OP) should be ok, either NPN or mosfet. Strange you are having difficulties, noting I have not needed to try this MOD myself as my 8050 display is good. Maybe post a schematic of how you wired yours precisely. Other possibility the 4054 is faulty. 

[gcwills]

You are using a separate LED ?. If so it can be used as a 'common anode', as in the arrangements in the first two links (OP) should be ok, either NPN or mosfet. Strange you are having difficulties, noting I have not needed to try this MOD myself as my 8050 display is good. Maybe post a schematic of how you wired yours precisely. Other possibility the 4054 is faulty.

Yes I think the pin 4 output in U11 is faulty. I'll check it out. 

[gcwills]

Another update!

Further tests on my 8050A to determine the non-functional "minus" display issue revealed that U11 was not faulty.

Rather, the issue was traced to an error in the PNP transistor configuration that drives the minus LED. The schematic I linked earlier shows the transistor emitter connected to U11 pin 5 (the + output) and the base connected to pin 4 (the - output) via a 3K9.
Those connections should be reversed so that pin 4 connects to the emitter and pin 5 connects to the base via the 3K9. Making this change now provides a working "minus" LED.

Some observations: there is some confusion created with other online 8050A LED display mods as to how this interface is wired. Mr Modemhead has the emitter connected to pin 4 (-) and base to pin 5 (+). However the https://lous.home.xs4all.nl/fluke/Fluke8050Asite.html mod shows the BS170 source connected to pin 5 rather than pin 4 and the Rich Williams schematic follows this arrangement. Better brains than mine could explain the differences?