Rich, perhaps these notes refer to another mod using common anode LEDs? Your earlier notes that I followed indicate the common cathodes connect to -5V and the 4054/4056 outputs drive the LED anodes positive to illuminate them.
I must have made a typo.
Let's try again. Take a 4056, Pin 16 is 0V Pins 6,7.8 are -5V So the 4056 outputs are at either 0V for high or -5V for low. So all the LED segment cathodes should be connected to -5V.
@gcwills Very much appreciate your help to get this right and your excellent writeup and photos on EEVblog :-
8050A mod by gcwills
Revised circuit
Good luck all trying to keep these old Fluke DVM's going.
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Let's try again. Take a 4056, Pin 16 is 0V Pins 6,7.8 are -5V So the 4056 outputs are at either 0V for high or -5V for low. So all the LED segment cathodes should be connected to -5V.
@gcwills Very much appreciate your help to get this right and your excellent writeup and photos on EEVblog :-
8050A mod by gcwills
Revised circuit
Good luck all trying to keep these old Fluke DVM's going.
-
Last edited:
Photo of a Fluke 8050A I converted to 7seg LED's using common cathode 7seg LED's
Procedure followed and drawings as follows:-
1. Forty wires! the 7805 regulator gets a bit warmer so put a small heatsink on that (20mm bolt), increase size of unregulated supplies capacitors to 1000uF (reduce ripple),
2. The DP's and the -ve sign work, didn't bother with the other indicators,
3. Didn't like the power on/off switch because it runs the unregulated supplies right through the main PCB and the display PCB to get to the switch and the line transformer is always left on,
4. Wasn't bothered about relative measurement so the Power and Relative switches have gone,
5. I used common Cathode LED's and no series resistors!
6. The leading "1" needs a little help from a pull up resistor to get the brightness right,
7. The -ve sign needs a PNP transistor to work properly,
8. Be careful working near the CPU chip, its very close to the unregulated supplies,
Main Circuit with power rails highlighted
Main PCB mods
This 8050A modified in May 2021 is used frequently on my workbench
Procedure followed and drawings as follows:-
1. Forty wires! the 7805 regulator gets a bit warmer so put a small heatsink on that (20mm bolt), increase size of unregulated supplies capacitors to 1000uF (reduce ripple),
2. The DP's and the -ve sign work, didn't bother with the other indicators,
3. Didn't like the power on/off switch because it runs the unregulated supplies right through the main PCB and the display PCB to get to the switch and the line transformer is always left on,
4. Wasn't bothered about relative measurement so the Power and Relative switches have gone,
5. I used common Cathode LED's and no series resistors!
6. The leading "1" needs a little help from a pull up resistor to get the brightness right,
7. The -ve sign needs a PNP transistor to work properly,
8. Be careful working near the CPU chip, its very close to the unregulated supplies,
Main Circuit with power rails highlighted
Main PCB mods
This 8050A modified in May 2021 is used frequently on my workbench
Last edited:
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. Rich your schematic 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. This is incidentally how the A733 transistor is shown connected in the photo of your mod. 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 Rich's schematic follows this arrangement. Better brains than mine could explain the differences?
And also, when wiring up an A733 PNP transistor I discovered that the A733 in my stock was in fact a 2SA733 and not a KSA733. The 2SA733 pinout is emitter collector base and the KSA733 emitter base collector - another trap for young players!
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. Rich your schematic 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. This is incidentally how the A733 transistor is shown connected in the photo of your mod. 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 Rich's schematic follows this arrangement. Better brains than mine could explain the differences?
And also, when wiring up an A733 PNP transistor I discovered that the A733 in my stock was in fact a 2SA733 and not a KSA733. The 2SA733 pinout is emitter collector base and the KSA733 emitter base collector - another trap for young players!
I'm sorry if my error lost you time. Most likely I soldered up the KSA733 first (after working out the somewhat strange + - logic) and then went back to redraw the Fluke 8050A circuit. Thanks for finding that.
The other mods are also important, everything marked in red or green on the 8050A LED Version circuit. I also think its important to cut those unregulated supply lines because they run all over the PCB and are just waiting for the unwary to blow the entire circuit. The power button on the front panel is unwise in my opinion because power is still applied to the input mains transformer and the unregulated lines are hot even with the power button off. The mods to smoothing capacitors are also important because the LED display takes more power more smoothing is needed to keep mains ripple low.
I'm attaching the corrected circuits showing all the mods as follows:-
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Thanks Rich for your response. No problems my end re the transistor lead reversal - I was just pleased to resolve the conundrum to assist others who may be planning to convert their 8050A. And thanks again for taking the time to describe your LED conversion - I think it is the best and easiest way to use readily available common cathode displays to resuscitate these DMMs.
