Monday, January 20, 2014

Fluke 8050A display repair

I had an old Fluke 8050A from 1979 with a broken LCD display laying around. LCD-problems are very common with these old units, and since there are no replacement parts to be found, I tried to rescue the unit from the junkyard by replacing the LCD with a 7-segment LED display.

A few people have done similar repairs successfully (e.g this one and this guy), but i found this one particularly interesting since it uses a ATMega328 to interface with the Fluke, and I happen to have most of the components laying around.

Fluke 8050A with a new LED display.

In short, the 8050A uses multiplexed data from a 3870 microcontroller. The multiplexed data lines are all available on connector J1 on the display board. These are connected to the ATMega, which demultiplexes the data and communicates to a MAX7219 LED driver which again drives six common cathode 7-segment leds. You should read The Belfry blog for instructions on how to do this. I just downloaded his code and did not change anything.

Breadboarding the circuit connected to the Fluke 8050A
The first I did was to breadboard the circuit. I did this mainly to make sure the display was working and to familiarize myself with the MAX7219. I found that the power supply on the 8050 was very unstable, and traced the problem down to the NiCad batteries. They were from 1979, so no wonder they had to be replaced. Original battery-packages are hard to find, but I replaced them with four sub C 2500mAh 1.2V NiCads, and after some hours of charging, the meter was running fine.

The circuit soldered on two stripboards. It is a tight fit.

The circuit was then soldered on two stripboards, one which contained the ATMega328, 16Mhz crystal, capacitrors and the MAX7219 with some mandatory components; and a second stripboard with the six 7-segment LEDs. The two circuit boards were fitted (almost) in the same space as the original display. I used the glass and the plastic frame from the original LED, and with some sugru, it all fitted quite nicely and sturdy inside the Fluke 8050A.

So, I now have a 1979 Fluke with fresh batteries and a brand new display which is way better than the original LCD, ready for another 30 years of duty.