Converting a BK Precision 2831A bench multimeter to 220V supply

I just came across a BK Precision 2831A bench multimeter (for free). I do not know the age or the general reputation of the unit, but it seems to be a rather low-end 3-1/2 digit multimeter. Nevertheless, another multimeter might come in handy.

An old BK Precision 2831A (on top of something else)

The device is supposedly only meant for the US marked and is labeled 120V, so I could not test it right away. First, I thought of buying a 120V/240V transformer, but good quality ones does not come for free.

It is labeled 120V on the back side

Based on the labels on the circuit board (GDM-558D), it seems like this unit is a relabeled GW Instek multimeter of some kind. It is probably ins the same family as the GW Instek GDM-8034 (or the GDM-8135, although it has a circuit board marked GDM-625A). The accuracy of those two devices (DC volt) are 0.5% and 0.1% respectively, whereas the BK is reportedly 0.1%.

The circuit board also has some markings indicating that the transformer has two primary windings that can be coupled in parallel for 120V usage (marked as 114V on the circuit board) or in series for 240V usage (marked as 234V on the circuit board).

Typical configuration for switching between 110V and 220V with two primary windings.

Normally, units with such a transformer has a switch on the back, making it usable for both 120V and 240V mains supply, but this multimeter has no such switch. However, there are some resistors on the board that can be replaced to allow for the higher voltage setting.

Zero-ohm resistors configured for 120V (117V)  mains supply

Zero-ohm resistor configured for 240V (234V) mains supply

All there is to do is to remove the two resistors (and resolder one of them), and voila, the multimeter is ready for european voltages. In addition, the main fuse should be reduced to about 2/3 of its original size.

As indicated on the first image, the multimeter now works on 230V (or 220V/234V/240V or whatever). I cannot report on the quality on the device itself, such as the accuracy in taking measurements, but it seems to be fairly close to my Fluke 8050A so at least it is not damaged or anything. Hurray!

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.