Mastech MS8040 bench DMM back-light problem [solved]

[enut11]

Hi. I have just purchased a Mastech MS8040 bench multimeter. It has 5 digits and good accuracy for the price (~$200AU). Sits on my bench and gets used most days.

The MS8040 has a large, easy to read LCD display with back-light capability.

My problem: the back-light has to be switched on manually and only stays on for less than 1 min, despite the unit being powered from 240vAC.

When I am taking lots of measurements, it is a pain to continually press the back-light button to read the display.

Any suggestions on how to modify the unit so that the back-light stays on permanently or at least 10 min or more?
enut11

[tautech]

If there's no menu option to permanently enable it a hack should be possible.
If it lights instantly with a push and hold of the button a bodge wire across the pads or terminals nearby should do the trick.
Have you tried multiple pushes to see if there's a lock on feature?

[xrunner]

Sometimes the controller IC expects to see a pulse from the switch, so simply making that line closed all the time with a wire might not work, you'll have to try it. It probably has an EL backlight so you might have to look at wiring that permanently to power.

[enut11]

If there's no menu option to permanently enable it a hack should be possible.
If it lights instantly with a push and hold of the button a bodge wire across the pads or terminals nearby should do the trick.
Have you tried multiple pushes to see if there's a lock on feature?

The yellow button toggles the back-light on and off. Holding the button on just makes the buzzer scream. Multiple pushes don't work either.

[enut11]

Sometimes the controller IC expects to see a pulse from the switch, so simply making that line closed all the time with a wire might not work, you'll have to try it. It probably has an EL backlight so you might have to look at wiring that permanently to power.

What do you mean by EL?

[TheSteve]

EL = electroluminescent panel.

[xrunner]

The yellow button toggles the back-light on and off. Holding the button on just makes the buzzer scream. Multiple pushes don't work either.

That's what I thought was going to happen. It's under the control of a microcontroller or microprocessor. You'll need to hard-wire the back light to it's supply if possible.

[enut11]

Thanks All. I decided to hard wire the EL back-light panel so that it comes on with the main power switch. Works well.

The MS8040 back-light panel works off ~6 volts and draws ~60mA. The pins were marked A and K so I assumed a diode array. I disconnected the array from the main PCB.

I then wired an LM317 in constant current mode by soldering a 20 ohm 1/2W resistor between adjust and output pins. The input to the 317 was at the main filter cap (+25V).  The adj pin on the 317 was wired to the 'A' terminal on the back-light panel. The panel return 'K' was taken to a convenient earth point. Dissipation is about 1W in the 317.
enut11