Follow-up to my previous posting.
WARNING!
The design of the MS8040 display lighting circuit appears to overload the LEDs used, aging them rapidly resulting in reduced brightness (and unnecessary high drain from batteries when activated). Continuous use while on mains power significantly accellerates this aging because dissipation is increased. This posting describes my fix.
UPDATE
Having used the MS8040 on mains power for half a year (incidently, not continuously), I noted that display backlighting brightness had degraded significantly.
Note: IMG numbering continued from my previous posting.
The modification I had made earlier to keep display lighting turned on while on mains was still ok, so something else had to be wrong. Series resistor R1 was still 47 ohms. I measured 3V across it, so the current was in excess of 60 mA.
Removed the display module, removed its front bezel and the LCD display to get access to the light plate assy.
Opened the lightplate assy which exposed a small pcb strip with two - each size 0603 - LEDs (1.6 x 0.8 mm) connected in series via a 0 ohm resistor.
The LEDs had seriously discoloured by overheating (see IMG-5), which is no surprise when they are operated @ 60+ mA using mains voltage.
Connected the strip via a 47 ohm resistor to a variable power supply and recorded the u/i characteristic, see LED Module YDGHY7448H original.
Looked for new high brightness @ low current size 0603 LEDs and chose SLOAN P/N: SLO-230W-00007 (old P/N SMD-W0603-02), Peak Wavelenght: 6000-7000K, intensity min: 460 mcd, typ: 600 mcd, angle: 130 deg, Voltage min: 2.7, max: 3.3, lens diffused, If typical: 20 mA.
The repaired strip is shown on IMG-6, dimensions in [cm].
Driving it again with 60+ mA was no option, so some sort of current limiting was required.
From visual evaluation a LED current of approx. 15 mA would be more than adequate. At this current the voltage drop across the strip was 6V.
The supply voltage for a.o. the lighting circuit is either derived from mains (12V) or from battery (9V). These voltages are ORed via diodes.
For the available voltage drop of the current limiter, the battery voltage where the low battery indicator turned on was determined to be 7.6V.
As the various source voltages are diode ORed (0.7V drop) the the minimum available voltage would be 6.9V, so the series part of the limiter had to operate down to 0.9V.
The updated circuit diagram is shown in MS8040_Backlighting_mod_R1.
With the components shown, the LED current is 14.2mA, slightly temperature dependent, decreasing with temperature.
In order to keep MS8040 circuit changes to a minimum, the current limiter was inserted in place of R1 (the original current limiting resistor). On its soldering pads two new wires (white and yellow) were soldered, see IMG-7.
The piece of circuitboard containing my original modification (3 rows of conductors) was replaced by a slightly wider piece (7 rows), see IMG-8.
The connectors changed from 3 to 5 wires/pins (one wire to the power supply board, 4 wires to the PCB area under the display module). Near the solder points, the wires were fixated with a drop of glue. The opening I made in the side of the display module for the 2 wires of the previous modification was adequate for the additional 2 wires. See IMG_4 Previous posting.
The results are shown on IMG-9 and IMG-10.
Display brightness is more than adequate at significantly less current, saving both LEDs and batteries.