BC780XLT Backlight out from polarity reversed

[gregtheham]

An apparently common (idiot) problem. 12VDC incorectly wired and blows protection diode. Also bakclight no longer works correctly.

I have replaced the protection diode, but ONLY remaining problem is backlight operation. Using MENU to test backlight options I find: HIGH and LOW setting produce NO light. Setting to OFF turns light on but a reduced brightness (perhaps lower than the low setting..I cant recall how bright LOW was)

I have tested the obvious transistor used for the dimmer, D1667, mounted on the side of case, ( as claimed on various msg boards and as per schematic) and it tests OK using usual DMM method.

I am also getting a change in voltage there, when changing the menue setting from off/low/high, but only by bypassing the dim cct at the D1667 Transistor, does full brigthness get restored. This would indicate my suspicion of the MO522 double series diode device in series with the R551 100Ohm 1/4w resitor thats parallel to the leds, is OK? ( Note I could NOT find this device onthe board as per the schematic..Wonder where it is!)

Note that diconnecting the DIM transistor changes the sysmptoms, ( light out in all settings hi,med and off)

There are 3 other gen perpose transistors in the dim cct as wellas a dide, but they seem to test ok, (while IN CCT).. Before I start unsodering SMT microscopic parts, does anyone have any experience with this problem OR can I direct you to the schematic for a look?

I think I am close, but am not a bench tech.

thanks for your input

 

[bctrl1]

The base of the D1667 (pin closest to the back of the radio) should have about 8.5volts (ref to gnd) in "HIGH" and about 6.8volts on "LOW", on "OFF" it should be nearly zero. The actual voltage will vary slightly, but that's the type of swing you should be seeing. The emitter (pin closest to the front of the radio) should be about 0.7v less than the base.

If you're not seeing this type of voltage swing at the D1667 then there is something wrong with one (or more) of the transistors on the main board. For reference, Q42 controls the "OFF" function and Q48 controls "HIGH". Q40 is used for both HIGH and LOW.

I'd suspect that Q40 is bad and/or D67 is popped. Q48 is probably OK, you would only lose the "HIGH" setting if it was bad. Q42 could also be shorted, which would also result in the same symptoms.

The quickest way to check Q42 is to just pull it from the radio. If the light stays off then you know that your problem lies with Q40/D67.