Yaesu FT-2400 display lights burned out.

Hi, I myself have never owned, had, nor had I repaired a YAESU 2400 radio but, having a brief look at the schematic the problem (as I understand it) is located on the PCB that holds the LCD screen in place. It is marked on the drawing as (CNTL UNIT). So,  as there are four  9V incandescent lamps (PL2001 – thru to – PL2004) wired in parallel and each lamp has a current limiting resistor in series. Now the whole group of lamps are fed from a 10.2V DC voltage source but via the collector terminal of a transistor (Q2010). Now the question is: when it happened, the lights died all at once or gradually ? In other words, since they are 4 lamps wired in parallel it is unlikely that they have all died at the same time unless some kind of over-voltage has surged thru Q2010 to them. Now the first hing I would do is to verify if you have around say 8-10V at the collector of Q2010 to ground and if so it means that all lamps are indeed dead and simply have them replaced with new originals/equivalent ones or you could put high brightness LED’s (with the color of your choice) and change the value of the existing resistors accordingly to further limit the current so the LED won’t burn. However,  I see that this radio also has an additional circuitry in relation with the Q2010 that has to do with dimming down the lamps intensity according to the ambient light. There is a component marked as CS2001 that seems to be a sort of photo-resistor that is exposed to the ambient light via the front panel. His signal gets buffered/amplified with an op amp (Q2009) and gets pushed to the base of Q2011 that in turn coupled with Q2010 will do the dimming thing. There could be a problem with that part of the circuit too. Even though I would say, it is …kind of unlikely but, who knows ? . I hope it helps. PS: if my interpretation of the drawings is wrong somewhere along the line, anyone…feel free to correct me.

@ovi4 Thank you for the very detailed reply! I will check the circuit out and let you know what I find. The lights all went out at once by the way.

https://imgur.com/a/kpy4mam

@ovi4  2 of 4 bulbs were bad. Replaced with blue LED’s.

[img] [/img]

@rs-electron wow !!!!   What a change in looks !  So you finally got to the bottom of it. Very good.

@ovi4 Project got put on the back burner for awhile, I’m happy the way it turned out!