Hi.
I have repaired my old Philips PM3212 oscilloscope.
Only one of the two lamps illuminate the reticule, so, after that I saw a video on youtube, I decided to do the same thing.
Now I want show you how I have modified the circuit for the illumination of the reticule.
My first observation is that I need to direct the led light to the tube.
The incandescent lamps was directing his light in all directions, while the LEDs has an irradiation beam of few degrees.
My second observation is that if the light of the led remain closed into his beam, the light on the reticule is not uniform.
So I clip the top of all the led and then I polish it with a super fine abrasive on an accessory of my minidrill.
It's to be note that the 5 mm LEDs that I want use, was higher than his space, while now these led can stay in the support.
Always speaking about the uniformity, I'm sure that if instead of 2 LEDs I will use 4 LEDs, I will have a better result.
Furthermore, if I mount 4 LEDs also in the white plastic support under the tube, the illumination will be better more.
I want remember that the current for 4 LEDs will be the same that with 2 LEDs. Doubling the number of groups of LEDs (two instead of only one) the current needed also will double.
Every group keep a 100 Ohm in series, hold inside the white plastic support.
I've do it all, but my opinion was that in the "minimum" position of the potentiometer, the light was too much for the cases of a trace not so much intense. I think also that in the "maximum" position of the potentiometer the light was too much.
So my small modification it's been to subtract current to the potentiometer with a 18K in parallel to it.
I have changed the value of the emitter resistor to 120 Ohm, having finally the regulation of the LEDs current adjustable from 1.25 mA to 37 mA. Be quite, we have to remember that every LED group sink one half of the total current, so into the LED will flow currents adjustable from 0.62 mA to 18.5 mA.
Now I show you my schematic of that circuit.