PM3212 - Reticule illumination - Page 1
Forum section: electronics / repair

author: Greenray
author_rank: Contributor
timestamp: January 08, 2017, 09:25:08 am
content: 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.

author: Greenray
author_rank: Contributor
timestamp: January 08, 2017, 10:20:06 am
content: And now some detail.I'm sorry to haven't take pictures of the "squared" LEDS.

author: Greenray
author_rank: Contributor
timestamp: January 08, 2017, 10:22:51 am
content: How to keep stable

author: Greenray
author_rank: Contributor
timestamp: January 08, 2017, 10:26:35 am
content: The LED are too high so I need to grind a lot the dome and then polished.It's a pity have no photo.

author: Greenray
author_rank: Contributor
timestamp: January 08, 2017, 10:27:52 am
content: Not so good.

author: Greenray
author_rank: Contributor
timestamp: January 08, 2017, 10:29:00 am
content: This is the arrangement

author: Greenray
author_rank: Contributor
timestamp: January 08, 2017, 10:31:19 am
content: Minimum and maximum intensity of the illumination.Obviously my camera adjust the exposition, so you have to compare the contrast because as you can see the light of the room seems different, but it's due to the automatic exposition.One note: the red color of the reticule depends from the coloration done to the part of the tube where the LED are applied. For this I've used permanent transparent red marker.I've try other solution as a sheet of red transparent plastic, but it was absorbing too much light.