I'm starting to move all my pages away from this ISP by saving everything to PDF format and storing it in Dropbox. Nothing will be lost, just stored in a way that I can manage a little easier.
You should be redirected to my new website in 30 seconds.
If the redirect doesn't work or you can't bear the suspense
you can go there right now.
I only wanted to do this once and not have to think about changing lamps in the future so I thought I would use leds instead of incandescent lamps. Now whilst the green display looks OK and I'm sure there are many arguments regarding color selection to minimize eye fatigue etc. I don't expect to be staring at the display for hours on end. I decided to give the set a cosmetic boost and settled on the currently very fashionable blue for the display. I went out and purchased four high brightness 3mm blue leds. These leds are rated at 1000 mcd and are quite stunning.
The display assembly is quite easy to access and with a little care it is possible to remove the old lamps and install the new leds without taking the display assembly out of the radio. The display lamps are all in parallel and the open circuit supply voltage is around 7.3 volts. This is supplied via the black and yellow wires that plug in to the back of the display board. These wires run to the power supply pcb.
I don't have a service manual for the set so I had to trace the wiring out to see how it worked. A bit of snooping about with a multi-meter showed that the dimmer switch toggled a control voltage on the supply pcb which probably meant there was a transistor involved somewhere. Tracing the circuit a little it seemed that there was more than one transistor involved in the process and as it didn't look like the last transistor in the chain had a collector resistor I decided to put a 330 ohm resistor in series with the leds. This gave a total led current of 12mA which was just fine.
I cut the track leading from the collector of the transistor to the yellow lead and put the resistor in series with it. A bit of tape over the tracks and some sleeving for the resistor legs makes sure there are no shorts.
I took a bit of time bending the leads for the leds to make sure that there was plenty of length to absorb some of the heat during soldering. I've never had much success with soldering very close to the body of a 3mm led. I've found the closest I can get is about 10 - 12mm from the body ever with a 600 degree tip. Anyway this is how I bent the leads and as it turned out it made it easy to hold whilst soldering and the extra lead length makes it easy to push the led centrally into the hole. Make sure the led goes all the way into the hole.
The finished product looks magnificent and the blue is particularly striking. It took about an hour to complete but the results are worth it. The picture doesn't really do it justice. The dark patches you see in this image are probably due to the camera. Whilst the dark areas are visible by eye they are not as prominent as in the image.