20-07-2011, 09:37 PM
When I repaired my first DAC90a I needed new dial lamps for it. I knew that these had a reputation for only being slightly more effective than a couple of fag-ends behind the dial and the ones I saw on sale seemed expensive and I had no idea of their quality.
On checking just what voltage was available it didn't seem far out for LEDs.
I got ten super-bright white ones for less than a couple of dial lamps would have cost and, providing my idea worked, they should last effectively for ever.
As the power going to the lamp-holders is AC there was slightly more to it than just connecting them instead of incandescent lamps as they'd need a diode facing the other way for the half cycle when they'd be facing the wrong way.
A simple way around this would have been to connect the LEDs in inverse-parallel but I was loathe to do that as it would have necessitated changing the wiring and I wanted to leave this alone so 'Proper' lamps could be refitted if I, or a subsequent owner, wished to. For the same reason I intended mounting the LEDS in old MES bases.
This was the arrangement I eventually settled upon.
The diodes drop 0.6V which was a useful bonus as the voltage was a bit high but like this both it an the current seem well within acceptable limits.
I used BA148 diodes simply because I'd got them to hand and 1N4001s or whatever would be just as good.
The LEDs I'd bought had quite a narrow 'viewing angle' which made two spots on the dial without much light in between. This was largely mitigated by angling them to shine onto the rear of the light box.
Another concern had been that the light they produced would be too white and I did consider using yellow ones. In the end the fact that the celluloid defuser was yellowed with age as was the interior of the light box meant that this wasn't a problem.
If I were doing a set where the defuser had had to be replaced I'd either paint the interior of the light box a creamy colour or, in the interests of reversibility, line it with something of appropriate shade.
The only down-side is that when the set is switched on they immediately light up at full power and remain thus. There is slight half-wave flicker which keeps a bit or tradition at least alive.
Since then I've fitted LEDs to a second DAC90a and probably will to any others which cross my path.
On checking just what voltage was available it didn't seem far out for LEDs.
I got ten super-bright white ones for less than a couple of dial lamps would have cost and, providing my idea worked, they should last effectively for ever.
As the power going to the lamp-holders is AC there was slightly more to it than just connecting them instead of incandescent lamps as they'd need a diode facing the other way for the half cycle when they'd be facing the wrong way.
A simple way around this would have been to connect the LEDs in inverse-parallel but I was loathe to do that as it would have necessitated changing the wiring and I wanted to leave this alone so 'Proper' lamps could be refitted if I, or a subsequent owner, wished to. For the same reason I intended mounting the LEDS in old MES bases.
This was the arrangement I eventually settled upon.
The diodes drop 0.6V which was a useful bonus as the voltage was a bit high but like this both it an the current seem well within acceptable limits.
I used BA148 diodes simply because I'd got them to hand and 1N4001s or whatever would be just as good.
The LEDs I'd bought had quite a narrow 'viewing angle' which made two spots on the dial without much light in between. This was largely mitigated by angling them to shine onto the rear of the light box.
Another concern had been that the light they produced would be too white and I did consider using yellow ones. In the end the fact that the celluloid defuser was yellowed with age as was the interior of the light box meant that this wasn't a problem.
If I were doing a set where the defuser had had to be replaced I'd either paint the interior of the light box a creamy colour or, in the interests of reversibility, line it with something of appropriate shade.
The only down-side is that when the set is switched on they immediately light up at full power and remain thus. There is slight half-wave flicker which keeps a bit or tradition at least alive.
Since then I've fitted LEDs to a second DAC90a and probably will to any others which cross my path.
