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Using LEDs to Replace DAC90a Dial Lamps


Using LEDs to Replace DAC90a Dial Lamps

Post by Joe » Sun Jul 24, 2011 2:45 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 what 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 connected 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 within acceptable limits – 23mA at 3.6V.

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.

- Joe

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Post by crustytv » Sun Jul 24, 2011 3:01 pm

Thanks for adding this Joe, an elegant top tip indeed. I will have to give it a try.


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Post by Spot-Wobble » Thu Jul 28, 2011 12:07 pm

It would be quite easy to design a simple delay so that the LEDs only lit up slowly.
Feed the LEDs from a transistor with a nice fat electrolytic from it's base to ground. With a suitable value bias resistor from the +ve rail.
should work in theory.


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Post by FRANK.C » Thu Aug 04, 2011 7:51 pm

Hi Joe
What a great idea. I am restoring a Mullard MAS6 which needs a 4 volt bulb. I have ordered a few LED's to try it out. I will have to mount the two LED's in the same base, I will need a series resistor as well. I am interested to see how it will work out.

Thanks for sharing the idea

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Post by Michael Watterson » Sun Aug 14, 2011 12:15 pm

Badge Engineering... been going on a long time.

I'd use back to back LEDs.
If you want apparent  simulation of lamps you could use RGB Leds and adjust the R, G & B separately. You could even have the G & B resistors smaller with a Thermistor in series placed near dropper resistor or PA valve and then at power on the LEDs are orangy and get brighter and whiter as radio heats up :)

I've tried Orange /Amber and yellow LEDs as meter lamps on Amateur radio gear and it was unsatisfactory.

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Post by Electrical » Sun Aug 14, 2011 4:53 pm

Hello Frank
If you would like a few 4 volt 2 watt lamps, I can send you some. The glass bulb is quite large, about 14mm in diameter and was intended for use with Murphy radios. I can put some in the post for you if you would prefer the real thing, instead of LED’s. Please see attached photo.
Regards Stan.


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Post by FRANK.C » Sun Aug 14, 2011 5:51 pm

Hi Stan
I would love one of your bulbs,  as this radio belongs to the Mother-in-law and has been in there family for a long time, it would be nice to have a proper bulb fitted before I hand it back to her. I will send you a PM.

I ordered some ultra bright LED's from a seller on ebay. I got some yellow, white and some warm white.
The yellow were a dark yellow closer to orange so not of much use for this purpose.
The white were indeed white and of a similar colour to a daylight bulb.
The warm white were the best of the lot and there colour was close to that of a incandescent bulb.

I wired two of the warm white back to back and used a 22R series resistor. The flicker was very noticeable and I thought it may be a problem, but once fitted in the radio the diffuser behind the scale seams to even things out and it was hard to notice the flicker.


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Re: Using LEDs to Replace DAC90a Dial Lamps

Post by Goleudu Gwyn » Sat Mar 30, 2013 1:56 pm

I recently bought a Philips 22RB292 radio for the the princely sum of 99p which only needed a shot of servisol to bring it into full working order. This is one of those wide bookshelf models with a large tuning scale but unfortunately both bulbs were blown. Inside the case was a taped down A4 schematic :D which showed that the bulbs were 4V 100mA and in series! The bulbs were removed and wrapped in a rag and the glass smashed and the bayonets cleaned up. 2 yellow leds and two 100R resistors were soldered into the bayonets but the result was too dim even in a dim lit room.

I bought 50 hyperbright leds for £2-60 and two of these replaced the 'old' style leds with only one 100R resistor to drop the voltage (hyperbrights drop 3.5V not the 2V of old style leds) the resulting current being 10mA. The display is now excellent, aided in no small part by the large aluminium reflector behind the panel.

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Re: Using LEDs to Replace DAC90a Dial Lamps

Post by ianj » Sat Mar 30, 2013 2:43 pm

Yes,Ive done this before on a set,much nicer,brightly lit dial,looks good.

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Re: Using LEDs to Replace DAC90a Dial Lamps

Post by Doz » Mon May 20, 2013 1:08 pm

Is there any concern about UV emissions from LEDs degrading the plastics used for light boxes/ dials etc?

I had some early white LEDs I used in my server computer case for the HDD activity LEDs. It yellowed the white plastic case in a very short time.

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Re: Using LEDs to Replace DAC90a Dial Lamps

Post by Michael Watterson » Mon May 20, 2013 2:14 pm

I'd be concerned. :)

Anything "collectable" rather than 1980s or later every day use I'd put filament bulbs of suitable rating. On some 1980s Amateur gear prone to blowing meter lamps I've put green/yellow LED. White LEDs don't have a great life as they are really near UV and a mix of phosphors. The Phosphors degrade faster than CRT or CFL types and also most have a nasty purplish hue. True Scale lamps are under-run designs (unlike similar voltage torch bulbs) to give longer life, hence much more orangey than 3V or 6V Torch Bulbs, or CFL, Halogen, ordinary filament mains bulbs or "White" LEDs.

Even where I fitted Green/Orange/Yellow etc LEDs as scale / meter lamps it was only due to the "wheat" bulb of correct rating unavailable. For a DAC90a or other Series filament radio usually the CURRENT rating is the important figure, not voltage. Thus a pair of 6V bulbs of correct current look fine vs the original 3.5V type (a little brighter). It means mains voltage tap is out by about 7V or 8V (as they are additionally under run). The ballast resistor across the 6V bulbs can be increased or left as is.

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