NAD 4020 Display Lamp Replacement

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Hello -
I have an NAD 4020 tuner and the display light has gone out. Would anyone happen to know what value the bulb is and if it's still available (source for replacement)?

Anyone replace with LED? As the current bulb appears to be to one side of the display I'm wondering if it would still light the full display as the bulb does today. (I'm assuming the LED has less lit surface area to shine on the display.)

Thanks,
Comsat
 
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The bulb on these appears to be fed directly from an AC winding on the mains transformer… which means you can measure the voltage across your failed bulb (AC voltage range on meter) to estimate the voltage.

An LED might be possible but you would need to power it from a DC rail (easiest solution) and because the current needed is so small it shouldn't cause any problem. A super bright white LED and suitable limiting resistor would be worth experimenting with.
 

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PRR

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That same power winding feeds an LED! (With AC!) 220r series resistor. So we have some rough idea what it is.

A too-simple analysis: assume 20mA in 220r, 4.4V. Add 1.6V for LED. 4.4V+1.6V- 6.0V. It may be a 6V winding.

Less-simple: feeding AC to an LED makes a funny waveform, and the average current will be less. But they may not have needed full 20mA in a small power indicator LED. So maybe 6.3VAC, a standard value.

1.6V is for Red LED. White LEDs (such as you want for dial illumination) need more. You want to start from more than ~6V.

There is raw +23V at C401. A 1K 1W resistor from here to LED to ground (other end C401) would feed an LED (or several LEDs in series) with safe steady current.
 
> There is raw +23V at C401. A 1K 1W resistor from here to LED to ground (other end C401) would feed an LED (or several LEDs in series) with safe steady current.


My level of expertise here is _low_ but I follow where you are going here and likely have a 1W resister kicking around in a parts drawer. In a world without Radio Shack where does one go for such small parts orders?


And, out of curiosity what are the chances the incandescent lamp is still available? Slim to none?
 
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Post a picture of the lamp and we'll see if it looks like anything common. There are lots of small lamps available in lots of different shapes and sizes ;)

LED's are something you would have to experiment with. Modern ones can be blindingly bright on a millamp or two. Its a case of experimentation. A little opaque plastic can work wonders as a diffuser to give a more even light. Its an option well worth trying imo.

Digikey for parts ?

Lamps - Incandescents, Neons | Optoelectronics | DigiKey
 

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The bulb on my NAD 4020A Tuner had blown the same as mentioned in this thread. I wanted a quick fix and to keep it fairly original, so I bought a similar bulb and replaced it.

The original bulb measures 5mm diameter, and 18mm long. The power measured on the two white wires that feed it was 6.6V ac, but this is without any load/lamp. I therefore thought it must need a 6 - 6.3V bulb of the dimensions above. I bought a 6V 50mA E5 lamp (2 for £1.49 + postage (£1.20 - UK) on everyones favourite auction site, and soldered it on the original wires with some heatshrink to protect it all.

I read on another site that the bulb doesn't light the display in the traditional way you'd expect it to (shining light "onto" the numbers), but aligns with the side of the glass that the numbers are in, and uses it as a sort of prism. This is why the bulb is pushed through a rubber grommet then sealed in place. The alignment is said to be quite critical to get it to shine down the glass okay. The other site suggested turning all the lights off and adjusting the bulb position that way, to get it optimum. DON'T DO IT THIS WAY! The mains AC switch is right near where your fingers would be when you're adjusting the bulb.
I already had the front panel off, so decided to align it with the power off, and using line of site to position it. Then powering it on and see if the brightest part of the filament was square on to the glass, and adjusting further if needed, with the power off.

The display isn't bright in normal lighting, and I had to turn all the lights off to check it was on. It may be brighter with a stronger bulb, but I was happy with it.

Hopefully the attached pictures will help other too. Rob
 

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Neat job with the heatshrink :)

The display illumination looks very even in the picture... and if it is not bright enough then its really a case of experimentation with different bulbs I would guess.

LED's could be an option. It is easy to rig up a high brightness one with a resistor and a 9 volt battery as a test rig and see if it gets you better light output.
 
I installed the bulb but it's so dim it might as well not exist. I think I want to move to an LED so I won't have to open it up again. Someone suggested they used an LED (sku: 119202 from here: WJOE Radio) and it was a drop in. Do I assume it has a resistor in-line? Would polarity matter with the LED?
 
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An LED is definitely NOT drop in. LED's are current driven rather than voltage driven and tbh the easiest solution is to run it from one of the DC rails within the the unit, for example from the supply across C401 or C402.

Polarity is important and you also need a suitable resistor in series with the LED.

The current draw is small and modern LED's would be bright at just a few milliamps. For example a 20v supply would need around 1k5 to 2k to give about 10ma LED current.

You would have to experiment.

LED's are cheap, those individual ones are very expensive (edit... actually not so bad, I was looking at the X10 price). Get a few and experiment.
 
After some trial and error I was able to bend the leads of the LED at a 90 degree angle, attach a resistor to the positive lead, and thread the works through the grommet that held the original bulb. At this point I had the LED pointing toward the display. I wired the LED to the two white (6V) leads and used some hot glue to keep the leads of the LED from coming into contact with anything else and hold the LED in place. It looks great.
 
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You're welcome :) and if you can see any flicker (such as when looking out of the corner of the eye) then just run it off the DC rails instead after increasing the value of the resistor to suit... or adding a second in series to what you have now.
 
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