Replacing the incandescent light bulbs

Summary (TL; DR)

This article shows the results on the experiments of creating a plug and play (PnP)/snap on replacement component for the old incandescent light bulbs originally placed in the RPM adjustment part of the keypad on the Beogram 4002/4004/6000. 
Since most light bulbs burn out after 40+ years and since the original parts are no longer available the scope was to make a component using only then newest technology primarily for durability and ease of use. 
These experiments are based on various results from other sources combined with knowledge within the domain of 3D printing and PCB development.
As a part of the experiments, various PCB designs and 3D models were made to engineer a component which would imitate the light of the incandescent bulb as much as possible. The is a PCB with two multi-colour LEDs and a custom-designed 3D housing which makes it possible to snap and fix the replacement in the same position as the old light bulb resulting in an experience as close as possible to the original.

Intro

When restoring old Hi-Fi equipment it is not unusual that the light bulbs normally used in the 70ies are burned out. This is especially visible on the Beogram 4000 series since the part that is used to adjust the RPM has two incandescent light bulbs and is placed on the keypad which is one of the most visible parts of the Beogram.

When restoring a Beogram without replacing the light bulbs we risk that they burn out in the near future which would be too unfortunate. Therefore I was looking for a replacement part which was harder than I thought. I was looking on eBay but couldn’t find a 24V light bulb with the same dimensions as the one used in the Beogram. Furthermore, the few I found didn’t emit light from the same part of the visible spectrum as the originals. After evenings spent on trying to find the right replacement I decided to make my own.

The Problem

Before developing the replacement I needed to take the following issues into account:


The Solution

With all the problems defined earlier in mind, I started brainstorming over possible solutions.

Preparation

My initial thoughts, since it was the easiest way, was to use a through-hole LED in series with a resistor so I ordered a couple of warm white LEDs because of their spectrum and a 1W resistor at 1kΩ 5% which should reduce the voltage to ~2V.
When the items arrived I quickly soldered it in place to check how it performed. Unfortunately, it was easy to see that the light didn’t matched enough with the original spectrum. The size of the components was also a problem for me. It would be hard to create a 3D printed housing small enough to hold the LED and the large 1W resistor in place and I’m not a fan of letting the component float only by the wires.

Therefore I quickly moved on to my next idea; Using a custom made PCB with SMD components and some sort of 3D printed holder to keep correctly it in place.
In my initial research, I stumbled upon a red-green SMD LED which would, if given the right voltage, emit light even closer to the spectrum of incandescent light. 
After receiving the LEDs I made various tests with different resistors to find the right combination so it would emit light as close to the original spectrum as possible. This was done using two resistors, one for the green LED and one for the red LED. It was also necessary to add two multicolour LEDs so the component would emit enough light. This seemed to work as expected since it emitted enough light and the light emitted was close to the original spectrum.

The Process

The next step was to design the PCB for the components. The PCB should be small enough to hold all the components while still having room for the custom printed 3D housing. It should also fixate the LEDs in the correct place so the component wouldn’t move e.g. under transport. Therefore I created a lot of different PCB designs but ended up ordering two candidates which differed in shape as you can see in the image below.


Various tests for the two different designs made me select the square PCB since the rectangular PCB didn’t allow the LEDs to get close enough to the aluminium profile which it should. 

All that was left for me to do was to create the housing for the PCB. It was important that is was as small as possible and easy to snap in place. I tried multiple different designs since it shouldn’t be in the way of the wires and since it should lift the PCB a couple of millimetres so it wouldn’t short circuit with the PCB below.

Below you’ll see one of the first designs I made. As you’ll notice it has a notch with the purpose of snapping the component in place.


Unfortunately, the wall was a little too thick in the rear part which encloses the PCB on the keypad, so it was back to the drawing board.
My second attempt featured two smaller notches (instead of one large) which allowed easier assembling, a thinner rear wall and only one small notch to enclose it to the main PCB since the original three notches were colliding with the wires. As you also can see I needed to add a small piece of heat shrink tube since we otherwise risked a short-circuit.

The Finished Product

Well, that’s it! This was surely one of the more interesting components to develop since it involved a lot of different areas.

Below you’ll find some pictures showing how the component is fixated by the housing and finally you’ll see an image showing how the light looks with the Beogram assembled. Judge for yourself but I’m satisfied! If you would like to get hands-on this part yourself you can contact me here.




With almost everything assembled the last thing for me to do before this Beogram 4002 is ready, is to restore the brushed aluminium part of the keypad. It has the classic fingerprints...