Beocord 9000: Peak Program Meter Scale Illumination with a Red/Green LED

After I put in the rebuilt 7-segment LED display back into the Beocord 9000 that I am restoring right now, I had to realize that the light bulb that illuminates the dB scale of the peak program meter broke off one of its leads. This happens often to these old light bulbs. They seem to mostly die this way. The leads oxidize and then break off when stressed. And there was definitely a bit of stress when wrangling the display bezel back over the display PCBs.

This is how it looked before the repair. No numbers show in between the PPM LEDs:

I examined the situation. The bulb sticks into a total internal reflection wave guide that feeds light into the space between the PPM LEDs. Matching the positions of the numbers in the mask in the bezel the surface of the wave guide is roughed a bit to diffuse some light out, hence back-illuminating the numbers. Here is a picture of the bulb as it sticks its head into the guide:

The bulb is held in place by a 'cork' that is pressed into an orifice below the bulb. Here is a picture from the other side of the PCB:

I extracted the bulb and the cork:

I aways try to replace these sensitive bulbs with LED based replacements to achieve better shipping survivability as well as longterm stability (I hope they will still sell cassette tapes in 30 years...;-). The bulb has a 5mm diameter, and the waveguide has an opening that is a bit larger. I encountered a similar situation when I upgraded the tonearm position indicator illumination of the Beogram 4000 that I rebuilt recently. For that case I developed a special PCB that holds two red/green SMD LEDs to achieve an incandescent look and a red scale indicator. I checked and found that these PCBs perfectly fit into the cavity in the waveguide. So I adapted one of them by just soldering one LED on it and bridging the space of the second one that is in series on this board.

The Beocord 9000 scale illumination also needs red and green LEDs since the positive dB values show in red, while the negative ones appear in green. An amber LED would not work since LEDs generate their light by a semiconductor transition, i.e. there is only the specific wavelength of the particular LED color present. So if red or green light is filtered out to achieve red or green scale illuminations amber does not fit the filters, i.e. the light will be suppressed or show amber at a low intensity, but not red or green.

Here is a picture of the adapted PCB:

I used a high brightness red/green LED (Newark 75R0619)  in combination with a 499 Ohm resistor for the red LED and a 2k resistor for the green one, which has a higher luminescent efficiency. This adapted the LEDs to the 6.2 V bulb supply and achieved the proper brightness.

This picture shows it in action after soldering it into the light bulb spot: 

It is held securely in place by two strong leads from below. Luckily the for-Beogram 4000-designed PCB had the "+" and "-" tabs in the right orientation, so the jumpers are not crossed:

Here is a shot of the wave guide under LED illumination:

And with bezel installed:

For comparison: This is the display in my own Beocord 9000 that still has the original light bulb:

Very similar appearance. The LED version is actually a bit more color-true since incandescent light bulbs are not very good at making green light. The LED illumination is also a bit more even since the light is piped into the guide in a directed fashion due to the narrow emission angle of LEDs.