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Wednesday, September 2, 2015

Beogram 4000: Replacement of Incandescent Light Bulbs in Control Panel with LEDs

The Beogram 4000 that I am restoring right now had three broken incandescent light bulbs in the control panel. There are four bulbs, two for the position indicator and one each to illuminate the RPM trimmers. When I initially turned this deck on, no light came from any of the bulbs. Nonetheless, one of the position indicator bulbs was still o.k., but since the two bulbs that illuminate this indicator are in series, this did not help. At any rate, this is a perfect demonstration how fragile these bulbs are at this age, and that a replacement of them with LEDs is a great way to enhance longevity and reliability, and to make sure the unit survives shipping.

I recently developed dedicated SMD LED based replacement units that drop in for the position indicator bulbs. I assembled two and replaced the bulbs with them. I also replaced the RPM trimmer bulbs with high intensity red through hole LEDs fitted with appropriate 3.3k current limiting resistors. This brought them down to the proper intensity, making for a gentle but clearly visible low power illumination of the trimmer slots. 

Here are some pictures of the process. This picture shows the original light bulbs in place:

This picture shows the SMD LED PCBs that I used to replace the two position indictor bulbs:

and here after installing them and putting in the two red LEDs to replace the trimmer illumination bulbs:

After I was done with this I tested the functions of the keypad, and I had to realize that the deck does not switch to 45 RPM. Pressing the key did turn on the 45 RPM trimmer illumination, but the motor stayed at 33 RPM. This meant that the control system worked fine, but that there was an issue at the executive end of things. I traced the signal and found out that the relay that is responsible for changing the resonance frequency of the drive oscillator seems to have given up its ghost. It will be interesting to find a replacement.

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