Beogram 4004: START Key Not Working

A customer from New Zealand recently ordered a set of Beogram 400x transport bushings from the Beolover Shapeways Store for his 4004 (5525) model. He sent me a picture after he installed them:

Pretty! I like them in white!

A while later he contacted the Beolover Customer Service Department ;-) about an issue with the START button on his deck. When pressed nothing would happen, while he was able to start the deck by first pressing the 33 button and then using the "<" button to drive the tone arm towards the start groove of the vinyl. Furthermore, the START button worked once the arm was on the vinyl. This indicated that the switch itself was o.k.

The fact that the 33 button worked told me that the circuit mostly worked, and that the fault must have its root cause in the SO-switch override circuit that is activated by the START button. Here is the relevant section of my annotated version of the circuit diagram:

This is how it works: When the carriage is in its 'home' position the SO (switch off) switch pulls the base of IC1 down which disables the 21V power supply, turning the deck off. When start is pressed, 30V are applied to the base of IC1 via R5/D9/R3/D6 (follow the red dashed line from the << ON (=START) switch). The 22V Zener diode D7 makes sure that the voltage at the base is 22V. This turns the deck on. D5 prevents a short circuit to GND via the SO switch, while this process is unfolding. The <<ON switch also turns on the carriage motor (via the red dashed line to he left at the branch point below the <<ON switch) and then the carriage starts moving. After it clears the SO switch the <<ON override is no longer needed. C2 makes sure that the override is in place long enough until the SO switch is open.

The 33 button provides another way to override the SO switch, but it does not turn on the carriage motor. Only the platter starts turning. That is why my customer had to press the < button to move the carriage while pressing the 33 button to drive it far enough until the SO switch was open. After that the deck would work normally.

The above considerations immediately suggested to look for a problem in the R5/D9/R3/D6 path. And indeed, there was a dead solder joint on D6, which prevented the override voltage from making it to the base of IC1. A bit of soldering, and the START button started working properly.

Wonderful! Another Beogram back in business! This is Beolove...;-)!