Beogram 8000: Repair of Platter Brake Circuit

After replacing the electrolytic capacitors of the Beogram 8000 that I am rebuilding right now and fixing the scraping subplatter issue the unit was again functional and ran smoothly at 33.33 RPM. However, I noticed that after pressing stop and the return of the arms to the home position the platter continued to spin for a long time until it finally came to a rest. The 8000 has a low friction bearing and there is no belt due to the linear platter drive. So there is very little friction overall once power is cut to the motor stator coils. That is why the designers of the Beogram gave it a motor brake that reverses the motor phases to essentially run it in the opposite direction for bringing it to a fast stop. That makes the humming noise when the platter comes to a fast stop.

This shows the relevant portion of the circuit diagram as shown in the Technical Product Information manual:

The motor stator coils are OL1. This are the two big coils that 'hug' the sub platter (which is the rotor of the motor). These coils are phase shifted by the motor cap 4C1. This is basically the same setup like in an AC motor 4002. The motor is driven by the Drive System, which is essentially a push-pull stage that follows the grid frequency taken from a dedicated transformer coil (4T1A) and which regulates the current through the stator coils that is produced by transformer winding 4T1B (the winding shown above the stator coils in the above schematic).
The brake circuit is is essentially formed by TR 31, TR32 and TR33. Once pin 37 goes low, TR31 is turned on (and the drive system turned off via TR27 and TR28). In the process TR32 turns on which then via D39 pulls down the base of TR33. TR33 turns on and the positive half wave of the 60Hz signal from the 4T1B winding is fed into the opposite end of the left coil of OL1 via D40. This reverses the motor phase relative to the normal signal.

The 'reverse drive' aspect of the brake system can be seen by a simple test: Connect the base of TR31 to ground with the unit in standby and the platter starts spinning backward!

It turned out that the brake malfunction in this unit was caused by a broken trace. The trace that connects the collector of TR31 to R110 was cut through, probably the consequence of a previous 'repair' attempt. This broke the chain of command between TR31 and TR32, so when the microcontoller said "brake!" TR32 did nothing, and TR33 remained off, which prevented the reverse phase signal to be applied to the stator coil. Since power to the coils was still cut via TR28, the platter simply spinned until mechanical friction finally stopped it...

Below is a photo of my fix: I soldered a small piece of 'magnet wire' between the relevant solder spots: Magnet wire is good for such tasks since it is coated with a special polyurethane coating (so one can wind a magnet without making short circuits between the windings) that burns off when touched with the soldering tip. Very convenient for making connections with short pieces of wire where it would be difficult to take the insulation of mechanically with a stripper tool.

After this repair the unit 'fired on all 8' again (ah the good old days when most real cars had a V8!)

On to mechanical adjustments and fixing the cosmetic issues of this unit!