Beogram 4004 Issue: STOP (>>) Function Permanently Engaged

A customer from Canada recently sent me the boards of his Beogram 4004 for trouble shooting. The fault the deck exhibited was this:

When the START (<<) button was pressed the deck very briefly came on, the carriage moved a few mm and then it immediately returned home and the deck shut off. This issue typically arises when the STOP (>>) function is permanently engaged. Normal examples would be a stuck actuator under the STOP key or a damaged OFF switch on the board under the carriage that contains the carriage position sensor. 

In this case these standard root causes did not check out. When I received the boards I first tested the main PCB in one of my Beograms together with a Beolover RIAA pre-amplifier board that I had already installed in this Beogram. The RIAA board works together with any 4002 or 4004 main PCB. When I plugged the Beogram in and pressed START everything worked normally. This immediately told me that most likely the 4004 output board was the trouble maker.

I replaced the RIAA board with the 4004 output board and tested again. This indeed brought about the described malfunction. I looked at the circuit diagram of the 4004 output board and identified two transistors that are able to initiate the STOP function, TR3 and TR4:

They essentially are used by the Beomaster 2400 remote control function to initiate STOP/OFF when a different source is selected on the Beomaster or it is turned off.

P10(8) connects directly to the OFF switches, i.e. the Beomaster simply 'simulates' the actuation of the OFF switch under the carriage when it wants to shut the Beogram off:

Since TR4 needs to produce a pulse for OFF initiation it was most likely not the culprit. Dead transistors usually do not make pulses, they are either permanently off or on. So I replaced TR3 with a new BC547B and put the board back in. I pressed START and the carriage moved normally to the LP setdown point and the solenoid actuated. Perfect! I tested all the other keypad functions and everything appeared to work normally. This Beogram 4004 appears to be back in business!