Beogram 4002 (5524): A Fun Afternoon with the Electronic Switch

I am making good progress with the restoration of a Beogram 4002 (Type 5524) that I recently received from Singapore.

Excitingly this unit exhibited an hitherto unexperienced issue:

After rebuilding the PCBs I tested the unit. After pressing START (<<) the following happened:

The carriage started moving and the solenoid immediately engaged lowering the arm. When I let the START key go, the carriage immediately stopped with the arm down. Only continuously pressing the START key kept the carriage moving. The arm would always go down as soon as I let the key go.

The STOP (>>) key behaved similarly: The arm lifted and the carriage started moving back home, but as soon as I let the key go, the carriage stopped.

Normal behavior would be that the carriage keeps going after pressing the START or STOP keys, and only <, >, and up/down would stop it.

After a bit of head scratching I thought that most likely one of the two transistors in the Electronic Switch responsible for arm lowering/lifting (TR14/15) might be malfunctioning. I replaced them on this hunch and tried again. But no cigar, this did not fix it!

Very strange, I thought! So I implanted the main PCB of this Beogram into my own 4002 and pressed START. Everything worked perfectly: The carriage kept moving after I let the START key go, and it stopped at the LP setdown point and the solenoid engaged to lower the arm.

This immediately suggested that the board was o.k., and that the problem originated from a component not on the board! But what could influence the Electronic Switch and solenoid in this way? There is really nothing off the main board that participates in the active control of the Beogram except the keypad and the board underneath the carriage that contains the carriage position sensor. 

I put the board back into the unit from Singapore and exchanged the keypad with one of my own. Nothing changed. Then I replaced the carriage PCB with the position sensor. Also no effect. Same behavior.

The only connector on the main board untried at this point (except the RPM panel, which I ruled out based on its purpose in this design) was P4. So I pulled the header out and tried again. This restored the normal << and >> behaviors, i.e. the carriage kept moving until I pressed another button. Of course the arm would not lower with P4 unplugged since the solenoid gets powered via this connector. But this told me that something connecting via P4 was most likely the culprit! Promising!

So I broke out the P4 connections with some jumper wires:

This allowed me to disconnect each of the 10 signals individually and see if this would change the behavior.

And lo and behold, when I got to pin 6 and tried again after disconnecting it, everything worked normally!

This was a bit of a surprise since P4-6 carries the signal for the output relay on the output board, i.e. it is not an input to the main board circuit, but an output from it for controlling the output board!

I looked at the circuit and it occurred to me that this pin has access to the base of the solenoid activating transistor TR9 as well as to the bases of the Electronic Switch transistors TR12 (<<) and TR13 (>>) via R69 and R73. The plot thickened!

This shows the relevant section of the main PCB circuit diagram (click on the diagram to see it in full resolution):

Pin 6 of P4 connects to the base of TR9, which needs to be pulled up to activate the solenoid. This connection basically causes the output relay on the output board to activate as soon as the solenoid gets activated. The base of TR9 also connects to the electronic switch since it gets activated from there (dashed red line). If we turn this around, of course P4-6 could be used to control the solenoid and influence the electronic switch!

This diagram shows the circuit on the output board:

Normally the signal for the relay activation comes in on pin 1 of P8 on this board. It basically turns on 8IC1 once 8C1 is charged up sufficiently (this results in a delay until after the needle pops into the groove). Once 8IC1 is conductive, the relay engages and the outputs are opened up.

After I understood this connection it occurred to me that most likely 8IC1 was the root cause of the observed malfunction! I measured the voltage at P4-6 after pressing << and got 13.5V, which is plenty for activating TR9, TR12 and TR13. 

I replaced 8IC1 with a MPSA13 Darlington and tried again. Everything was working again properly! The carriage found the LP setdown point and the solenoid activated. Beolovely!

On to completing the restoration after an afternoon of fun exploration of the Beogram 4002 circuit!..;-).

Addendum in Proof:

I guess I was not done yet. After a few more tests the newly implanted MPSA13 Darlington died again and the immediate solenoid activation returned.  I should have asked myself yesterday why 8IC1 had died in the first place!...;-). I immediately knew where to look: The spark snubber diode 8D2 that short circuits the inductive voltage peak when the relay turns off when the arm is up. I extracted it and tested it with my transistor tester:

It had become a 6.6 Ohm resistor! I replaced it with a beefier 1N4004 and put in another MPSA13 (a great idea to buy such components by the bag...;-):

And everything was ok again! Let's see how long this will last! The joys of beoloving!...;-)