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Wednesday, November 25, 2020

Beogram 4000: A Fun Evening with a Smoking Solenoid Resistor

This is another installment of my 'A Fun Evening with...' series...;-). A Beogram 4000 that I restored this summer came back with a non-working arm lowering mechanism and the emission of a 'burnt smell'.

Oh well...I extracted it from the box and put it on the bench and removed the aluminum plates for a closer look.

After I plugged in and pressed ON, the unit started normally, but at the LP set-down point the arm would not lower. Instead the unit gave me a vivid demonstration of 'burnt smell', while a thin plume of smoke rapidly developed on the left side of the PCB that carries the carriage switches.

I quickly pulled the plug and lifted the PCB and found a browned solenoid resistor (7R1) and a blackened board:

I immediately thought the culprit might be the solenoid power transistor (0TR4) that regulates the current through the coil:

This TIP41 is usually a pretty resilient device, but in the case of the solenoid it also sees some pretty significant currents. I extracted it and put it into my transistor tester. And it tested seemingly o.k.:
This threw me a bit off the right track. First, I thought maybe the resistor itself had an issue (unlikely since it is wire wound, and it measured at 9 Ohm), then I suspected the solenoid (but it had its prescribed ~9.5 Ohm, i.e. seemed o.k.). 
I plugged the unit in for another measurement, and now the solenoid immediately activated after I plugged it in. Something had gotten worse. Since the solenoid is directly connected after the rectifier of the 24V rail this now really suggested that the transistor now was a permanent short circuit. 

So I replaced the transistor with a new TIP41C: 
Then I measured the original one again, and now it came up as a voltage divider, supporting my hypothesis:
Another good example to not blindly trust a transistor tester. They test at low currents and this can mean that results are displayed that have nothing to do with the actual situation the device is seeing in its circuit.

After bolting the TIP41C to the enclosure bottom, I plugged the unit in, and the solenoid did not activate anymore. Encouraged, I pressed ON and now everything worked as it should. The carriage went to the LP set-down point and the arm lowered. At that point I decided that I better also replace the solenoid resistor

and the two transistors that drive the TIP, 1TR10 and 1TR11:
I replaced the TR11 with a NTE128 and TR10 with a new BC547B:
The reason to replace these two was that in an earlier 4000 restoration TR10 was out of spec with a too low gain, and I increasingly think that transistors where the gain is critical or that carry a high current may need to be replaced during a full restoration of a Beogram 4000. Life is a learning curve! This is Beolove!

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