Beogram 4000: A Fun Evening with a Prematurely Activating Solenoid

I am making good progress with the Beogram 4000 that I recently started to work on. After rebuilding most of the deck, I finally plugged it in for the first time and started it up. The carriage started moving in search of the LP setdown point. It found it and stopped, but then nothing. No solenoid activation to lower the arm. I performed a visual inspection of the wire harness and the solenoid transistor 0TR4, which is mounted directly to the enclosure bottom in search of heat transfer. I immediately saw that a thin grey wire had come off from one of the terminals of the SK3054 NPN power transistor (later models usually seem to have a TIP41 in this position):

An interesting question at this point was, however, to which terminal should it be connected. I did a bit of contact tracing and comparing with the schematic in the manual:

It turned out that the emitter of 0TR4 has three wires connected, a yellow one creating a feedback to the base of the initial control transistor 1TR10, a black one to the current limiting resistor 7R1, and a thin grey one to the switch that short circuits 7R1 during initial activation of the solenoid. This explained the non-action of the solenoid at the LP setdown point: If the solenoid is not connected directly through ground via the grey wire during the initial lowering of the arm, not enough current flows and the solenoid is not strong enough to act against the springs in the arm lowering mechanism. I reattached the grey wire to the emitter,

which restored the solenoid function. So far so good.

Unfortunately, I immediately realized when I tried out starting the unit that the solenoid now briefly activated right after pressing the ON key. After this brief 'twitch' of the solenoid everything proceeded normally. The arm stayed up and at the LP point it was lowered.

First I thought there was a start-up delay issue with the delay circuit that sets the flip-flops in the logic control circuit when the unit is powered up after ON is pressed. But a check of the Q1' signal with the oscilloscope revealed a normal behavior:

The yellow trace shows the Q1' logic signal, and the blue trace shows the signal at the base of 1TR20, which is responsible for generating a short delay to set the flip-flops to the proper start states. The yellow trace essentially confirms that the Q1' signal that controls the solenoid is indeed going 'high' when the ON key is pressed, i.e. the solenoid should not twitch as far as the logic circuit is concerned.

After a bit of head scratching it occurred to me that the fault most likely was within the solenoid circuit itself. So I measured the signal at the base and the collector of 1TR10, in an attempt to follow the execution of the Q1' 'command'. This shows the traces I measured:

The blue trace is the signal at the base of 1TR10 and the yellow at the collector. This explained the behavior. The collector only sluggishly follows the signal at the base. The base signal turns the transistor on, but the voltage at the collector only collapses over about 30 ms, in effect creating a situation that for a brief moment is able to turn on 1TR11, and with that 0TR4, and so the solenoid gets a brief current boost, which made it twitch. After the 30 ms the signal is close to 0V, and the solenoid turns off.

I had a closer look at 1TR10, and the flux residue on its solder points already suggested that some previous intervention had taken place:

I flipped the board around and found this transistor installed:

Clearly a non-spec type, a SK3122. The manual prescribes a BC239B and a few alternate BC types. I pulled the data sheet for the SK3122, and it suggested, however, that this type should be ok for this circuit. But when I checked it with my transistor tester, it turned out that its hFE gain value was too low:

It should be at least 120 according to the data sheet. Out of spec gain seems to be an increasingly common issue with small transistors of this vintage.

I replaced it with a new 1N2222 with an hFE of 210, and this fixed the issue. No more twitching of the solenoid after pressing start. Out of curiosity, I measured the collector signal at 1TR10 with the oscilloscope, and indeed the reaction of the collector signal was now much more swift:

So it seems this Beogram has a working solenoid circuit again!