The oscilloscope shows that the DR signal resolves to 6.5V when the detector arm isn't detecting the platter ribs (so it thinks there is a record present). When the detector arm detects the platter ribs, it drops DR to a logic low signal (0.08V). That low DR signal prevents the turntable logic from dropping the tonearm. The bad Beogram 4000 unit is stuck with the DR signal always 0.08V so that is a big problem. My guess is the probe mishap yesterday burned out a component or two in the arm detector circuit in the main board. Tomorrow I will start trouble-shooting those components.

The

The detector arm circuit on the main board is where I suspect a failed part is that is preventing my Beogram tonearm lowering solenoid and position detection sensor logic from working. This detector arm circuit sends a key signal (DR) to the control logic board and the non-functioning Beogram 4000 has that signal stuck low (0.08V). That tells the logic board that there is not record present.

I spent some time today to use my oscilloscope to measure each of those points on the schematic where a signal is shown. I measured them on my working Beogram 4000, then checked and compared the measurements from the non-functioning Beogram. Later I may post some pictures of each of those measurements.

Fortunately the non-functioning Beogram is getting the signal from the detector arm. It also processes the signal through the first three measurement points (up to the 1VR7 trimmer) before 1TR15. The measurement between the collector of 1TR15 and the base of 1TR16 fails. Instead of a waveform I get a constant 6V. So here is the root of the problem.

I checked 1TR15 first and it passed. That leaves 1TR16. It fails -

I continued on and checked the other transistors that go to the +6VDC rail. They were good but if you look at the schematic, 1TR16 is different in that it connects directly to the +6VDC rail where the others have a resistor between them to the rail. My probe mishap blew out two 6V lamps and now I see it also took out a transistor. The components it blew out were all components without a resistor to limit the current through them. Only the 33 RPM (and not the 45 RPM) lamp blew because the Beogram was in 33 RPM mode. The detector arm lamp blew because it is directly connected to the +6VDC rail and finally, 1TR16 is the lone transistor connected to the rail.

I currently have 1TR16 removed from the main board. With it out of there the Beogram of course does not function correctly. Interestingly though, it does now stop at the 30cm LP position and the tonearm lowering solenoid engages. So I believe I am close to having this turntable back where I can start final adjustments. All I need is to find a replacement NPN transistor for 1TR16 (B&O part: 8320095). The service manual says it is a BC239B or BC184BK transistor. I'll have to see if I have a spare or a substitute.

HI

I may need to get the logic circuits checked in my Beogram 4000, are the ares highlighted in green responsible for the solenoid and tone arm lowering function?

thanks

Kevin

The Ǭ₁ signal is generated by logic on the board and controls the tonearm lowering and raising solenoid. There are a number of signals from the tonearm transport buttons and the control panel buttons that factor into that logic. So if Ǭ₁ is not firing (it should go from around six volts to zero volts to lower the arm) when a signal calls for the tonearm to lower, then the solenoid won't react. However, since Ǭ₁ is controlled by several signals you have to check all of them. Start by checking OFF, ES, DS and W0. The first three (OFF, ES and DS) have to be high. Then the W0 signal (normally six volts) briefly goes low, then back high, when the command to lower the arm occurs. That short, low pulse causes Ǭ₁ to go high which, in turn, latches itself in that position until an event happens to raise the arm (ES, raise arm, forward, reverse or off buttons).

Currently my tonearm lowering solenoid is firing again but I have other control logic problems involving the tonearm drive motor that I have to trouble-shoot now.

Good luck with checking your signals and be careful with the measurement probes. It is best to unplug the Beogram when moving probes. I found out the hard way.

This has been an amazing thing to read and see, I'm truly startled by your dedication to this beogram 4000.

I have done many beogram lids and have made really poor lids look so much better, even getting many scratches out.

The only thing i noted was that the beogram 4000 lid was harder to work on, I have one lid I was going to do some restoration on again

after spending some time on it before, but I want to do some more work on it.

I am also needing to get my Beogram 4000 tested, I've done as much as i can but the solenoid is not engaging.

i suspect it could be the logic circuits but hopefully I will be able to get that checked soon.

Great work

cheers

Thanks Kevin,

I have the materials to fix the Beogram dust covers but so far I haven't done one. I need to go ahead and learn the technique. I even have a badly damaged one I can practice on so I don't really have an excuse not to.

Tonight I went over my latest problem, the tonearm motor having ceased working. Luckily the problem was minor and typical. A wire had broken off one of the terminals of the main circuit board. It was hard to see and I found it while working back through all of the control signals. The wire is back on now and the tonearm drive motor is working fine again.

I think I am down to two electrical problems. There is still an issue with the solenoid on the tonearm lowering. The symptom is when I start the Beogram playing the tonearm correctly moves to the 30cm position and stops. Then there is about a 30 second delay before the solenoid finally engages and lowers.

The other problem is the detector arm is not detecting an empty platter. Even though I could see activity on the detection arm circuit on the main board, I think the signal is too weak. I may just need to reposition the detector arm lamp to improve the detection sensitivity. I guess I could also have a failing photo resistor in the arm. To test that theory out I used a powerful LED flashlight to boost the amount of light on the platter when the detector arm was trying to detect a record. That did the trick. When I hit the correct spot the tonearm assembly keeps going until it determines there isn't a record. I was also able to observe it change speeds to 45 RPM at the 17cm spot for the first time.
 

It looks like one of the two problems is fixed now. I resoldered and repositioned the detector arm lamp and it started correctly detecting the platter ribs. The tonearm travels the entire platter now and returns home when it doesn't detect a record. The final test was to check if it detects a 45 RPM single. It does so the 17cm sensor switch is in the correct position and the detector arm appears to be working fine. The tonearm also lowered but it still has the 30 second delay.

Is it the relay or the damper cylinder that causes the delay