Bang & Olufsen Beogram 4002 Turntable 3 Volt DC Motor Speed Not Constant

[jfrancis]

Hello H H,

I am inserting an image of the good motor motor I have (the one I am sending to you is in pieces and I need to reassemble). Also I have the mounting bracket which I will send along to you for this if even if the motor types are different. Let me know if it is the same looking motor because I am not sure I have a good plastic pulley for it

Regards,

John

 

[H H Scott]

Hello H H,

I am inserting an image of the good motor motor I have (the one I am sending to you is in pieces and I need to reassemble). Also I have the mounting bracket which I will send along to you for this if even if the motor types are different. Let me know if it is the same looking motor because I am not sure I have a good plastic pulley for it

Regards,

John

Hello John

It is 03:12am here in Cape Town and I am such an imbecilic idiot I don't deserve a motor. Two days ago, I did the impossible and made new brushes for my motor and after also discovering and repairing one of the rotor coils which was open circuit and actually got it to work although I couldn't check if the speed was OK.

For the past two days, I battled to get the control panel to work and I can't tell you how many times I took the top stainless plate off and put it back on as one cannot check its correct operation without assembling it fully.

The problem was 3 broken switches and little red rods inside which are activated by pressure on the various controls corresponding with the segments on the stainless steel slitted plate.

Anyway, I managed to repair this as well late yesterday evening.

I then fitted the motor in its outer housing and the boomerang-shaped mounting to the housing.

I mounted the motor, fitted the belt , the sub-platter and the platter and pressed "Start" and the platter started turning.

I went to collect its cartridge, a strobe disc and a record in another room.

After a minute or so later, I returned to the 4002 and found smoke pouring out of the motor and from the top left corner of its pcb.

I instantly removed the mains plug but the damage to the motor is severe.

The rotor coils are all open circuit, three tinned wires with 3 disc like varistors? had desoldered themselves, I think the tachometer coils are also damaged and there's melted solder flakes everywhere.

I guess I screwed up from tiredness as I have been working on the 4002 practically 24hrs at a stretch for days already.

The cause of this tragedy is that I plugged the motor into the P2 socket in error - really, really stupid after all this work.

I think there's not too much damage on the main circuit board as the burned area is only about 1cm square.

Your motor appears to be the same as mine and I have a good pulley. The mounting bracket is no problem as I can easily make one. There is no need to re-assemble the motor as I now know them almost blindfolded.

I suppose I can rewind the coils on the rotor but the big conundrum is those 3 'blobs' soldered to the thick, tinned wires around the rotor's commutator.

I have tried to measure them as diodes but the results don't add up. I've asked around the local forum and I've asked a knowledgeable friend with much electronic knowledge and they seem to agree these discs could be varistors of an unknown type.

I'm attaching 2 pics of the motor and will upload a third to show you how I made new brushes. All of it now gone up in smoke.

Regards

H H Scot/andre

 

[jfrancis]

Hello H H,

We won't start talking about making mistakes here on the repair of a Beogram 4002 or I would be going on about mine forever. I checked and found that I have a pulley and a bracket so will get that boxed up in the next day or so and off to you. I hope you are able to sort out the other problems as well.

Regards,

John

 

[H H Scott]

Hello H H,

We won't start talking about making mistakes here on the repair of a Beogram 4002 or I would be going on about mine forever. I checked and found that I have a pulley and a bracket so will get that boxed up in the next day or so and off to you. I hope you are able to sort out the other problems as well.

Regards,

John

Hi John

Thanks again for your generosity. I don't think there are other problems now besides the motor and the burnt top left corner of the printed circuit board, which I haven't looked at yet.

It is very easy if one is half stupid like myself, to plug the motor into the adjacent P2 socket as I did. I don't know what that socket is for - perhaps for the ac motor as fitted on some models. Who knows.

Impossible as it may sound, I have lived with this 4002 for over 20 years but it never worked correctly but then, I have never really made a concerted effort like now, to diagnose and repair its problems.

So, if you have any questions which I may be able to answer eek, please let me know.

Hereafter, I will add a pic of my 4002 and you will be able to see why I kept it all these years.

Regards

H H Scott/andre

 

[(*steve*)]

Post a detailed picture of the circuit board showing the problem area from both sides.

I'm sure someone will be able to give you a pretty good diagnosis of it...

 

[H H Scott]

Post a detailed picture of the circuit board showing the problem area from both sides.

I'm sure someone will be able to give you a pretty good diagnosis of it...

Hi (*steve*)

A good (and rainy) morning from Cape Town to you

Thanks for the kind offer. I will do so asap. From above, the affected area is small and it looks like some components have desoldered.

Regards

H H Scott/andre

 

[H H Scott]

Hi Steve

The circuit damage pics. Fortunately localised and mostly diodes.

Regards

H H Scott/andre

 

[jfrancis]

Hello H H,

The turntable looks really nice, glad you are getting it back to functioning. The key pad looks pretty good in the photographs as well which is unusual on these tables. The key pad has a painted coating I believe which wears off eventually. That wass one of the cost cutting measure over the Beogram 4000 which had brushed aluminum rocker type switching. I think someone could make a mint if they new how to refurbish the paint on the key pads.

Your motor will go out in tomorrow's mail, I got it boxed and a label printed out. Hope you see it soon.

John

 

[H H Scott]

Hi John

Thanks so much, yet again. If I can send something you need, please let me know.

The keypad on mine is definitely brushed stainless steel, Even on mine, the brushed effect has worn smoother from the previous owner's finger marks, which is clearly visible in certain light falling on it.

It may well be that B&O cut cost as the entire deck - the 4002 - was a cost cutting and simplification on the original 4000 series with their brushed aluminium control sections. So your control cover may well have a painted surface.

I however think you have the same, brushed stainless cover as mine which may be restored by 'brushing' mechanically with those 3M sanding pads available from hardware stores.

I am a bit reluctant to say it's possible to restore the original finish by hand and using a 3M sanding pad as it is quite difficult to get the 'brushed effect' in a straight, even fashion as the originals had been given this finish with a belt sanding device to get an absolute even effect.

Mine for example, just has a smooth normal bare metal sheen to it on the reverse.

It is also very tricky to get the control surface off without damage to the bits underneath.

I had no choice as it wasn't working and after some thought and a bit of daring, I removed it with a putty knife. Very carefully inserting it through the side's centre incase the knife touched the slits.

If one has to do this, it is almost 100% certain the loose rockers below will get damaged or disturbed and the control pad will malfunction, like mine did and cost me an entire day's fiddling to repair again.

Regards

H H/andre

 

[(*steve*)]

The circuit damage pics. Fortunately localised and mostly diodes.

Yes. It looks like a bridge rectifier made from discrete diodes.

I would replace all 4 diodes -- they as 1N4..something -- I'd suggest 1N4001, or 2,, or 4 etc. Read the part number. These are very easily available parts.

The board looks like it may be burnt to the point that it may be conductive in parts. If so, then you should cut that piece of board away. I'm not certain that it has got that burnt though.

Fortunately it appears that the traces are shown on the top of the board, so if any of these are damaged, you should be able to see where they were connected.

I don't see a filter capacitor near the bridge, so perhaps this is very fancy reverse voltage protection, or perhaps the current is so low that the small tantalum capacitor is enough.

Take a note of which way around the diodes go before you remove them

Remember that it is also possible that some other component may have failed (e.g. a transistor) which has resulted in the heavy load on the diodes and their subsequent (possible/probable) failure.

edit: looking at that picture again -- yes, I'd certainly suspect the board is damaged sufficiently to consider it charred and probably conductive.

 

[rwootan]

How did you rebuild the brushes?