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Beogram 4004 problem

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riis
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riis posted on Fri, Feb 5 2016 1:00 PM

I have a Beogram 4004 which I haven't used for many years. I took it out of storage the other day and tried to get it going. The tone arm didn't lower, but after cleaning and lubricating and giving the solenoid a little "massage" it worked almost perfectly.

The problem is that the arm doesn't always raise as it should on return. Sometimes it works ok but sometimes the arm doesn't raise before it has travelled 1-2 cm on return.

/riis

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sonavor
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I did as Martin suggested and added a small washer (nylon M2) to the end of the plunger that makes contact with the sleeve. That should prevent the problem from coming back. Having restored a lot of Beogram 400x turntables and owning quite a few myself I don't think this is a widespread problem. I think it warranted the B&O service bulletin that Martin mentioned but it isn't something every Beogram 400x turntable has. The one I fixed is the first one I noticed an issue on. However, now knowing it can be a problem I will plan on adding an M2 washer to insure the problem doesn't happen on any new projects.

Just a note about the nylon M2 washer...The thickness of those are about 1mm. I found some fiber type M2 washers like are used as insulators for screws on circuit boards. Those washers are 0.5 mm so they might be a better choice. I am going to try that type on another solenoid.

Thanks for bringing the problem to our attention. As I said, it wasn't something I had encountered so it is great to learn something new about restoring the Beogram 400x turntable.

-sonavor

Doofer
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Brilliant! Could you upload a photo, just so as I can see clearly what you did?

sonavor
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I posted the photos here.

-sonavor

sonavor
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I have an update.

The M2 nylon washer (that is 1mm thick) does not work. It interferes with the Beogram circuit's ability to keep the solenoid engaged.
In testing it I found that the solenoid keep disengaging.  I removed the washer and now it works perfectly again.
I think I will try out a 0.5mm thick fiber washer (also and M2) and see how that does. However, I am leaning to the solution to this problem being to disassemble the plunger from the brass lever, demagnetizing the plunger and the solenoid metal frame (including the sleeve component for the plunger and the lock ring).

-sonavor

sonavor
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I updated the blog post. The 0.5mm fiber M2 washer also interferes with the solenoid operation by the Beogram 400x circuit. It is best just to demagnetize a problematic arm lowering solenoid then leave the components as they are. 

-sonavor

Orava
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Orava replied on Tue, Apr 6 2021 8:43 AM

0,2mm electrical tape is ok, do not know if it is any help anyway.

 blah-blah and photographs as needed

Doofer
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Doofer replied on Tue, Apr 6 2021 11:22 AM

The way the circuitry does the initial 'pull' at 30V then 'holds' at around 3V says a lot for the sensitivity to distance here! I guess it's no surprise that the circuit was designed to hold with zero separation between the components. The residual magnetization issue is considered to be to do with that physical contact, so it may be abolished by much smaller separations, but of course how physically durable would a washer 0.1mm (or less!) be in the long run? So you're probably right about demagnetization being the solution... 

Dillen
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Perhaps it would be better if some metal was removed to minimize the metal-metal contact area, rather than fit something in between.

Martin

Doofer
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UPDATE. Apologies for the pause, I had to wait many weeks for my MC20EN to be 'renovated' after I had noticeable distortion towards the end of 12" singles. Finally it has returned (and now appears to be labelled 'MMC4000"... the murky world of refurbishment...) and can play the end of 12" singles without distortion.

Anyhoo, I can confirm that demagnetization in situ @ 3V pk-pk, as described above... DOESN'T WORK.

I note that just testing using the buttons all still seems well: Press the button, the arm immediately lifts. However, play a full side of an LP, and at the end of the side it drags the needle - ouch! My just-refurbished $$$ needle! - up to halfway across the LP - Ouch! My lovingly-preserved 40+ year-old LP!. I'm still baffled that Bang & Olufsen didn't think it wise to inhibit 'Sled return to Start' before 'Needle lift' is confirmed by a little  switch somewhere.

- I will try demagnetizing at a higher voltage, given that the solenoid does need about 30V to initially pull in.

- I will also look further into providing that (needle & vinyl-protecting) 'Sled return inhibit' function, which will solve the problem permanently.... if slow things down a little...

Doofer
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Doofer replied on Tue, May 11 2021 3:01 PM

Progress on the 'safety switch'.

When R92 (see portion of schematic) is pulled high, the sled motor moves (relatively) quickly back to the start position.

I cut the PCB track before R92 (see photo - PCB just to the left of the control switches) and soldered a switch in place instead. When this was closed everything works as usual. When the switch is open, everything works as usual EXCEPT at the end of the LP side the sled doesn't start moving back to the start position until this switch is closed again.

I then fitted a leaf switch so that the leaf contacts close when the arm running up the left side of the solenoid is in the 'Arm Up' position. This arm even has a convenient lug sticking down on the side, allowing me initially to just cable-tie my leaf switch to the wire bundle next to the solenoid: The leaf switch needs very little force to close the contacts. I ran the wires to the main PCB alongside the existing bundle.

At the moment this works. The sled won't start moving until the solenoid releases. Since my engineering is somewhat basic (cable ties, heat shrink etc.) I will see if this works for a while before releasing any photographs of the switch fitting(!).

Doofer
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Doofer replied on Mon, May 31 2021 12:57 PM

Update: Still working i.e. no more dragged stylus. It still lifts at an angle, since my leaf switch is detecting the solenoid return NOT the oil-damped lifting of the arm. 

It occurred to me that if the leaf switch fails (stays open when the solenoid returns) I would be in a MAJOR pickle, since the sled won't return... imagine trying to remove an LP with the sled stuck in the middle, how would you lift it over the platter pin with the arm in the way?

So, I soldered a very small tilt switch - on the PCB in the place shown in the photo above - mounted at a 10 degree tilt; if the leaf switch fails = sled doesn't return, I just have to tilt the whole turntable up about 10 degrees and the sled will return as usual. (I hope!)

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