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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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riis
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riis replied on Sat, Feb 6 2016 10:21 AM

I have measured the platter height and there is only 20-21 mm from platter to top of the tone arm. 

Can this be the cause of my problem do you think. As you can see I'm desperately hoping for a mechanical issue that I can fix to solve the problem :-)

sonavor
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riis:

I have measured the platter height and there is only 20-21 mm from platter to top of the tone arm. 

Can this be the cause of my problem do you think. As you can see I'm desperately hoping for a mechanical issue that I can fix to solve the problem :-)



I don't think so. That height doesn't affect the arm transport detection from working. You can observe what is going on by removing the Beogram deck panels and platters. When you operate the turntable this way the Beogram should think there is a record present, stop at the 30cm position and lower. From there you should be able to gently nudge the tonearm (towards the spindle center) as if it were moving while playing. When the arm reaches the point where the end of record would be detected you should see the arm lift and return to the rest position.

Doofer
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Doofer replied on Wed, Mar 24 2021 2:49 PM

I have this problem too.

- Arm raises immediately if I press the right buttons on the deck

- Carriage detects end of record and moves back to start

- BUT sometimes there is a delay between the carriage starting to move back and the arm -> needle can be dragged  several cm across the record surface before lifting. Sometimes the delay is very short e.g. the arm swings a few mm to the left as the carriage starts moving, before the stylus leaves the record.

This is clearly the wrong order of events! The arm should lift, then after a short delay the carriage should start moving... what with vinyl often being irreplaceable and replacement cartridges for these decks costing $$$... 

It's perhaps not an issue with the mechanical raising of the arm, since this responds fine, immediately, every time I press Up/Down or Stop.

It's not an issue with detecting the end of the side or the carriage motion, since the carriage does this fine every time.

So it could be a problem with how the 'end of side' signal gets to the 'raise arm' input, since both functions work fine, independently.

Of course, since the problem is intermittent, it could be that the raise mechanism is a bit sluggish, although this deck was recently serviced by Beolover, and even more recently the various linkages in the lifting solenoid setup were lubricated by myself, during which excercise I could see no problem with the lifting mechanism. 

So, question:

- Is anyone familiar enough with the Beogram circuitry to advise on where such a problem might be located? i.e. where do I put the probes of a two-channel oscilloscope to detect the 'end of side' output, 'start carriage return' input and 'lift arm' input, to see if this delay is electronic, rather than sluggishness in the lifting mechanism?

(Given that it's all discrete components, I guess a partial component failure might mean the 'end of side reached' signal, which is getting to the carriage motor system fine, might be being degraded on the way to the 'lift arm' circuitry)

Finally, if anyone is familiar with the circuitry, is there an obvious point to increase a timing capacitor value to make a more obvious gap between 'raise arm' and 'start carriage return'?

Dillen
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Dillen replied on Wed, Mar 24 2021 4:51 PM

It's a known problem - also described in a B&O tech note.
It's because the solenoid plunger gets "permanently" magnetized and won't let go when the solenoid power is removed.
Instead it hangs up for a varying amount of time.

Because you cannot take the plunger out of the solenoid without breaking something (- else you could introduce
a thin insulating washer inside to stop the plungers physical metal-metal contact),
the solution is to replace the solenoid with one from a newer deck.

Martin

Doofer
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Doofer replied on Wed, Mar 24 2021 5:56 PM

Ah, so it happens when the solenoid is pulled in for an extended period (such as playing a record side), but not when the solenoid is briefly pulled in (such as testing/pressing one of the buttons)?

Quite a design flaw!

I wonder if connecting the solenoid to AC for a few minutes (resoldering the wires possible without breaking anything) might demagnetise it, sort the problem out for a bit? What voltage would you recommend for this particular coil?

Doofer
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Doofer replied on Sun, Mar 28 2021 4:26 PM

When it's in the 'down' position, I measure 1.49V across the (10 ohm?) coil. So I'm thinking of de-soldering the live end, connecting it to a small audio amplifier with a signal generator attached, then supplying 1.5V pk-pk @100Hz or so for 30 seconds, then gradually turning the amplitude down to zero.

This may of course not work in removing residual magnetization in the plunger, but more specifically is there anything I'm missing here?... before I do some damage!...

Dillen
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Dillen replied on Sun, Mar 28 2021 5:21 PM

It wouldn't provide a permanent fix.
The problem is the metal-metal contact.

Martin

Doofer
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Doofer replied on Mon, Mar 29 2021 10:29 AM

Nor would replacing it from a dead 4004? This seems likely to be a problem that gradually worsens over time (years).

Demagnetizing might sort it out for a while.

 

Anyway, I've performed the following procedure:

- de-solder solenoid at PCB

- attach it to an oscilloscope (to measure voltage across solenoid) & the output of a small audio amplifier

- connect sine wave signal generator to input of amplifier

- from zero, turn up signal generator until +/- 1.5V across solenoid

- push plunger into solenoid

- leave it there for 45 seconds

- gradually turn down signal generator to zero

- disconnect everything, re-solder solenoid

Everything works okay now BUT my problem was intermittent, so initially this proves nothing!

 

I will report back whether this actually seems to have achieved anything (!) after using it for a while.

 

(I still think it's nuts that the Beogram 4004, for all its sophistication, can start moving the carriage back to the start when the stylus is still on the record. I know that modifying the B&O circuitry is anathema to a true Beofan, but there must be a simple way of fitting an 'arm up detect' switch to prevent this from ever happening. Any/all suggestions 1. where to fit the switch and 2. where to wire it into the control electronics gratefully received!)

Orava
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Orava replied on Mon, Mar 29 2021 1:30 PM

Simple AND circuit? End of record AND solenoid out switch signal to go on.

 blah-blah and photographs as needed

Orava
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Orava replied on Mon, Mar 29 2021 1:30 PM

Simple AND circuit? End of record AND solenoid out switch signal to go on. 

And double... There should be delete option on More menu

 

 blah-blah and photographs as needed

Doofer
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Doofer replied on Mon, Mar 29 2021 1:49 PM

Undoubtedly

sonavor
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It seems like the best solution is to solve demagnetizing the solenoid.  Either through the method you used or figuring out a way to disassemble the solenoid. For the latter it appears that the metal bar that actuates the lever for the arm lowering damper would have to be bent straight so it could be removed (after also removing the lock washer). 

I have a solenoid right now that appears to have this magnetization issue. It was kind of sticking when it tries to engage...I could feel it resist moving when I had the solenoid out by itself and operated it manually. I checked it out with a DC power supply and it required about 700mA at 7 VDC for the solenoid to engage.  Again, that is with the solenoid removed and unloaded.  When I tried a solenoid that does not stick and is properly working it only required 300mA at 3 VDC for its solenoid to engage.

I am going to see if I can do anything to correct the first solenoid.

-sonavor

sonavor
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Actually...the solenoid components disassembly quite easily.
I will post a photo in a little bit.

-sonavor

 

sonavor
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I made a quick repair and a blog post about it here.

Initially I tried to post the photos here in this thread but I could not get my photos to upload to my Beoworld folders.

-sonavor

Doofer
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Doofer replied on Wed, Mar 31 2021 3:52 PM

Nice piece of work. I wish my workshop had spare Beogram parts lying around (!).

Your efforts suggest that removing & disassembling the solenoid is not too tricky. That being the case, I wonder how hard it would be to replace the solenoid liner with something non-magnetic? As long as that remains potentially magenetisable, the residual magenetism issue will keep coming back.

(Alternatively, if my method of 'demagnetization in situ' works - nothing conclusive to report yet! - I suppose one could add a little PCB that demagnetizes the solenoid every time a record is finished, as there is always that 'spare time' when the arm is up and the slide is returning to the 'rest' position at the end of a side)

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