Beogram 4500 doesn’t respond to buttons or beolink

[Andy Cousins]

Just got a Beogram 4500 turntable to complete my 4500 set. It was sold as not working, but I figured it was worth trying to repair.

Having opened it up and checked it over. I found a bunch of components dislodged and the tone arm drive wire missing. I suspect someone tried opening it with the arm in park, but there don’t appear to be any broken parts.

I’ve fabricated a new arm transport drive wire, and relocated the dislodged parts and springs. The unit powers up and spins the platter briefly, but the unit doesn’t respond to the buttons or beolink control.

Turning the pickup arm travel and lift/lower gears, the switches engage and spin the main platter drive motor, the pickup arm transport motor and the lift/lower mechanism.

At certain points, the solenoid plunger will shoot out. Sometimes the transport will bring the arm to the ‘single’ (45) position and lower the arm. Moving the arm (I don’t have a cartridge installed, so I’ve just added a piece of blu-tack to give it weight to allow it to lower). Gently pushing the arm to the left triggers the transport motor. When the arm reaches the centre of the disk, nothing happens (the platter continues to spin, and the arm stays in the lowered position)

The above proves that the motors and solenoid appear to be functional, and the mechanical components aren’t damaged, but it the unit will not respond to the buttons or beolink.

With a mutimeter on pins 5-11 of the main IC, I can see that the buttons are working and drop power from 12v when pressed (press to break switches), so the problem seems to to me to be either with the chip or downstream of its outputs.

Other info – the platter light initially stayed on all the time, but doesn’t light up now, and the disk speed indicator LEDs on the arm transport both come on from time to time, but there doesn’t seem to be any logic to that.

Reading up on the spec sheets for the main IC, it looks like it’s a programmable chip, and the B&O parts list identifies it as a ‘customised part’, so I’m guessing it was programmed by B&O and isn’t a straightforward swap?

I don’t have a cartridge, and there’s no point getting one yet, if the main IC (and therefore the board) is effectively dead.

Any advice on what steps to try next to diagnose whether the main IC is dead, or if the problem is downstream of it?

[Dillen]

Have you tried it with a record on the platter?
With no record it will only spin briefly – while the record sensing system operates.

The light staying on at all times could be as simple as a shorted transistor.
A bad batch of BC337 was used in production, so it’s not uncommon to see some fail.

Martin

[Andy Cousins]

Thanks.

Yes, I’ve tried it with 7″ and 12″ records. It doesn’t respond to inputs in any case.

I need to keep notes about what it does in each test, but I’m pretty certain that whichever size disk I put on the platter, the tone arm LED displayed ’33’. Without a record I think it displayed both speeds, or neither.

I’ll trace back through the schematic and test the transistors that control the light as well.

[Andy Cousins]

I made some progress. A cheap ebay digital oscilloscope proved that the main IC isn’t dead (I could see activity on the System Ocillator Output and Input pins), but something didn’t seem right with the Reset and Voltage Reference signal.

Digging around, while checking the power supply output current, I found a diode that tested bad. Replacing that, a whole load of functionality woke up, but it’s still not working correctly yet.

It will now ‘play’ once (I don’t yet have a cartridge and I’m not going to buy one until I have the unit working correctly).

Pressing play, it’ll move the arm to the relevant place for a 33 or 45 and lower the arm. Nudging the arm to the left will engage the tracking motor and move the arm in to the centre. Pressing play lowers it again in the new position.

The forward and reverse buttons will trigger the unit to raise the arm and move it left or right. The play and move left and right buttons will also make the light come on for four or five seconds. The 33/45 LEDs are working and the speed selector buttons change the speed of the main motor speeds up or slows down and the LEDs switch over as appropriate.

Allowing it to ‘play’ and tracking the arm in to the end of the disk, it’ll get to the endc centre, raise the arm and start to return it, but it always returns it to the start point of the disk, not to the far right ‘home’ position.

Once it’s done that once and the arm has reutned to the start point on the disk, it won’t play again and the main motor continues to rotate the platter.

Pressing play, stop or the move left and right buttons after the arm has returned to the start point on the disk causes the the light to come on and the tracking motorto runs for several seconds, but the metal lever doesn’t push the tracking worm gear in to contact with the main bobbin that the tracking wire is looped round, so nothing moves.

So, it seems to have the potential to work correctly, but it’s almost like it doesn’t know how to go through the start and end part of the cycle. I’m still thinking that something is amiss with the input signal to the Reset pin. The voltage on it and the Voltage Reference pin are constant 5.4V, unless I unplug it and the input voltage to it slowly drops. Plugging it back in doesn’t cause the main motor to spin up and reset the arm though

Any pointers?

[Andy Cousins]

In the end; I bit the bullet and bought a salvaged main PCB, which fixed the initial problem. Looks like the problem was with the main IC. It didn’t generate the necessary signal to trigger the play elements of the circuit. Replacing the PCB, the play elements of the circuit all work.

I then found another problem, this time with the mechanism that took me a couple of days to figure out.

The turntable would work fine simulating 45 (7 inch) disk, but, the mechanism would jam when I simulated a 33 (12 inch) disk. The tone arm carriage would travel a short distance back from the center, then jam. The jam would then cause the fuse to blow.

It took me ages to work it out, but eventually I noticed that the jam was being caused by the center, upper left and top center cams all meshing together at once. Studying the movement; it looked the upper left cam shouldn’t have been engaging at that point in the cycle for a 33/12-inch (That upper left cam moves drives the mechanism to move the tone arm carriage between the stop and play positions for a 45/7-inch disk).

For some reason, the top center cam (the pulley with the carriage wire wrapped round it) was nudging that upper left (45/7-inch) cam a few mm just before the carriage reached the centre of the disk, so that the first tooth on the raised section of that cam came very close to the teeth on the center cam. Normally, the top left cam rests with the toothless section adjacent to the centre cam after playing a 33/12 inch, so they don’t mesh at the start of that return cycle. When the center cam started turning at the start of the 33/12 inch return cycle, the center cam would catch the first tooth on the upper left cam, causing all three cams to mesh together and jam. The only way to release the jam was to pop the three cams off and reset them.

After watching it several times, I worked out that the pivoted arm (part 1609) which sits under the upper left and top center cams was nudging the upper left cam in to the incorrect position where its teeth could get caught by the center cam. The arm has a pin at one end that follows a track on the underside of the top left cam. I think it’s purpose is supposed to be to lock that top left cam in a position such that its toothless part lines up with the other two cams at the end of a 33/12-inch play cycle, preventing the cam engaging at the wrong time. The arm has a pivot in the center and the other end of that arm contacts a projection on the underside of the top center cam. That projection was catching and causing the arm to rock, which nudged the top left cam those few mm in to the wrong position (the opposite effect to what it seems like it should be doing).

I removed arm 1609 and the problem went away. The turntable played 45/7-inch and 33/12-inch records correctly.

I’ve just googled that part number and found other threads here blaming that arm for mechanism jams and indicating that removing it doesn’t seem to cause a problem, so – fingers crossed I can get away without it as well. Seems like it was added as a safety feature to prevent jams, but actually causes them.

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