JimT wrote:

So the power supply looks OK. Zener diode OK. That leaves us with the opamp? or maybe the caps?

Yes, the power rails are good, the CPU is good.
That leaves the Zener diode and the opamp. Reading only 3.64V at pin 2 of IC2 points to a Zener issue! How can a 2.7V Zener rated at 500mW show an 8+ Volts voltage drop without burning up?? There's something fishy there!
Of course, opamp failure is possible too...but since this chip is obsolete and hard to find, focus on the Zener diode first.

You’d likely see that voltage drop if the Zener is open. Time to replace. BTW, 4558 OpAmps are readily available. 

Thanks Mark and Manfy for the troubleshooting,

I'll replace the Zener and will share the results.

If you're ordering that part, you may want to look at the next bigger diode! At 500mW the Zener can provide a max. of 170mA and that seems a bit low for a motor and an opamp that is spec'd at 700mA output current! I'd probably go with a Vishay BZX85 with 1.3W, ie. max 360mA  -- that gives you some headroom!

Mark wrote:

[...] BTW, 4558 OpAmps are readily available. 

4558?? Is that a typo? According to the datasheet that's a run-of-the-mill 20mA dual opamp - and it's not even pin-compatible!

Some electronics blogs recommed the TCA0372. That should be a drop-in replacement for the L272M. However, the usual online shops (Mouser, digikey, Farnell) don't seem to carry the PDIP8 version any longer, they only offer SO16 SMT versions.
Google popped up with one German supplier, Reichelt, that does have a TCA0372DP1 by ONsemi available ex stock (approx eur1.50). I'm not sure where you're at and whether they ship to your country, but Reichelt is definitely more trustworthy than some no-name ebay shops! I've bought from them before.

Sorry, my mistake - wrong IC2. Should be L272M which also are available. I’ve always hated B&O’s reuse of reference numbers across boards. 

One more thought: Before you test the circuit with the new Zener diode, you should probably disconnect the motor to avoid any unwanted damage. Just measure the voltage at the OpAmp output from P1-1 to P1-2.

You should also measure the internal resistance of the motor (simple resistance measurement across the 2 terminals) and if possible, the idle current of the motor at 7V. This way we can see whether the motor is the source of the failed Zener or opamp. I guess you should see approx. 10-20mA at 7V and in no-load condition.

OK the parts arrived and finally i found some time to replace them.

I replaced the Zener first and had the same readings from the OpAmp. I then moved to replace the OpAmp itself. (thanks manfy for the alternative)

I now get to P1-1 to P1-2 +2.3V and -7.3V when i operate the < and > buttons. I also get 8.64V from pin2 of IC2 to ground.

As for the motor... i measure 60.7 Ω across the terminals and 25mA at 6.5V (with no load on the motor).

I connected the motor to the board and it spins with the belt off. Once the belt is on it looks like it has no power to turn it.

I probably also need a replacement motor right?

Glad to see you are making progress. I have never seen a servo motor that worked but did not have enough torque. I have seen where the there is binding due to shaft or bracket alignment, pulley not secured or a worn belt. I would check all of those.

JimT wrote:

I now get to P1-1 to P1-2 +2.3V and -7.3V when i operate the < and > buttons. I also get 8.64V from pin2 of IC2 to ground.

As for the motor... i measure 60.7 Ω across the terminals and 25mA at 6.5V (with no load on the motor).

I'm not quite sure how to interpret your voltage measurement...is that across the motor terminals? +2.3V looks strange. Can we stick to the same procedure as before, ie. each terminal measured against GND. Here's what you measured with the faulty OpAmp:
upon pressing " < " : P1-2 = 3.51V, P1-1 = 0.15V
upon pressing " > " : P1-2 = 0.15V, P1-1 = 3.62V

The terminal resistance of 60.7 ohms doesn't sound right. If that were correct, the stall current would be 7V / 60.7 ohm = 115mA. Comparing that to the idle current of 25mA we get a ratio of less thqn 1:5 !! That can't be right considering that the stall current is always much bigger than the nominal current at maximum power output -- for any DC motor!

I agree with Mark, those little DC motors are usually very robust and don't die easily. I googled the net quite a bit, trying to find the specs for this Matsushita motor but without success. While doing so I found multiple blogs that showed mechanical problems with that mechanism but not a single thread that talked about a faulty motor.
But anyway, that's no proof that your motor is fine, of course! I could imagine that there's something wrong with the brushes or the commutator.
I'd like you to make one more measurement: Clip your probes to the motor terminals and measure resistance again, but now slowly turn the rotor and see how much the resistance value fluctuates.

PS: If some other member has a Beogram with the same motor (according to the service manual that should be BG3000/3300/3500. BG4500/5005/5500/6500/8500/9000/9500/TX2), it would be great to get the terminal resistance of a confirmed working motor!

manfy wrote:

I'm not quite sure how to interpret your voltage measurement...is that across the motor terminals?

Yes it was across the terminals by operating the < and > buttons.

I did the above mentioned test again more carefully and i have the following readings.

upon pressing " < " : P1-2 = 2.8V, P1-1 = 0.48V

upon pressing " > " : P1-2 = 0.48V, P1-1 = 7.7V

As for the motor i stand corrected. I measure 10.4mA when i supply 6.42V with no load on.

Resistance is 59.5 ohms across the terminals and by turning it slowly it gets to 70 (or 45 depending on the direction of the rotation)  and then settles again to 59.5. But really is goes all over the place depending how fast or slow i rotate...

Now the strange thing is that now i measure 3.34V from pin 22 of IC1 to GND and 5.16 from 21 to GND. 😂

The components that have been replaced since the first time are the Zener, the OpAmp, and caps C28 and C2

By the way with the motor disconnected from the board when i connect it to an external supply providing 6.5V it spins perfectly and traverses the sliding chassis with no hesitation at all. So i eliminate any mechanical problems there. After all i have cleaned and re-lubed the relevant gears.