ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hi all,
I have an issue with my beogram 4500. The servo motor does not work. When i operate the arrows, the deck responds and does what it should (starts spinning, raising tonearm turning the light on) but will not move the sliding chassis. Also the sliding chassis will not advance as the record progresses. The string is OK, and so is the servo motor, it seems ike no commands reach the motor. Any suggestions from where i should start?
Thanks.
When you state "the string is OK." are you referring to the rubber belt that connects the servo motor to the pulley? If you are then check that the motor is actually not spinning as the pulley may be loose. If it is not, then are you capable of performing electronics troubleshooting and repair as there is likely a faulty part that a tech will need to replace?If you have a meter you can start by checking whether the servo motor is getting 15V across it as you use the controls to move the arm.
Thanks for the reply Mark,
With ''string'' i mean the... 'wire' that connects the pulley with the sliding chassis. But the rubber belt you are referring to is also OK. Good hint on that pulley being loose but not in my case unfortunately.
Basic electronics troubleshooting are within my reach so throw your suggestions. I metered the servo motor and it gets 0.5V when i operate the controls or when the tonearm gets deflected. That doesn't look good, does it?
By the way, the servo motor has a label on that says "max.8V" are we sure its supposed to receive 15?
I’m not sure about the Schematic annotation of 15v, however you should see 7V when pressing either “<“ or “>” across the motor, so that is you issue, Pin 21 and 22 provide the signals for the controls which should be +5v when the respective one is high. If it is then I would look at the inputs and power supply to IC2. BTW, if the servo motor was pulling down the voltage, then you could confirm that by measuring the 7V with it disconnected. In any case I would investigate that circuit area.
OK you are right the motor was puling down the voltage. With the motor disconnected i measure 3.6V from the board and not 7... Do you think it could be a bad capacitor?
First, I would check that the motor works using a 9v battery. It should change direction with polarity. As to the unloaded measurement, you did not mention whether its polarity switches with the arrow and I’d also check if it is higher with >> and <<. I would also check the overall power supply voltage and the power pins on the OpAmp as per the service manual. If those are low, then I’d go after the electrolytic caps. Overall my bet is on a bad motor.
JimT: Thanks for the reply Mark, With ''string'' i mean the... 'wire' that connects the pulley with the sliding chassis. But the rubber belt you are referring to is also OK. Good hint on that pulley being loose but not in my case unfortunately. Basic electronics troubleshooting are within my reach so throw your suggestions. I metered the servo motor and it gets 0.5V when i operate the controls or when the tonearm gets deflected. That doesn't look good, does it? drift hunters By the way, the servo motor has a label on that says "max.8V" are we sure its supposed to receive 15?
Basic electronics troubleshooting are within my reach so throw your suggestions. I metered the servo motor and it gets 0.5V when i operate the controls or when the tonearm gets deflected. That doesn't look good, does it? drift hunters
Thank you for your kind sharing
Thanks Mark,
Yes the motor is spinning and changes direction with reversed polarity (i gave it 5.6V and is works nice). The unloaded measurement switches polarity with << or >> and gives the same 3.6V either way. I will check the OpAmp and will keep you posted... If it is the caps i was considering starting from C28 and then moving to C30-34-35.
JimT: [...] The unloaded measurement switches polarity with << or >> and gives the same 3.6V either way. I will check the OpAmp and will keep you posted... If it is the caps i was considering starting from C28 and then moving to C30-34-35.
[...] The unloaded measurement switches polarity with << or >> and gives the same 3.6V either way. I will check the OpAmp and will keep you posted... If it is the caps i was considering starting from C28 and then moving to C30-34-35.
hmmm...What test points do you measure when you say 'unloaded measurement'? CPU outputs 21, 22 toGND or between opamp outputs 1 and 3 with motor disconnected? 3.6V seems strange at either of these points.Those caps you mention are unlikely to be the source of the problem. What dou you read on the 5V and 12V power supply output. And what's the voltage at pin2 of IC2? The latter should be around 9V.
-------------
Oops! After actually reading the whole thread instead of just skimming it I realized that you're measuring 3.6V across the motor terminals -- and that's surely off. My recommendations above still stand (and Mark has suggested the same steps already).Additionally I'd measure both motor terminals to GND instead of measuring just across the terminals. This way you can see whether the problem lies with one of the opamps or the zener diode or power supply.The schematics show the nominal values:upon pressing " < " : P1-2 = 7V, P1-1 = 0.15Vupon pressing " > " : P1-2 = 0.6V, P1-1 = 7.5V
OK further to your suggestions the CPU outputs 21 and 22 give 5.15V to GND, the same goes to opamp 7 & 8. Opamp outputs 1 and and 3 give 3.51 and 3.62 to GND respectively.
Pin2 of IC2 to ground outputs 3.64V
The 5V and 12V supply output give 5,15V and 12V.
upon pressing " < " : P1-2 = 3.51V, P1-1 = 0.15Vupon pressing " > " : P1-2 = 0.15V, P1-1 = 3.62V
So the power supply looks OK. Zener diode OK. That leaves us with the opamp? or maybe the caps?
JimT: 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: [...] BTW, 4558 OpAmps are readily available.
[...] 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: 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 = 0.15Vupon 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: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.
JimT: But really is goes all over the place depending how fast or slow i rotate...
But really is goes all over the place depending how fast or slow i rotate...
Yes, that's normal. When you spin the rotor manually, the motor turns into a generator and the generated voltage will throw off the multimeter. You're not supposed to spin the rotor but "reposition" it, ie turn it slowly and gradually in steps of 45 degrees or so.Anyway, that's just an info for the future. Apparently your motor is fine since an external power supply can drive the whole mechanism without problem.60 ohms seem still high to me, but maybe this motor is designed as low current, low torque model.
JimT: upon pressing " < " : P1-2 = 2.8V, P1-1 = 0.48V upon pressing " > " : P1-2 = 0.48V, P1-1 = 7.7V [...] 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
[...] 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.
Thanks! That clears things up. The 3V from the CPU is the problem. The opamp works as a differential amplifier and the level of the input signals determines the output. With your unit unplugged, measure resistance from pin 21 and 22 to GND. If pin 22 somehow finds a low-ohm path to GND, it would explain the reduced voltage. Normally you should read a high resistance value on your meter.You should also check for cold solder joints, damaged PCB tracks and anything odd/suspicious in that circuitry around IC2 and the path from pin 22, IC1.
Afterthought: When you measure 3.3V on pin 22, is the supply voltage on IC1, pin 9 still at a solid 5 volts or is it being pulled down??.
The pin 9 on IC1 stays solid at 5 volts whether i operate the < or > button.
As for resistance from pin 21 and 22 to GND with the unit unplugged i measure 46.6KΩ and 44.5KΩ respectively. Both numbers are not solid, and keep rising (capacitors?). With the unit plugged the measurements are 31.4Ω and 30.4Ω and solid.
JimT: With the unit plugged the measurements are 31.4Ω and 30.4Ω and solid.
With the unit plugged the measurements are 31.4Ω and 30.4Ω and solid.
Forget these values, they're bogus. You cannot measure resistance in a live circuit. The multimeter uses its own current and voltage to calculate the resistance; any external voltage results in arbitrary nonsense measurements.
Right now it looks like one of your opamps is spoilt. If you have another chip you can try to replace it -- but before soldering in the new one, measure the output voltages on pin 21 and 22 again. When the dual opamp IC2 is removed there is practically no load on pin 21/22 and both pins should be 5V when the respective button is pressed.If not then isolate pin 22 completely and measure again. "Isolating it" means removing resistor R53 - and if that doesn't help also C29.I don't think that the CPU port is spoilt since ports are well protected internally, but the only way to confirm this is to remove any load from that pin.