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
greetings,
a recent project/find which is almost complete......the problem I am fighting at the moment is the automatic arm drop.
Without a record , play activated- symptoms:
carriage starts, wont stop at the drop-down point and moves right to the end without auto-return
with record on, play activated - symptoms:
Same
I can stop the carriage with << or >> at the drop-down point, press play, arm lowers and record will be played.
at the end arm will not lift and return automatically straight away ....only after some wait the carriage will eventually return via auto - shut down.
main board has been re- capped, connectors have all been resoldered, opto-element been replaced at the opto-device board at spindle and sends perfect rectangular signal, detector circuit has been checked, 5V and 15V present, detector arm bulb is fine.......
I am running out of ideas ..... would someone care to give some pointers
ALF
Hi Alf,
It sounds like the Beogram is not reading the arm drive spindle's opto sensors. I read where you replaced them but the symptoms point to a problem with that area of functionality. The manual cueing makes sense as it is a user override of the automatic operation. I would bet that the pause and resume functionality doesn't work either right now. Something is preventing the Beogram microcomputer from sensing the drive spindle rotation (slide tacho) so it can't determine the drop down or stop points. Use your scope to monitor those signals going in and coming out of the two sensor counter OpAmps (1IC1 and 1IC2). The outputs go to the microcomputer pins 28 and 29 respectively (slide tacho).
John
Hi ALF,
In addition to John’s suggestions I’d check the Detector Arm Circuit.
A simple test to do is with a record on and press Play, watch the speed of the arm as it moves towards the set down point. Does the arm slow down as it approaches the set down point or does it travel at the same speed? If it doesn’t slow down then the detector arm circuit is not working properly to control 2IC1 pin 33. Pin 33 should be low when there is no record on and go high when a record is detected. If pin 33 stays low it will not allow the arm to lower.
Regards
Lonnie
Hi John,
detector arm circuit has been check and should be fine, all 3 transistor tested fine, bulb in the arm is fully iluminated.....that was my first suspicion.
you are right, the slide tacho seem to have a problem: I should get a signal at P2 pin 4 and 6 ?!
pin 4 is a flat line, pin 6 a nice steady rectangular signal.
looks like one of the upper “receiver transistors” had it . I replaced the sender diode with a OP240C component - would a OP240A or B version be
a more suitable replacement ?
what type are the 2 “receiver-transistors “ in OPE1 ? Nowhere to be found in the SM.
With the current scenario the counter can not work and the uP does not know where the carriage is ! Makes sense!
if that assumption is correct I can not imagine the carriage would slow down near the drop down point in order to check the proper working detector circuit
as Lonnie suggested ?!
any further thoughts on this ?
The arm sensor is only read after a certain amount of pulses from the slide tacho has come in.With no valid pulses from the slide tacho, the arm sensor will never be read.
Martin
Makes sense to me, that is why I have to get the slide tacho pulses sorted.
it seems the matching opto transistor to the OP240A is the OP550
The Optek OP240 emitting diode documentation says that the matching phototransistor for it is the OP550 and OP560 series. The letter designator on the end of the part number references the sensitivity level. A is the highest sensitivity. D is the lowest. I haven't had to change out those parts yet so I can't tell you which one to use. You might have to try a couple and see what the output measures. On my most recent Beogram 8000 project I measured those 0PE1 phototransistor signals (original parts in my case). You can see those results here.
Thanks John for confirming that.
I also mentioned that in my last post with a ? after studying the data sheet of OP240. 🙄
parts ordered and I will report back after the trial.
regards
Looks like you’re on track to get this sorted.
If you haven’t already removed the failed phototransistor it would be worth taking some in circuit measurements to confirm the failure mode.
If you measure the voltage drop across R2 (the good phototransistor) you can see what the correct voltage drop and hence current drawn on the working circuit is and compare it to the voltage drop across R3. I can think of three scenarios on the failed circuit:
1. Voltage drop on R3 of ~15V (~15V- ~0V) which would indicate a short in the phototransistor which when you remove it you can confirm on a diode tester.
2. Voltage drop of ~0V (~15V – ~15V) indicates an open circuit. Check continuity of circuit between R3 and collector of phototransistor and also continuity of emitter and ground. If ok you can confirm an open circuit on the diode test after removal. If circuit continuity is nok find and fix the open circuit.
3. Least likely but if you measure a drop of ~10V (~15V - ~5V) and it's different from the drop on R2 it could indicate a short between P2(5) and P2(4).
Hi Lonnie and John,
good news: after replacing the two opto-transistors for Optek OP550C the carriage now stops at the correct position and arm lowers as it should.
I did use the combination of OP240C & OP550C as replacements.....OP240A would have a higher sensitivity, followed by the ‘B’ version but it works fine.
cautiously - as always - optimistic that was it and like to close the case 🙏
thank you once again for all your supporting advice - much appreciated.