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
Just wondering if anyone here knows how a beogram works.
- how does it detect the edge of the record?
- How does it know when to lift?
- how does it keep the tone arm at a 90 degree? What if the record moved the tone arm faster than another one, does it do spot checks on where it should be?
Hi,
The edge is not detected, but the carriage motor has a tachodisc that counts the number of turns of the pulley, which the microprocessor knows. When the correct number is reached, it then sends the information to the motor feed. It is a loop system.
It knows when to lift when the carriage speed increases at the end of the record - the motor draws more current and this is detected.
There is a photo-detector at the base of the arm, with a shutter made of tiny slots, one fixed and one that moves along with the arm lateral movements. When the light diminishes, that is when the shutter slots are not perfectly aligned, the parallelism is corrected, normally at every turn of the record. It is an optical servo-system. So the perfect alignment of the shutter must coincide with the arm being parallel to the detector arm.
The job of the detector arm-platter ribs is to sense the diametre of the record and to know when there is none. When it detects that an EP is on the platter, the number of turns to the edge of the record will be higher, which again is recorded in the processor's ROM. And of course the table's speed will be changed accordingly.
In effect, the speed of the record and the inter-groove space does not matter.
In older Beograms there were micro-switches (Beogram 4000) or an optical system (Beogram 4002) with a photofilm on which vertical lines were printed instead of the switches.
This system was invented and patented in the late sixties (1969) by Pierre Clément for his turntable type A1. B&O copied it and mass-marketed it!
Jacques
Nice, thanks for the information. I thought there was only a photo cell at the end of the sensor arm, but there is another set hidden near the base of the arm? Wonder if they need occasional dusting off?
Yea I found out that the lines on the platter have to do with record detection. I read where a guy took steel wool to his platter to make it all stainless looking. After he did that his tone arm would drop on the platter while playing say a 45 rpm record. Not too good on the needle.
Let me see if I am understanding this correctly.
Lets say for some reason the groves of the record cause the tone arm to get ahead of the worm drive. Giving it a slight toe in (with respect to the sensor arm) The sensors located near the worm drive would detect that the tone arm is not at a 90 degree and would send a signal to the worm drive motor telling it to speed up. I would do this until the tone arm caught up and was 90 again then go back to the default worm drive speed?
RAJOD: Let me see if I am understanding this correctly. Lets say for some reason the groves of the record cause the tone arm to get ahead of the worm drive. Giving it a slight toe in (with respect to the sensor arm) The sensors located near the worm drive would detect that the tone arm is not at a 90 degree and would send a signal to the worm drive motor telling it to speed up. I would do this until the tone arm caught up and was 90 again then go back to the default worm drive speed?
Yes, that's what happens. The phenomenon is clearly visible in the lead-in grove section. But the worm-screw doesn't turn continuously under normal operation, it moves step-by-step.
Yes having the worm drive go on/off would make some sense. If you ran into a skip in the record if it had a default speed the worm would soon drag the tone arm past the skip.
So the way they have it set up during a skip the tone arm does not advance and since no advance the worm drive never gets a signal to turn.
RAJOD:So the way they have it set up during a skip the tone arm does not advance and since no advance the worm drive never gets a signal to turn.
chartz: This system was invented and patented in the late sixties (1969) by Pierre Clément for his turntable type A1. B&O copied it and mass-marketed it!
Really interesting fact.. ( and nice illustration too!) i always thought that the process was created by b&o.
Is there an official french name for this process? (So far i've found "Système optoélectronique de correction d’erreur"
Thanks for sharing
guillaume
So if the hidden sensors near the worm gear do the pick up and advances of the gear. What does that front sensor do? Just detects if a record is on the platter or not?
Lol, Dumb Dumb me was adjusting that front arm to be perfectly aligned thinking it had something to do with edge detection and lift off. All for nothing.
Did not know about the other sensors. One of my 8002s if you place the needle on say the last song it sometimes will pick up before the end of the song. If played from start of LP it always finishes. Do those rear sensors have anything to do with that? I mean can they be adjusted some?
In fact the detector arm needs to be aligned perfectly horizontally with the platter because it shines a light on it that the photoresistor right behind sees. If that arm is not perfectly alligned the spot it makes on the platter might be blurry or misaligned.. If it is, it wont detect the ribs ('ll let you guess what kind of disaster this could make...)
as for your pick up problem: check carefully: from the moment the needle drops on your selected song, to the moment it picks up. Did the carriage moved? If not, it might be a simple adjustment of the photoshutter control, under the tonearm. As i said earlier, if the carriage has not move during a certain amount of time, the turntable shuts off.
the procedure is written in the manual. I personnally find it really hard to get it right. If you do this adjustment by yourself, take extreme precaution with your pickup!
Guillaume
Yea I still don't trust them to play a 45, I put a 33 under the 45 just in case. No sense ruining a 700.00 cart due to sensor malfunction.
RAJOD: Yea I still don't trust them to play a 45, I put a 33 under the 45 just in case. No sense ruining a 700.00 cart due to sensor malfunction.
This actually spoils the tracking angle so is not particularly good for the stylus.If correctly adjusted, even an accidental lowering outside the record won't damage anything.In high-rib models (Beogram 6006/8000 etc.) the ribs are cut out to make room for the cartridgeand - again if correctly adjusted - the stylus cannot reach the ribs (or the platter) on the low-rib Beogram 8002.The stylus can rub a little on the edge of a record if it missed the correct landing spot but the adjustments arenot that difficult to get right.
It's a very well thought-out machine.
The Beogram will also initiate a lift, park and stop sequence if, while in play mode,no pulses are received from the threaded rod opto for a certain amount of time.If f.e. the record is skipping indefinitely, the stylus tip breaks, the servobelt or circuitry fails and/or if something blocks the carriage movement.
Martin
Yes it would not be a perfect angle with the 33 under the 45 but it was new at the time and I have only played one 45 on the unit just to test it. But good to know it won't trash the needle. I figured it would just drag a groove in the alum platter. So I figured 1 2mm lift from the 33 would be a safer first test.
Martin is quite right about the cut out in the ribs, but my 4000 will drop in this way and certainly makes one's heart skip a beat if forgotten about!
Peter