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--- projects:repair:beogram8000 [2018/01/20 02:01] – admin
+++ projects:repair:beogram8000 [2018/01/24 01:24] (current) – admin
@@ Line 3: / Line 3: @@
   * sudden speed increase, without any apparent reason. Originally about every 20 minutes, for a duration of 5 seconds.\\
   * Schematic diagram can be obtained free of charge [[https://www.vinylengine.com/library/bang-and-olufsen/beogram-8000.shtml|here]] (Free for personal use.)
-After measuring with an oscilloscope on some meaningful points in the circuit, there were no anomalies found. Signals looked as expected. Since the speed control circuit as a whole is in a closed loop, the cause could be anywhere as well within the loop. A better way to measure therefore was to place measuring probes before and after several functional blocks in the closed loop and see for each block how it behaves when speed increases suddenly. For this a Saleae capture device (Logic Pro 8) was used. With that the reason for the speed error was found:
+After measuring with an oscilloscope on some meaningful points in the circuit, there were no anomalies found. Signals looked as expected. Since the speed control circuit as a whole is in a closed loop, the cause could be anywhere as well within the loop. A better way to measure therefore was to place measuring probes before and after several functional blocks in the closed loop and see for each block how it behaves when speed increases suddenly. For this a [[https://www.saleae.com/|www.saleae.com]] capture device (Logic Pro 8) was used. With that the reason for the speed error was found:
 | {{ :projects:repair:beogram_8000_33.33_not_ok.png?direct&600 |}} |
@@ Line 11: / Line 11: @@
 | {{:projects:repair:beogram_8000_33.33_ok.png?direct&400|}} | {{:projects:repair:beogram_8000_33.33_ok2.png?direct&400|}} |
-|  Normal operation: Only SLOW UP/DOWN  |  Zooming in  |
+|  Normal operation: Only SLOW UP/DOWN  |  Signal detail  |
 So to find the cause for this, the circuit which handles the speed sensor was analyzed:
@@ Line 20: / Line 20: @@
 The B&O schematic diagram from above is a bit different then what this turntable has: Phototransistor OPE2 is pulled up to 15v with resistor R47 (22k) and fed via resistor R48 (100k) to opamp IC1, which is configured as a 5v comparator (with a feedback of R49 (1M0). The output of the comparator is fed via a voltage divider (consisting of resistor R50 and R51) into the microcontroller. This is also different than what is designed in the circuit diagram from above. Maybe the 4013 schmitt-trigger was added in a later revision as a modulator for the speed signal.
-With the limited triggering possibilities of the oscilloscope, measuring both signals for a long duration while staring at it and wait until one could see where the root cause was, was quite hard. Also, while investigating this, the cause did not appear a single time while observing. Still without repairing anything specific, it was difficult to understand that defects would be fixed somehow spontaneously. Due to this long duration, the Saleae analogue capturing device was not suitable either, because of the limited sampling time. Then, to find the root-cause (Or at least prove that the cause was not anymore present), a simple but effective Arduino Nano microcontroller as watchdog was taken. With some software it was able to guard both signals and indicate if one or both signals would fail and if speed was deviating from 33 1/3rpm.
+With the limited triggering possibilities of the oscilloscope, measuring both signals for a long duration while staring at it and wait until one could see where the root cause was, was quite hard. Also, while investigating this, the cause did not appear a single time while observing. Still without repairing anything specific, it was difficult to understand that defects would be fixed somehow spontaneously. Due to this long duration, the Saleae analogue capturing device was less suitable either, because of the limitations with maximum sampling time: If the error would not happen a single time within at least 24 hours, then the issue could be declared as resolved. Then, to find the root-cause (Or at least prove that the cause was not anymore present), an Arduino Nano microcontroller as watchdog was taken. With some software it could then guard both signals and indicate if one or both signals would fail and if speed was deviating from 33 1/3rpm.
 To reduce the influence of the Arduino measurement system, two opamps as voltage-followers were taken, fed with 20v to allow some margin above the 15v level to be measured:

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