Beomaster 8000: Sudden On or Switching to FM Preset Phenomenon - The Story Continues

I am still in head scratching mode regarding the Beomaster 8000 that occasionally turns itself on to a FM preset, usually P1. Despite improving the decoupling of the processors from the power supply by adding 100nF capacitors to their power pins, this Beomaster finally turned itself on to P1 while I was present, and a few days later it switched itself to P1 while it was playing from my iPad on Tape 1. While this was frustrating news, it also meant that I was finally able to reproduce the issue, and that the phenomenon happens independent from where the Beomaster is plugged in (one of my theories in absence of being able to reproduce the phenomenon was that maybe the location of my customer's house is prone to a different level of line voltage fluctuations etc...I was grasping for straws, I know...;-). 

Anyway after it switched itself spontaneously from Tape to P1 I thought that the issue must be originating from the P1 input. A look at the circuit diagram showed that the only way P1 could accidentally trigger itself would be the absence of the 4.7k pull down resistor R1 (as long as no key is pressed on the keypad, the pins are electrically only connected to ground via the pull downs and to the input pins of the uProcessor). The interesting thing about the pull down resistors for the keypad inputs is that they are part of an integrated 7 resistor array with a common pin, i.e. the entire array has only 8 pins. This conveniently allows connecting all 7 resistors to ground with just one solder point. My hypothesis was that the array may have some intermittent issues, permitting some of the keypad inputs to float (which is known to cause erratic input conditions in microprocessors) hence potentially explaining the sudden switching to P1. 

I opened the Beomaster up again and took the uProcessor board out. Before unsoldering the array I removed the slave processor IC3 where it is connected to the input pins for the keypad. I usually do this when I work around the processor ICs since new parts may carry a charge which may accidentally fry some pins on the processor while implanting the part. The black 8 pin flat package in the center of the photo right in front of the IC socket is the array.

I extracted the array. This shows it together with the new part (yellow, Newark 62J2860):

While taking the original array out it occurred to me that the common pin (on the left in the first picture) is soldered into a via. And that made me think that maybe not the array is the problem, but the via, whose intermittency is a common problem on this board. The via is needed since the ground connection of the common pin is made on the composite side of the board, but they only wave soldered the board only from the solder side. This means they had to put vias around such pins that capillary forces would draw the solder into the via and through to the component side of the board, making the connection to the top copper layer. In the 1980s they were apparently not yet able to through-plate vias with electroless copper deposition technology, like it is done today.

I soldered the new array in paying attention to soldering it also on the top side. This shows the final result:

I put the board back in and fired the Beomaster up, and it still worked. So far so good...only in a few weeks we will know if this finally cured the problem.

Beogram 4002 (5501): Random Stop Issue Fixed - Defective Photo Resistor in the Spindle RPM Sensor

A Beogram 4002 (5501) that I restored last year developed a strange new problem: When playing a record the OFF (>>) function  would be triggered randomly during play as if someone pressed the >> key on the keypad.

Before I will discuss the fix, let's have a look at the circuit diagram (click on the diagram to get it in full resolution):

TR17 in the 'Electronic Switch' is responsible for triggering the >> function of the turntable. Whenever its base is pulled to GND its collector goes high to ~18V and then the arm is lifted and the carriage is driven home. There are three ways this can occur: Via two mechanical switches, one being the "End Switch" (ES) under the carriage and the other the ">>" key on the keypad. The End Switch is the one that sends the carriage back whenever it is driven all the way to the left (i.e. manually via the << key or automatically if there is no record on the platter and the arms go all the way searching for one).

The third way to trigger >> is via the end groove detection mechanism ("RUN-OFF STOP" on the diagram). This works via TR20 whose base is pulled up when the end groove is detected. That connects its collector to ground, and with that the >> function is activated.

The end groove detection mechanism works via an 'analog rotary encoder' that detects light flashes impinging on a photo resistor (OR2) from a bulb (OIL1) that shines its light though four holes in the carriage pulley as they pass by (creating light fluctuations on OR2). 

These light pulses short OR2 to GND causing TR21 to shut down, which increases the voltage at the collector of TR21 to about 19V. These voltage pulses (see oscilloscope trace schematic on the circuit diagram) charge C33, which, via the voltage divider formed by R88/89, pulls up the base of TR20, triggering the >> function.

It is interesting to to note that the bulb OIL0 only comes on when its switch to GND is closed. This switch is activated by the carriage when the tone arm gets close to the record label. This switch is activated by the same tab on the carriage assembly that activates the 17 cm (singles) set-down point switch ("B" in the service manual). This is the reason that the tab that activates B has a long flat shape. Anyway, this switch aims to prevent triggering the the RUN-OFF STOP mechanism when the carriage briefly moves faster between two tracks of a record when the carriage is still far away from the label.

Why does the mechanism not trigger RUN-OFF STOP when the carriage advances normally while playing the last track when OIL1 is already on? C34, R91 and D30 form a network that discharges C33 when there is no light on the sensor. This means there is a competition between charging during illumination episodes, and discharging when there is no light on the sensor. So if the pulley rotates only slowly and occasionally during playback of a track, discharging 'wins' and the base of TR20 is not pulled up high enough. But when the light flashes happen quickly like when the carriage moves fast pulled along by the end groove, then the charging mechanism wins and TR20 is turned on. The beauty of analog control systems!!

Ok, back to the 'Random Stop Issue': After verifying that the ES and OFF switches were working properly, I traced the signals from the base of TR17 into the RUN-OFF STOP circuit. This shows the oscilloscope traces that I measured:

The traces are assigned as follows:

• Yellow: TR17 collector (when this signal goes high the measurement was triggered - the graph shows ±5 sec around the >> event)
• Blue: TR20 base
• Green: TR21 collector
• Red: TR21 base

These traces show that before the >> event happens there is a random statistical fluctuation of the voltage at the collector of TR21 (instead of being close to GND if there is no end groove event). These voltage flashes obviously correlate with the signal at the base of TR21 (suggesting that TR21 is working properly). This leaves only one conclusion: The sensor OR2 randomly short circuits to GND causing these voltage spikes. These spikes can be enough to charge C33 and when the voltage one C33 gets high enough due to these random events, the >> function is triggered. This is seen in the blue trace, which shows that the voltage exceeds the ~0.6-7V threshold needed to turn on a standard silicon based transistor when TR17 is activated.

All this told me that OR2 was faulty. This meant it needed to be replaced. Since this is a special B&O part, which is not available anymore, I decided to design a 3D printed assembly to replace the entire bulb/sensor assembly on the pulley. This shows the setup after I implanted an LED to replace the light bulb last year:

The orange part contains the LED (Newark 78R6602) and its 2k current limiting resistor (Newark 26R3983). It was designed to stick onto the OR2 sensor housing. The current design replaces the entire encoder setup. These are the structural parts of the assembly:

Assembled they look like this:

And with LED, resistor and photo resistor (type "5516", ~10k resistance when dark, ~500 Ohm when illuminated) installed:

And after installation and in action:

Once the set-up was installed I measured the signal at the collector of TR21 when the LED came on:

This signal looks pretty much like what the manual demands. I tested the Beogram by playing some records, and it seems everything is working again! So I am hopeful that this fix took care of the issue.