Beogram 4004: START Key Not Working

A customer from New Zealand recently ordered a set of Beogram 400x transport bushings from the Beolover Shapeways Store for his 4004 (5525) model. He sent me a picture after he installed them:

Pretty! I like them in white!

A while later he contacted the Beolover Customer Service Department ;-) about an issue with the START button on his deck. When pressed nothing would happen, while he was able to start the deck by first pressing the 33 button and then using the "<" button to drive the tone arm towards the start groove of the vinyl. Furthermore, the START button worked once the arm was on the vinyl. This indicated that the switch itself was o.k.

The fact that the 33 button worked told me that the circuit mostly worked, and that the fault must have its root cause in the SO-switch override circuit that is activated by the START button. Here is the relevant section of my annotated version of the circuit diagram:

This is how it works: When the carriage is in its 'home' position the SO (switch off) switch pulls the base of IC1 down which disables the 21V power supply, turning the deck off. When start is pressed, 30V are applied to the base of IC1 via R5/D9/R3/D6 (follow the red dashed line from the << ON (=START) switch). The 22V Zener diode D7 makes sure that the voltage at the base is 22V. This turns the deck on. D5 prevents a short circuit to GND via the SO switch, while this process is unfolding. The <<ON switch also turns on the carriage motor (via the red dashed line to he left at the branch point below the <<ON switch) and then the carriage starts moving. After it clears the SO switch the <<ON override is no longer needed. C2 makes sure that the override is in place long enough until the SO switch is open.

The 33 button provides another way to override the SO switch, but it does not turn on the carriage motor. Only the platter starts turning. That is why my customer had to press the < button to move the carriage while pressing the 33 button to drive it far enough until the SO switch was open. After that the deck would work normally.

The above considerations immediately suggested to look for a problem in the R5/D9/R3/D6 path. And indeed, there was a dead solder joint on D6, which prevented the override voltage from making it to the base of IC1. A bit of soldering, and the START button started working properly.

Wonderful! Another Beogram back in business! This is Beolove...;-)!

Beomaster 8000 Keyboard Restoration/Not Working Buttons

The final step to restore my Beomaster 8000 to its original glory was to fix a keyboard issue. I noticed earlier that the TP2 button only worked intermittently. I thought that this was a problem originating from corroded keyboard contacts since wiggling the button seemed to do the trick once in a while.... Hence, I took the keyboard apart to clean the contacts. While this was an interesting exercise, it turned out that this did not fix the problem. But more about that below. First a few pictures of the keyboard innards:

Removing the retaining washers turned out to be a pain. Since B&O used one-time use washers that need to be destroyed for removal I had to cut them off with a wire cutter:

This photo shows the buttons from below after removing the PCB and a plastic foil:

Here a shot of the PCB before cleaning:

For cleaning the contact pads have to be removed:

These contacts were quite oxidized (or probably rather sulfidized, assuming that the pads are silver coated)

Using a small head Eurotool fiber brush (I am grateful to Gunther at Beoworld.org for this excellent tip!!) I cleaned the contacts:

The above picture shows the PCB after reassembly.

I reinstalled the board using 3 mm external retaining rings from Grainger (113828) and #4 nylon washers. This is shown here:

So far so good. Plugging the keyboard back into the Beomaster and turning it on immediately revealed that this procedure did not fix the problem.

After playing a bit around I realized that the P4 button also not worked whenever TP2 was out. A look at the circuit diagram for the processor board immediately revealed that TP2 and P4 use the same input on the slave processor (it distinguishes between the two buttons via the strobe mechanism that also drives the LEDs of the display in three "phases"). The oscilloscope showed that after pressing the TP2 or P4 buttons the voltage at P81-7 (7th pin on plug 81 that goes to the microcontroller) was higher than on other pins connected to working buttons. This led to the conclusion that the connection between switch and microcontroller must be interrupted whenever the buttons ceased operation.

Hooking up an Ohmmeter between pin 7 and R103 indeed revealed a broken PCB trace, with an intermittent contact depending on the flex of the PCB in the vicinity of P81. I installed a bypass (pink wire in picture):

This seems to have done the trick. TP2 and P4 seem now stable.

After this I finally put the Beomaster back together. Tomorrow, I will swap it with the one I currently use in the living room and the next months will show if this rebuild is stable. At this point all features seem to work correctly. On to the next B&O!