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Thursday, November 1, 2018

Beomaster 8000: New uProcessor Crystals, Improved Decoupling of the uProcessors, and New IC Sockets

A Beomaster 8000 that I restored in 2014 recently returned to my bench due to some erratic behavior. Apparently it turns itself on spontaneously once in a while. While I was not able to reproduce this behavior for several weeks, I experienced it once with one of my own Beomaster 8000s, and there are some sporadic reports on the internet about similar issues.
Since I was not able to find anything wrong with this Beomaster, I am left with a hypothesis, namely that the uProcessor once in a while suffers from a power fluctuation or that one of the clock crystals is going bad, and that puts it into an unexpected state.
So I decided to replace the crystals and add 100nF decoupling capacitors to the power supply pins of the uProcessor IC, which only carry 1 uF from the factory. These days most of the time one finds 1nF in parallel with 100nF, which allows filtering a broader frequency range.

This shows the inside of the uProcessor can in original condition:
I removed the precious processor ICs from their sockets to ensure that they would not get damaged during the surgery. The crystals and also their capacitors can be charged with a high voltage out of the box, and that can damage the ICs. Therefore, I normally discharge all components against GND and I remove the ICs before working on this board. 
This shows one of the crystal setups in more detail:
I removed the crystal and their two 22pF oscillator capacitors and implanted modern crystals with their specified 18pF capacitors:
I also replaced the IC sockets, while I was in there:
This shows the an original and a new 2 MHz crystal in comparison:
When putting in the new 18pF capacitors, one needs to remember to solder one of the pins from both sides, since it serves as a via.
The final task was to add the 100nF capacitors. This is an easy thing to do since the 1uF cap is soldered between the 5V and GND pins on the back side of the PCB. This shows the original setup for IC3:
I simply soldered the 100uF cap in parallel across the 1uF cap:
After this step I put the board back in, and the Beomaster still worked...let's hope the new parts help suppressing the sudden-on phenomenon. Only the future will show...

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