Beomaster 8000: Step Six - Rebuilding the uProcessor Board

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ACHTUNG: Please, note that the crystal exchange procedure described here can damage the microprocessor chips on the uProcessor board. It is recommended to remove the chips before replacing the crystals and the capacitors. Due to the inherent capacitance of these devices, high voltages can be present between their terminals, which upon release, can burn out the gate that forms the oscillator together with the crystal. Make sure you short circuit the leads of the parts before installing them.

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After completing the display board rebuild with new SMD LEDs in the displays themselves, as well as replacing the incandescent light bulbs in the indicator cabinets with SMD LED based assemblies, it was time to do the uProcessor board.

The uProcessor PCB is the main source of operational trouble due to a few shortcomings in its design. In the 80s double sided PCBs were only beginning being standard in consumer electronics, and the earlier 8000s have boards without through plated vias. This made it necessary to insert metal plugs into the vias and solder them. Many 8000s have startup trouble due to these solder joints cracking. Luckily this Beomaster 8000 already had an updated board where the via plugs were replaced by soldered in colorful wire jumpers. This shows the processors, the jumpers and the crystals:

Another source of trouble are crystals that stop working reliably. This can cause severe malfunctions in the start-up circuit potentially damaging the main transformer. I started replacing them as a standard restoration item since I had a malfunction in one of my own Beomaster 8000s that caused the uninterruptible power supply (UPS) I run this Beomaster on to go in protection mode. This indicated that very high currents were flowing in the main transformer while the malfunction occurred. More here about this phenomenon. When the exact same condition happened in another Beomaster that I restored a few years ago, I took it as a sign that this is an issue that may be prevalent in many 8000s.

So here we go. This shows the old big crystals replaced with smaller modern 2MHz units. 

You can pretty much use any 2MHz crystal, as long as you also install the correct capacitors. The original crystals use 12pF capacitors to resonate, while modern units usually have 18-22pF. If you look closely, you will see that I also exchanged the two small capacitors directly underneath the crystals. They are now 18pF. 

I also replaced the two electrolytic capacitors on the board with high quality 105C grade Japanese units.

Here are some more photos: This is a detail shot of crystal X1

and a picture showing the solder side:

This shows the solder side of X2 for reference:

After this I plugged everything back together, and it was time for a test run! This shows the display board in all its glory:

Beautiful! Very Beolovely! By the way: One can 'provoke' the clipping light (left most) to come on, too, by setting the tuner to a station and then cranking up the volume. Above about 5.0 the clipping light usually comes on (make sure you do not have speakers connected during this test...;-).

On to replacing the speaker switches!