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Wednesday, February 7, 2018

Beomaster 8000: Replacing the Processor Crystals

A Beomaster 8000 that I restored a few years ago for an Australian customer unfortunately had to come back to my bench due to a malfunction of the processor board. It exhibited rapid-fire power on/off events, which are a sign for a dead crystal on IC4 (slave processor). See here for more detail on how I first ran into this issue on the 8000 that is in my living room. Anyway, here are a few pictures of how I fixed this one. This shows the original setup:
As you can see someone in the past soldered jumper wires directly to the pins of the microcontrollers (savages!) to bridge the often failing vias that connect some of the processor pins to other components on the board. I usually re-solder the vias instead of putting in jumper wires. The reason that these vias fail is poor initial soldering. In the 80s electronics manufacturing just started becoming fully automated and some outcomes were not as high quality as one would desire...

Anyway, here I had to deal with these wires since it is good practice to take the microcontrollers out before replacing the crystals. They can be charged with high voltages when you take them out of the packaging, and this can fry the ICs...and that essentially would mean that a donor Beomaster 8000 would need to be procured. I was able to remove IC3 (right) without trouble but IC4 on the left was stuck. When the wires were soldered on a lot of solder was used and some of it penetrated the lower regions of the socket. I unsoldered the affected legs of the socket and then pulled the IC out. It came out including the three soldered-on pins from the socket. Once it was out it was easy to clean the pins up since I was able to get to the solder without having to heat everything for a dangerously long time...much better for the survival of the precious IC.

I implanted a new IC socket
And then exchanged the crystals and oscillator capacitors (the original ones are 12 pF while the modern crystals need 18 pF caps):
I elected to not solder the jumper wires back on to IC4. Instead I fixed the vias. I left the wires on IC3 in place since every time one heats up a pin on an IC there is a slight chance that damage can occur...

Once it was all back together, I fired the Beomaster up for the first time, and it came on normally, no more relay rattling etc...so I hope this did the trick.

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