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Sunday, November 20, 2016

Beomaster 8000: Input Selector Trouble - Tuner Audio Mixed with Phono or TP1 Inputs

*************************Achtung: There is a follow up to this post. **********************

Oh well...I ran the Beomaster 8000 that I finished up recently for some time in my Beosystem 8000 setup and all was well, but then it developed a strange issue: When switching from FM tuner to Phono the phono signal was audible, but the tuner signal was still there in full strength. So instead of switching from one input source to another the Beomaster decided to mix them together instead! So back to the bench it went!

After I opened it up I discovered that I could provoke the issue by wiggling P82 on the uprocessor board. P82 transmits the microcontroller's wishes to the input switches on the preamp board. So this made immediately sense: The FM input did not get turned off when opening up the Tape 1 or Phono inputs due to an intermittent connection (the audio input switches on the preamp board are basically gates that open and close depending on high/low control signals from the processor).

First I thought it was a plug issue, and I cleaned the contacts a second time and bent the header bins a bit to give everything a bit more mechanical intimacy, but the problem persisted. So I came to the conclusion that the issue must be something more serious, like cracked traces or bad solder joints. This photo shows the original condition of the P82 header from the solder side:

Close optical inspection did not reveal anything abnormal, but I was able to demonstrate that at least Pin 1 had an issue since I was able to 'disconnect' it from the component it is connected with by flexing the board a bit. So I put in a magnet wire jumper and put the board back in. After firing the Beomaster up, well, no change...o.k. I took the board back out and this time I jumpered all five solder points to their next neighbors:

Magnet wire is really very helpful for this type of fix, since the polyurethane coating burns off when heated with the solder iron. This allows to make safe wire connections in tight spaces.

After this fix, the problem seemed to be gone. So I put the Beomaster back together...let's see if it is stable now! I usually test these babies for a few weeks until I send them back to their owners. There are just too many components and interconnections that can develop problems, and often they are intermittent. The only way to make reasonably sure that the experience on the customer end is a happy one is to give them a good testing under normal operation conditions. But I sure hope that this was the last post about this particular Beomaster 8000...;-)!

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