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Saturday, March 8, 2014

Beomaster 8000: No Tuning Above 93 MHz

A 'Beofriend' sent me an email about an interesting tuner issue, which is closely related to a problem I had last year with a Beomaster 8000. In my case, the tuner would not respond to the frequency setting with the rotary encoder, and when set to a frequency close to 91.7 MHz, I could hear the stations of the entire frequency band 'zoom by', either up or down, depending on being slightly below or above of 91.7 MHz. The relevant blog entries are these:

http://beolover.blogspot.com/2013/09/beomaster-frequency-counter-feedback.html

http://beolover.blogspot.com/2013/10/beomaster-8000-tuner-repair-exchange-of.html

Now, the problem described by Beofriend was that the Beomaster would properly respond to the frequency setting in the lower range (he said below about 93 MHz), while in the upper range the 'zooming' issue would occur. Very interesting!
It turned out that the cause for this was a broken flip flop chip, which is in the frequency feedback after the prescaler for a further frequency division by 4 to make the signal palatable for the ancient 6501 micro controller IC4 (which runs on a 1 MHz clock, i.e. anything above this frequency appears as a blurred whirl to it...hard to believe that there was a time when 1 MHz was considered 'fast'...;-).
Here is the relevant circuit diagram section:















After replacing IC8 everything worked again.
I really wonder how these ICs can fail (in my case the prescaler (IC5) needed replacement...In my opinion the most likely failure method for silicon is a too high operating voltage or too high signals. Probably another reminder to put our cherished Beomasters on uninterruptible power supplies (UPS) to put a buffer between them and the power grid/lightning strikes...



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