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...

Beomaster 8000 Tuner Repair: Exchange of Prescaler U264B

***********************this is a follow-up to this post****************************

Yesterday the U264B prescaler ICs arrived from Germany and the UK. I bought two batches of Telefunken NOS ICs from to make sure I would have something that actually works. It turned out that both batches worked. I guess now I have enough U264Bs to last me for the next 40 years. This does feel good...;-).

I unsoldered the old chip (series TFK 208). Before:

The new one I put in using a DIP8 socket. I wanted to test all the U264Bs I received, and this enabled a fast swapping of the chips. Here are the pics of the empty socket and with a new old TFK 225 installed:

I also got some series 234 and they also worked. This puts the question to rest whether the series number matters...it does not. The tuner works very nicely now. However, I wonder why this IC died. Maybe it is time to consider putting some TVS diodes on the power rails to prevent over-voltages.

On a general note, I always run my B&O on uninterruptible power supplies to keep them safe from the all to often occurring brown outs and voltage spikes in our 3rd world style power grid.

One more comment about the defect U264B: When I measured the voltage at pin 1, the meter showed only about 1.2V instead of the value of 3.3 in the circuit diagram. The replacement chips showed a value of 3.25V. Maybe this can be used to identify a broken chip if one is far from an oscilloscope.