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Beomaster 8000 frequency problem

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chartz
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chartz posted on Thu, May 9 2013 5:57 PM

For those who did not follow the saga two years ago, I have a problem in which the tuner won't tune at all past 97 MHz, it will go directly to 108 MHz, and I can hear all the available stations very briefly while it goes up there...

The tuner side is okay, I tried a (working) tuner module from another Beomaster.

The power supplies are fine, with new caps, and deliver the intended voltages with no ripple.

I've used the receiver every day since then, and apart from that, it works perfectly.

Old thread from 2011 (archived forum).

Still a bottle of champagne to win.

Jacques

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chartz
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chartz replied on Mon, May 13 2013 6:42 PM

Yes I do have a scope.

It clearly shows a break in the μP signal, which is gradually sweeping (base of 2TR7, pin 19 of μP ) until it loses it completely!

I changed the pre-scaler chip to no avail. 

Jacques

RaMaBo
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RaMaBo replied on Mon, May 13 2013 7:36 PM

Have you checked the signal the µP gets from the prescaler at Pin 21? Is it continuos sweeping or is there a sudden jump or break? Even if the µ264 is exchanged we don't know if it counts well from 98,2 to 118,7 MHz. Remember that the display doesn't show the frequency of the FM tuning oscillator, but the freq. the µP wants to tune it in. Also the signal form of the prescaled frequency should be near a clear square wave.

To check the points one should open the loop of the tuning system: The tuning should be set manual between 87,5 MHz and 108 MHz voltages with a potentiometer. This way you get the control over the tuning circuit as well as over the prescaler. If this is working well the processor may be the fault but before looking for a board which seems to be extremely rare you should try the steps above.

I think i remember the µ264 can be a bit difficult to use and in the old forum i think there was a problem with the same chip and also a prescaler chain in a beomaster 6000(or something similar)? This leads to the HEF4013BP ( or CD4013B) which could also be defective. But all this can only be checked if you manually control the tuning voltage with a pot and check the divided frequency after the 4013 (divider  by 4).

 

Ralph-Marcus

RaMaBo
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RaMaBo replied on Mon, May 13 2013 7:59 PM

 A quick search in the old forum showed these two things:

 

Prescaler supply voltage

and

Further Prescaler problems

 

Ralph-Marcus

chartz
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chartz replied on Mon, May 13 2013 8:52 PM

Yes, thanks, I had come across those before. Nothing relevant I thought. Or is there?

Jacques

RaMaBo
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RaMaBo replied on Mon, May 13 2013 9:04 PM

The first link says that the u264 is very sensitive for the supply voltage and the second one deals with the problems between the TTL divider and the processor input. Could be the same at your Beomaster 8000 and those are things that can be checked without a new processor board.

I guess you don't have a frequency counter? Maybe your digital multimeter can count frequencies. This way you could measure the otput of the prescaler circuit (u264 and 4013). Often a DMM can measure freq up to 1 (output of 4013) or 10 MHz (intput of 4013) at TTL level.

Ralph-Marcus

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