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Sennheiser SK50 re-tuning (and help with dual modulus prescaler)


Richard Thomas

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After picking up a very cheap Sennheiser SK50 transmitter and doing some research, I found it should be possible to re- tune it by replacing the PROM chip which contains the PLL prescaler values.

Sennheiser will re-tune it at the factory at a price, but I've go this far (and learned quite a bit about how radio mics work in the process)-

Firstly I've managed to read the PROM using an Arduino board: http://hackaday.com/...ade-rom-dumper/

and got the following values (HEX), corresponding (?) to frequencies (MHz):

48054385 724.9

48444385 724.1

40724385 721.4

48414385 719.3

48604385 718.1

400F3385 715.2

400E3385 713.6

406C3385 711.6

484B3385 709.7

485A3385 706.7

48683385 705.3

48183385 704.3

40673385 703.6

40373385 703.0

48073385 702.5

47663385 702.1

According to a bit of the service manual (found here http://dc207.4shared...70/preview.html) it uses a dual modulus prescaler, dividing the VCO by 128/129. This should require a 7bit A value and an 11bit B value.

The standard crystal oscillator reference is 6MHz and reference divider is between 5kHz and 25kHz (depending on frequency)

This is the part where I'm not 100% (I can't find the reference value):

After trying various combinations, the one which seems to make the most sense is to take all the first 5 digits worth and reverse them;

36674, 37084, 37304...

converting to binary and taking the first 11 bits as B and last 7 as A (ignoring the final 2 bits which = 00). A pattern I've found is all the frequencies which divide exactly into 128 correspond with those with an 'A' value of 1 (should this happen?), however I can't find a reference when dividing each frequency by (128*B+A).

Is there likely to be some other constant in the system, or am I doing something wrong?

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Would those be RF amplifier and filter? I'm looking at moving the tuning by a bit less than 100MHz and Senn only do 4 boards between 450-960MHz, so I'm hoping the component values are still OK.

If not, I can get it done at the factory (where they'll probably swap out the board)- I just find this way I learn something in the process

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