[time-nuts] FRS-C TTL / sine outboard filter question

Mike S mikes at flatsurface.com
Wed Jun 23 13:09:51 UTC 2010

At 12:54 AM 6/23/2010, Leigh L. Klotz, Jr WA5ZNU wrote...
>At the local flea market, I picked up what appears to be an Efratom 
>FRS-C.  It is marked "TTL" internally.  It has the passive connector 
>board, but not the active board with the 15 MHz synthesizer on it.

Connector board? No such beast described in the manual. Do you mean the 
A3 power supply board, which has the i/o connector on it?

Active board? Ditto. Do you mean the A4 oscillator board, which has a 
20 MHz VCXO on it? There's no 15 MHz anywhere in an FRS-C.

Here's a manual: http://www.to-way.com/tf/frs.pdf

>Mine is marked "TTL" internally.  The service manual has a chart 
>showing the differences between the sine and TTL options, and I 
>converted it to the sine version by changing a jumper to a resistor 
>and populating an LC filter with 10uH and 100pF (~5 MHz).  I also 
>terminated the RF connection on the connector board with a 47 ohm 
>resistor to ground.

It sounds like you made the changes to the A4 oscillator board, but not 
the ones to the A3 power supply board (several inductors, resistors and 
caps). I found that using a 1 uF for C16, instead of the documented 0.1 
uF, gives a better signal. See the top of page A-15 in the manual. 

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