Need SB-303 Part - Calibrator IC

[Chris Iwata]

The MC724P is a very obsolete digital IC and you'll be lucky to find a  
replacement. It is RTL which preceded TTL and CMOS digital and became obsolete  
quickly. The NTE equivalent is no more. 
 
The IC might not be bad if it still works for 100Khz.  Instead the  
odd-ball divide by 4 circuit is likely the culprit.  Check the values of  the 5% 
resistors and C611,  these are for timing and may have changed  over the 
years.  Also check that D601 is neither open nor shorted.  An  unlikely 
possibility is Q602 has gone bad. 
 
Maybe simply S704 isn't making a good connection in the 25Khz position and  
needs cleaning? 
 
Chris KL7DM 
 
 
 
 
In a message dated 2/16/2010 4:13:01 A.M. Pacific Standard Time,  
mvjohn at SYMPATICO.CA writes:

Heath  Team;

I'm in need of IC-601 for the SB-303 Calibrator. This is Heath PN  443-8, 
or a Motorola MC724P or NTE9924. 

The 25khz output is not  working and I believe that my IC is shorted. 
Anyone have one of these? I'll  buy an entire BFO/Power Supply board from a scrap 
SB-303 if required.  

Thanks!


Mark V Johnson  VE3DJU/VE3DDI


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