Need SB-303 Part - Calibrator IC

[Dave Edwards]

Mark....Just FYI....I have two SB303's, and I believe one of them has the 
exact same problem!
....Dave
----- Original Message ----- 
From: "Mark Johnson" <mvjohn at SYMPATICO.CA>
To: <HEATH at LISTS.TEMPE.GOV>
Sent: Tuesday, February 16, 2010 6:38 PM
Subject: Re: Need SB-303 Part - Calibrator IC


Hello Chris, thanks for the reply and your thoughts.

The two 5% resistors, R609, and R611,  had indeed changed value beyond 5%, 
so I swapped them both out with new ones. No change to the bug. I pulled 
C611, and checked it on my cap checker and it's OK. It's value may not be 
close enough to 1800pf so its still a possibility. On spec I replaced D601, 
no change.

Fortunately, a list member has come up with a NOS chip so I'll try that.

Its a weird bug. The 100khz portion works but in the 25khz position, I get 
calibrator signals every 20khz? Very strange. The actual output on the 
calibrator output reads 100khz on my freq. counter when the switch is in the 
100khz position and I get a reading of 60khz when in the 25khz switch 
position; hence the signal every 20khz on the band....

I'm still betting on that darn IC.....

Thanks Chris.

Mark V Johnson VE3DJU/VE3DDI


  ----- Original Message ----- 
  From: ChrisIwata at aol.com
  To: mvjohn at sympatico.ca
  Cc: HEATH at LISTS.TEMPE.GOV
  Sent: Tuesday, February 16, 2010 6:21 PM
  Subject: Re: [HEATH-TEMPE] Need SB-303 Part - Calibrator IC


  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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