[Heath] IC availability

[ChrisIwata at aol.com]

In a message dated 2/1/2011 2:13:07 P.M. Pacific Standard Time,  
lwiederh at worldpath.net writes:

Can anyone guide me to source of IC 442-626  MFC6030 or 78MGT4 for SB-104 
Power Supply? Unavailable at Mouser, Digikey and  Newark.
Thanks for any help.
 
Lou  Wiederhold

 
I tried to find this IC about 5 years ago with no luck.  Only  suppliers 
were surplus wholesale outlets, and you couldn't buy just one.   When the IC 
blew on my HP1144 PS here's what I did:
 
Replace (within the SB-104)  IC202/Q1and associated circutry with a  fixed 
three-terminal 5V regulator, freeing the IC for use in the PS. If you  use a 
LM7805K (aka LM340K-5) it has the same physical form as Q1, a TO-3  
transistor.  You can ground it to the chassis for a good  heatsink. These 
regulators are also obsolete, but back in the day of TTL  logic almost every PC board 
had one on it, so they're not hard to find. If you  can't find the TO-3 
regulator you can use the more common LM7805T which is  TO-220, but you'll have 
to figure out how to mount it. This mod is actually  better than the 
original circuit.  The three terminal regulator provides  more accurate and more 
stable voltage than did IC201/Q1.  Before I made  this mod, I blew a few TTL 
ICs on the SB-104 before discovering the IC202/Q1  voltage was about 5.5V, 
not 5.0V as prescribed. I blew IC101 which is  also obsolete! This is because 
R252 and R253 are 10% resistors whose values are  prone to change with time.
 
The reason the IC blew in my PS  was the computer-grade  capacitors became 
leaky with age.  This put an added strain on the IC  trying to regulate the 
voltage.  You should check this out. BTW,  computer-grade caps are easy to 
find surplus. 
 
A year later, Q3 blew on the PS.  This again killed the  IC.  I bit the 
bullet and did a total re-design of the regulator  circuit using a LM723.  
 
The HP1144 PS has a serious design flaw: It has no over-voltage  
protection.  If you blow one of the RF finals in the  SB-104 you'll have a helluva 
time replacing it, they're obsolete too. And  theoretically if you blow one, 
you have to replace all four! You should  seriously consider adding this 
feature, it only takes a SCR and a few other  parts. 
 
Chris, KL7DM 
 
 
 
 
 
 
 
 
 
 
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