REPAIRING THE B128 POWER SUPPLY 3-25-2013


           REPAIRING THE B128 POWER SUPPLY
           Latest updates and/or corrections: 3-25-2013


I've only worked on one B128 power supply so I don't have much 
experience with it. It is a switch mode type and, as old as these 
are, I expected to find some electrolytic caps dried up and failing. 
Not so. They all checked OK. This PS was intermittent... it would 
come on and go off at random, sometimes "flickering" between states 
with an audible buzz or clicks. The computer didn't work at all.
     I suspected the main switcher transistor, a Motorola TO-220 NPN 
type with nothing but a "house" number on the case: 3821-2. That 
number didn't cross to anything in my data books or online sources. 
It helped that the PS was at least partially working so I could 
measure the waveform on the collector. It would peak at over 900 
volts so I knew that standard switcher replacement devices wouldn't 
hold up. I found a generic replacement in a different case style 
(larger) that was normally used as a horizontal deflection output 
transistor or "HOT" in a tube type color TV set. Its generic number 
is BU508A. It's an equivalent to the Sylvania ECG 2300 (or Philips 
TCG 2300). 
     Because it is physically larger, the transistor would not mount
on the old heat sink. I drilled a hole just above the original one 
and mounted the new transistor with a mica washer (to isolate the 
transistor collector/case from the sink) and added heat sink grease. 
The transistors leads didn't easily fit the holes, so I cut them off 
near the body of the transistor and wired connections from the stubs 
to the board. The new transistor has a higher voltage and current 
rating and so should work well in this application. 

Ray Carlsen