C128 POWER SUPPLY REPAIRS 4-7-2012.


           C128 POWER SUPPLY REPAIRS
                 Latest updates and corrections: 4-7-2012



The power supply for the "flat" C128 computer is a "switching" type and 
therefore runs cooler and lasts longer than the C64 linear PS. However, the 
C128 supply does have a few shortcomings that can make it fail. One of them 
is the glue the factory used to hold components in place during assembly. The 
other is failing solder connections on a few components. Either of those can 
cause intermittent operation or outright failure to function. In addition, 
there is one fuse in the case bottom that will open if there is an overload 
or short circuit on the 9VAC line of the computer. A shorted SID chip caused 
that fuse to open in one C128 I recently repaired. Note there is another 
fuse inside the PS on its circuit board which protects the 5V source. 
     There are a few trouble spots on the PS PC board that should be checked 
and that often require resoldering whenever you open a C128 power supply. If 
any solder appears to be breaking loose, it should be resoldered. See photo. 
Always apply new solder... don't just reflow the old. Places to check are 
the terminals of the power transistors and large components like the filter 
capacitors. Check also for cracks in the PC board from rough handling. 
Bridge over any broken traces with wire soldered to both sides of the break.

THE MYSTERY FAILURES CAUSED BY GLUE:
     Factory glue looks like ordinary contact cement and is tan or light 
brown in color when new. As it ages, expecially near sources of heat such 
as power transistors, the glue turns brown and eventually black. Black glue 
is hygroscopic (absorbs water out of the air) and that is what damages the 
power supply circuit board. The resulting moisture under the glue can cause 
abnormal conduction between circuits and will eventually cause corrosion 
of copper board traces and component wiring. The failed glue must be removed 
before it causes permanent damage. When new, it's very adhesive and nearly 
impossible to remove, but when aged, it scrapes off easily. I use a dental 
pick to do the job. After the glue is removed, if the resulting corrosion is 
not too severe (no open circuits), the PS should work again. Of course any 
component destroyed by corrosion or damaged from glue removal efforts must 
be replaced. 

DISASSEMBLY:
     The case comes apart after removing four screws from the bottom. Clean 
the residual of plastic off the ends of those screws before you put them 
back... it makes that much easier. With the case open, you'll notice the top 
might be stuck to something inside. It's the double-sided tape that is stuck 
to the transformer. You may need to pull the chassis out of the bottom 
half-shell in order to pry it off the top. Don't stress the PC board or you 
may crack it. With the assembly apart, examine the board top and bottom for 
brown or black glue. Scrape the glue away from the wiring and components 
(see photos) and resolder any connections that look like they are breaking 
loose. 

REASSEMBLY:
     After the repairs are done, set the assembly back inside the bottom case 
half. The transformer has a recessed area it fits down into, and the PC board 
should sit flat against the bottom of the case. If it's tilted up on one 
side, there is likely a pinched wire underneath which must be rerouted out of 
the way. Now press the AC and DC cords into their places and put the top cover 
back on. Make sure the case top seats fully and evenly all the way around 
before you put the screws back in. If not, wiring is likely holding the board 
up. Once these repairs are done, the power supply should work fine again. 
Actual component failures are rather rare. 

Ray Carlsen CET
CARLSEN ELECTRONICS

Please let me know if you spot any errors here. I appreciate feedback!

 

Retro Computer Scene Competition celabrating  OUTRUN 10 year anniversary