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!