PRES AP3/4 Repair

This morning I switched on a couple Electrons to settle in to sorting some SD cards for my recently built SD card interface when one didn't give its traditional beep. Checking the CAP LOCK, which is off, usually points to the Electron PSU tripping due to a short circuit. I traced the cause to the PRES AP3/4 disc interface.

Fault finding a short circuit is a fairly easy process, check and/or remove each component to see if the short disappears. There are some components that are more suseptable to failing such as Tantalum capacitors and semi-conductors. Resistors very rarely fail without obvious signs of burning.

Delving in I removed the cover to have a visual check and to start removing each socketed semi-conductor. After checking the four IC's, RAM, EPROMs and 1770 disc controller I removed the circuit board from the case.

Checking the underside there are no clues so I started removing one leg of each capacitor and checked each time for the short - no luck. It looks like this board has been hand soldered with all that flux still on the PCB.

The only parts left to check are the seven 74 series IC's. This is not going to be easy as I have to remove one at a time to find the culprit, a one in seven chance to get it first time. I decided to start with the 74LS156.

I went along the bottom row one at a time and still no luck. Only three left so I removed the middle IC and lastly the one to its left, the 74LS174. Found it! I wish I started top left as I now have to remove all the IC legs and clean up the board.

Starting the process of removing the cut legs.

Ready to start cleaning out each pad.

Holes all clean. I just used a soldering iron and solder sucker.

Time to clean as much of that old flux off the board.

The board cleaned up well.

I decided to use IC sockets so I don't have to go through this process again..unless the 74LS38 fails

I had all the IC's needed to replace the ones I cut out so that's everything plugged in ready to test. The first test revealed that the two ROM's were not recognised and was down to a pin left unsoldered. Once that was corrected it's now working again.

Only thing left to do now is to put it back in its case and re-test.

With the amount of the replacement components it took the best part of the day - It just wasn't my lucky day.