|MC1488P x 5|
Having done a lot more reading I understand what the MC1488 is doing in this context. It is normally used in changing TTL voltages to RS232 voltages for serial communications. They contain four 'drivers', a driver being the conversion from TTL voltages to RS232. The MC1489 chip which sits above the MC1488 on the motherboard contains four 'receivers', which convert RS232 back to TTL voltages. In this particular instance, the first input appears is being used as a 'NOT' gate to change the 0v to 5(ish)v from the CIA into a more meaty +12v to -9v.
I won't go into any more detail, partly because it could become very technical but mostly because I don't fully understand it all. Suffice it to say that when the MC1488 has 0v on pin 2 there should be 12v on pin 3 and when there's 5v on pin 2 there should be -9v on pin 3. Given that my MC1488 has 12v - or thereabouts - on pin 3 no matter what the voltage on pin 2, this leads me to believe that there is an open circuit in the chip, hence, replace the chip. :)
So, tops off again chaps...
Unfortunately my phone camera failed miserably and didn't take pictures of the process of me getting the chip off the board. :( Basically, it involved cutting through the legs on one side of the chip - carefully - then bending up the chip to make it easier to do the same to the other side. A few minutes later, with the help of some solder wick and a pair of good quality tweezers and the chip and it's legs are gone.
|The dead MC1488 RS232 line driver chip|
This chip was silver for some reason. It's the only time I've seen one like this. In my A500+ it's black and all of the pictures of the various A500 motherboards on the Big Book of Amiga Hardware website show it as black too. Maybe it was special? Either way, it's dead.
|Awaiting pin removal|
I cleaned the old solder off with solder wick and then cleaned the surface with isopropyl alcohol with some cotton buds. Finally, I used a bit more alcohol and an old toothbrush to make sure no debris or other junk was left nearby.
|New MC1488P installed and ready for action|
Once the chip was inserted it was a two minute job to solder the fourteen pins. Far easier than Agnus.. Once again, when I'd finished I cleaned the board with some isopropyl alcohol and cotton buds, followed by a final scrub with the toothbrush.
Let's get this sucker tested!
I plugged in the floppy drive, power cable, monochrome composite output (who wants to mess with an A520 at this point? Not me.) and audio out left and right to my PC speakers. The A500 booted without any problem but then guru'ed when I tried to run OctaMED. Like a numpty I had forgotten to put the 512kb expansion in too. OctaMED won't run in 512kb...
|Testing - this time with 1Mb.|
It worked flawlessly. The difference in sound is now quite distinct with a definite treble and white noise that wasn't there before. Switching the filter on restores the 'muffled' sound, just as intended. Another repair successfully completed. :)
|Success! The filter is working!|