-= A DIY Guide to Computer Repair & Modification (PC and Amiga 500) =-

                  -=<* About This Document - Disclaimer *>=-

This document is intended to provide electronics hobbyists with some useful notes the author has gathered whilst working on various systems. It is not intended to train or encourage novices to attempt service or modifications to expensive appliances. It is especially not intended to encourage unqualified personnel to tamper with dangerous appliances such as monitors, power supplies and other devices which contain vacuum tubes, live mains or high-tension rails, or other hazardous areas. Any such work carried out is done at the reader's own risk and the author accepts no reponsibility for damage to property or person which occurs as a result of attempting any of the procedures described herein.

        Having said that, I can vouch for the workability of the modifications listed here. I have personally performed them all myself on my own machines (this is, after all, a chronicle of my own experiences, chopped up under suitable headings for your convenience). With regards to the service information, whilst I have described problems and the solution(s) I found, keep in mind that this is rather like a doctor prescribing medication over the telephone or from an encyclopaedia. What I mean by this statement is that different problems may well cause the same symptoms; to take a trivial example, if you turn your monitor on and nothing happens, perhaps the fuse is blown, but on the other hand, perhaps it is not plugged in, or perhaps there is a more sinister fault in the power supply section. For this reason, I suggest you use the service information as a very vague guide ONLY. Remember; it is very rare to find two machines which are exactly the same inside, and so what I have found to be a suitable solution to a given fault may be totally inappropriate to your situation.

                      -=<* The Commodore Amiga 500 *>=-

i. Serial Killer
----------------
Problems with Amiga serial ports generally fall into three categories; problems with the flow control lines, problems with the actual data transmission lines, and problems with the line level (caused by faulty line driver chips). The first two can be relatively costly to repair, due to Commodore's monopoly on the CIA and Paula chips, so I advise exploring the line driver section of the circuit before rushing out to buy new Paulas or CIAs. 

Faults in the line driver chips (MC1488 and MC1489, U38 and U39 respectively in the schematic on page F-7 of the book 'Introduction to the Commodore Amiga 500') are typified by either total loss of transmission, total loss of reception, or both. If you look at the flow control lines, (CTS and RTS), you usually see that these lines are behaving normally (exactly why these lines are rarely affected is a mystery to me); if not, then you probably have a faulty CIA. Since the 1488 and 1489 only cost around A$1.00 each, depending on your source, it is well worth replacing these chips before you investigate Paula. Unfortunately, they are not socketed so you will have to desolder them (see "Desoldering Small IC's" below). When you have the chips out, put in a couple of sockets to facilitate later repairs.
       
 If, after replacing the 148x's, you still experience problems, it is time to look further. If you have suitable software, you can check whether the problem lies in the flow control lines (in which case you have a damaged even CIA, U8) or in the data lines (in which case you have a damaged Paula). If you do not have, or are unable to interpret the results of, this software, you can make use of an old Amiga-tech wrinkle; exchange the CIA chips and see if your fault clears up. If it does, then ten to one the problem is in the CIA which started out as U8; if the problem persists, then it is most likely a Paula fault.