I tried more things, but with no positive results:
- Replaced all tantalum capacitors with new ones.
- Removed RAM ICs from banks 1, 2 and 3.
- Piggyback each RAM IC from bank 0 with another one, one by one.
The only 'signs' of life are two pops played by the PC Speaker, spaced about half second, when the power switch is turned on. Another pop is played when turned off.
I'm running out of ideas. Any advice is welcome.
Thanks again!
1. Test the potential difference between 5V and GND over the CPU.
-if it doesn't equal 5V; One of the other components may be shorting, there is a physically shorted trace somewhere, or either of the power-lines may be broken somewhere before the CPU. Try to find out where in that case, by testing potential difference between GND where the PSU plugs in and any other powerline on the board.
Else, if the potential over the CPU power-lines is equal to 5V;
2. Get an oscillioscope
3. Probe the low address and/or data-lines of the CPU in terms of Ground.
-If no or minimal activity, check the clock chip (8284) output.
--If it clocks, you may have a dead CPU, or some other component (problably one of the bigger chips) may be dead.
Else, if there are some activity; dump the BIOS ROM chips (note that they are not the same pinout as regular 27xx EPROMS). If they are OK, check other chips.
The noise in the speaker is certanly not a good sign, but it does at least show that there is a potentional difference somewhere on the board (indicating power). However, it shouldn't be over the speaker when the computer starts unless something is wrong with:
1. The speaker logic (U63, U95, C9, R10 or RN3)
2. The 8255 I/O port (U36)
3. The 8253 Timer (U34)
4. The CPU or system support logic (buffers, latches, decoders, etc...)
It's likely not 1, as the speaker has potential difference over it when you turn the machine off. However, all of the remaining posibilities should be possible. Try to probe the outputs from U36, U34 or eventually U63 and U95 if neither U36 or U34 outputs noise. The chip that outputs noise is at least failing. It may also be noise on the power-lines, but this should be prevented by the new capacitors you inserted.
Note that if U34/U36 makes noise, it may also mean that the CPU is randomly programming them. This may either indocate I/O errors, broken BIOS, or a broken CPU. Again, an oscllioscope would really come in handy finding out what.
