I'm trying to diagnose and fix a rare fault in an Amiga 1200 rev.1D1 and it's driving me nuts.
Testing the machine with Amiga Test Kit v1.20, all starts with the music module not being played at all (it happens also to a bunch of games, others play music fine) and the CIAB timer tests faults pointed out by ATK, it shows CIABTA, CIABTB and CIAB TOD IRQ as faulty.
The test displays exactly this screen:
Loss of int6 at even cia
So, a naive approach suggests that INT6 signal is missing and the CIA is bad as appeared in the net in pages like this (being CIA-A/INT2 this time):
A1200 CIA Timer Failure
As I've already socketed U8 (Even CIA), I've replaced it with at least 6 new CIAs, some of them sourced from working motherboards (I mean, at least the CIA subsystem...) and none fixed the errors.
The socket seems fine, I've checked the following:
Socket is in good shape, no cracks.
Every CIA-B pin seems correctly soldered and shows continuity with the rest of the components as shown by the A1200 schematics and Amiga PCB Explorer.
No bridges between neighbouring pins.
AFAIK, INT6 signal is generated by CIA-B itself and then injected to Paula (U3) and Gayle (U5). There's only a passive component (R954A) in the signal path and seems fine, measuring 1K. If I run the ATK music test and check pin 23 (INT) with an oscilloscope, I can't read anything at all, no signal, 0V.
So, I'm wondering the following:
Could it be that all my CIAs, even the ones working as CIA-A are faulty and can't generate INT signal?. I know that CIA-A INT signal must be a working IC as ATK tests passed OK 100%.
Are any other signals involved in generation of INT6 by the CIA-B and some are missing or out of spec?
Could a faulty Gayle or Paula pull down the signal and making it disappear?. I can try isolating the CIA pin; but I hate cutting tracks...
Any help will be appreciated...