A600 glitches after recap

[Jacksoft]

Hi all,
I encountered a strange issue during/after a recap on A600 rev. 1.5.
I've not repaired so many Amigas but fixed some 500's with minor problems.
This machine had floppy drive issue and no video on composite.
No video was caused by Z222 and I bypassed it just to make sure it works, floppy drive was recapped and now looks working' (not fully tested yet), so there's the main A600 plague, bad caps and some corroded traces.
Initially the machine worked ok, after I replaced all the capacitors (found about 4-5 ceramic caps on the 5V line in nearly short too and replaced them!) and removed Paula to check traces under the chip (some of them were "blacky") I putted all back and powered on the A600 with some ugly glitches on the half-down screen like in attached pictures.
For older machines like A500/500+ I used the Test Disk floppy but in this case, after loading the Test Disk, I could only see garbages on the screen, so I bought a DiagROM and did some tests. Most of all seems working ok, except option 1, 2, 3 in Graphics tests: it causes glitches, empty screen or no video at all. RAM, CIA, IRQ are ok, sound tests are ok.
I also tried disabling and removing Paula again (thinking about some wrong soldering or other) but nothing changed.

I've seen around that this kind of problems is caused by a bad Agnus, but I don't understand how it could be correlated the work I did.
Before starting messing anymore around and buying replacements there is some others checks and tests I can do?

Thanks in advance.

[F0LLETT]

How do you know, the ceramics were nearly short?
Did you remove them and read them, as in circuit they may well have some signs of resistance. Unless they are dead short, they are proberly fine.
Only way to check 100% is to remove them and test them.
I certainly would not be messing with paula. Its a video fault, so if leakage, check DAC, BT101 and Denise areas.
Bypassing things is not a great idea, did you actually solve that issue, as you do not say exactly.

PCB Explorer is anawesome tool, to help see where damaged track goes.
http://amigapcb.org/

[Jacksoft]

Hi! Thanks for the fast reply.
About the ceramic caps I noticed that one under the board was very hot. I removed and tested it in Ohm and returned me about 33Ohm. But one ceramic cap in short or nearly can happens. The strange fact is that other two caps on that line where also like this (60ohm, 100ohm, etc). Also the one on the Paula alimentation (after the big 1R resistor) had trouble so he messed with the tension partitor for the 2.4V.
I removed them all and tested and about 4-5 of them where faulty (this is REALLY strange).

I'll check where you suggested and report you asap. Thanks!

[Castellen]

Sometimes multi-layer ceramic capacitors can go resistive if they've been subject to physical stress (cracking) or contamination such as corrosion damage from capacitor electrolyte.  It's not uncommon for this kind of fault.  If the capacitor appears damaged/corroded or it there's any sign of heating, it should be replaced.

The valid clue here is that the 'insert disk' animation is getting screwed up.  The fact that the rest of the graphics mainly appear to be drawn OK suggest that the graphics output hardware (U4, U12, etc) is likely OK.

I'm not sure exactly how the disk animation works at the software level, but in hardware it relies on the level 3 interrupt (_INT3 on the schematic).  This is generated by U2/Agnus, which gets passed to the interrupt handler U3/Paula; presumably the software is relying on the interrupt to move the disk image data between different areas of memory to create the animation, not sure.  Based on the clues in the screen photo, something about this interrupt is broken.

While viewing the 'insert disk' screen, monitor what's happening with _INT3.  The line should be inactive all the time (high), and only strobes active (low) while the disk is 'moving', with an active duration of around 200nSec.  Make sure this signal is arriving at U3/Paula pin 20.  If that appears to be OK, there's possibly something broken with the interrupt priority level lines from U3/Paula and the 68000, so check continuity of the three IPL lines between U1 and U3.