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.

[Jacksoft]

I checked around Denise, there where some blacky traces but nothing so bad.
I checked the continuity from CPU/Angus/Paula for _INT3, IPL1-2-3 and they seems good.
I checked also the _INT3 with the oscilloscope and the active (low) signal is present just during bootup but rarely when the floppy animation runs. Most of when the floppy is "full out" and the screen looks good. In the rest of the time it is always high level. Also, if I power on the machine with the probe on _INT3 most of time the screen remains white. More rarely it shows "Software failure" like in the attached picture.
I forgot to mention before (just because I just reinserted the original Kickstart ROM to do checks) that after sometimes (randomly) the screen screws up, like in attached picture.

Thanks all.

[Castellen]

Also, if I power on the machine with the probe on _INT3 most of time the screen remains white. More rarely it shows "Software failure" like in the attached picture.

That sounds suspicious.  Since _INT3 is actively driven by a low impedance output from U2, connecting a scope probe (typically 10M Ohm impedance) shouldn't make any difference.  Sounds a lot like _INT3 might be open-circuit/floating, but if you've confirmed continuity, then the problem may lay elsewhere.  It's also possible that some of the graphics operations use DMA modes, so perhaps pay attention to the DMAL signal.  That should have activity all of the time (groups of 4 x active high strobes), as generated by U3.

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.

I gather the A600 was originally working correctly (no graphics display issues) before you did anything.  And it was OK after replacing the capacitors?  But the problems appeared immediately after reworking U3/Paula?  If so, it's very likely that the fault is a direct result of an issue you've introduced; such as a solder short between two lines, intermittent/unsoldered joint, damaged PCB track/pad, etc.  As opposed to reverse engineering the system by forum posts to track down a potentially very complex issue, my suggestion would be to very carefully go over everything you've done (again), inspect the entire board for a stray blob of solder, etc.

I don't know what your rework process was (i.e. temperature control, etc), hence there could be a possibility that U3 has been damaged.  Though I'd be looking for a connection issue or short between lines to begin with.

[Jacksoft]

I'll check DMAL signal tomorrow.
About U3, I already checked all (removed Paula again, cleaned pads, tested some traces by looking on amigapcb for reference) and seems ok, also redid soldering being more careful.
I'm also having suspects on Paula too, probably some ESD discarge damaged it and yes the problem happened after I removed Paula to check the blacky lines.
I usually remove big stuff at 380°C by moving the nozzle around the chip and grabbing with a pumpet to avoid using tweezer or other that could damage stuff.

Eventually I'll remove it again, recheck and take some pics to post here just for reference.

Thanks.

[Jacksoft]

I checked DMAL pin and I can see some toothsaw pulses.
I made some pictures of the Paula zone, removed it again. I'll re-do some continuity checks asap to make sure everything is ok. I would suspect, in case, of bad or little highter impedence on vias? (My multimeter have the "beeper" in diode mode, not in ohm. I'll check the impedence of the entire line too, in case).
(The green stuff on the PCB in pictures is solder mask applied with an artist brush to make sure it'll not be too thick.)

[Castellen]

I would suspect, in case, of bad or little highter impedence on vias?

Vias should be a normal part of the circuit, so will have a continuity close to 0.1 Ohms.  Anything much higher means the via is open circuit.

Don't rule out the possibility of an accidental solder short between two lines.  This can be harder to find, because the end-to-end continuity will measure OK, but it will also be joined to some other unknown line.  You can usually see this condition on the scope, since if two logic outputs are incorrectly joined, you'll see 'halfway' logic levels as opposed to the usual 0v to 4-5V waveform.

Solder joints on the PLCC look OK.  Rework temperature of 380°C is good.  Could have also been a coincidence that a weakened via somewhere has been damaged while you were performing the rework on U3.  Anything is a possibility when there's been corrosion damage.

Graphics issues could also be caused by a fault with U2/Agnus, as that performs the graphics data move operations during the 'insert disk' animation.  Though I don't see how that would have failed from what you were doing.

The fastest way to progress at this point would be to get a working A500, A2000 or another A600; power the good working machine up on the 'insert disk' screen, and do the same with this problem A600.  Doesn't have to be connected to a monitor.  Use the scope to compare each pin on U2/Agnus and U3/Paula between the good and problem machines to identify any signal differences while viewing the 'insert disk' animation on both machines, especially noting specific signal activity while the disk animation is moving.  Note there are some pin number differences, A500/A2000 have the 8364 in DIP48 package, the 8364 in A600 is PLCC44.  There are also a few minor pin function differences between the 8371/8372 in A500/A2000 and the 8375 in A600.