Faulty A1200 Motherboard

[RobSmithDev]

Hi Everyone, first post on here so Hi!

I have acquired an old A1200 motherboard (Rev 1D.4), that by the looks of it has had a dodgy recap done (picture attached, highres image here). Anyway, after fixing up some poor solder joints it looks like there's some issues to fix.

There's no output on the composite video (and it looks like there's been some dodgy solder work around U12 which I believe is the composite video generator), i'll look at that later.

I haven't got an RGB to SCART/VGA cable, but did have an A520 modulator which works and I can use to get a composite output from.

When powering up I experience the following:

1. Power light comes on
2. Pale yellow gray screen
3. Floppy light comes on (I only have an external one connected at the minute)
4. Pale green screen
5. Very faint Amiga screen (colour looks ok) with the disk animation, but it quickly fades to black
6. During this fade a few clicks can be heard from the external drive, then it stops.

My guess is there's a power issue somewhere. The power supply happily powers by A500+ unit (although its one of the old heavy ones from the A500) so more likely motherboard related.

Looking around I haven't found a board exactly like this one, but I don't know its history and as I said above looks like someone has been messing around.

I can see all the surface mount components next to the RGB connector have been re-soldered too.

Just looking for some pointers to start with, my first guess is to check all the capacitors are actually connected properly and then i'll start tracing out the power traces. If anyone has experienced anything like this your input would be most welcome.

Cheers

Rob

(PS: I'm the one who's doing this project http://amiga.robsmithdev.co.uk/ and am about to release the writer for it too)

[dlfrsilver]

try to get a scart cable. From the look of your highres pic, to me the soldering looks goods.
The problem is coming from somewhere else.

[Daedalus]

Often there are video issues due to corroded tracks to the left of the IDE port as a result of capacitor leakage in the area. Resoldering alone won't solve the issue if that's the case, the parts have to be removed, the board cleaned and damaged tracks repaired.

[RobSmithDev]

Quick update, been poking around with the multimeter, and sometimes with the A520 plugged in im able to keep a picture, its *very* dark, I even got it to boot a disk so there can't be that much wrong with it.
Probing the monitor port I noticed that when the picture goes the analogue out lines goto zero, but the digital lines and sync lines etc are still working properly. Something analogue related here. Next thing i'm gonna do is check around U30, the video DAC as I suspect this is the key. So far I can confirm it has power, but I think I need to hook it up to an oscilloscope.

Also, weird one, left it on a while with the disk animation running, and it crashed on its own (software failure message) - Not sure what this suggests, maybe faulty RAM!? I am starting to agree with you Daedalus that there are some corroded tracks somewhere.

Quick question, has anyone ever seen ther U30 video dac socketed before like in my photo? Im trying to work out if someone did this, or it came from the factory like this. I cant find a picture on the internet anywhere with it socketed.

[Daedalus]

Hmmm, nope, I don't think any A1200s ever left the factory fitted with sockets, but it's common when repairing machines to have one board fitted with sockets so you can quickly test other parts. I have an A600 that I've socketed for example. If this wasn't someone's test bed, it looks like someone's tried to repair the problem already by swapping chips.

As for the measurements, don't let the digital lines throw you off the scent - they're not used by the A520 (or any monitor from the past 3 decades), and so aren't loaded by the modulator's circuits. The syncs are driven independently from the analogue lines by standard digital outputs on Alice. I would have a close look at the various other components involved in the analogue side, such as the clamping diodes, resistors and capacitors, some of which might be on the underside of the board, though with the whole screen dimming together, I'd be more suspicious of the video reference voltage.

Crashing on the Insert Floppy screen is a pretty fundamental problem somewhere, either RAM, overheating, a problem with one of the custom chips, board damage, or even power supply. Your power supply does appear to be fine if it works with a 500+, but I'd stick a scope on it anyway, just to make sure it's nice and clean on the +5V line. Lisa could also be causing problems, but if it's already been replaced (which is likely given that it's socketed), that sort of rules that out.

The board doesn't look too bad from the photo, but really it's hard to tell without having a much closer look. There could be damage to traces under the parts if there was a leak at some point, it's worth having a very close look just in case.

[dlfrsilver]

About your PSU, has it been recapped ?

[fordav1]

All chips will probably be ok. I would not mess with chips unless you know them to be blown up. Generally the A1200 is a very reliable machine and chips don't go bad.
In almost all cases you will just have a broken via somewhere, or some damaged tracks caused by butcher repairer. All you have to do is go to http://beta.amigapcb.org/, load up the A1200 motherboard then sit there with a multimeter set to continuity for possibly several days and trace every pin on each chip to where it goes. After some time you will find the broken trace/bad via and simply join that with wire or patch the via directly with a very thin piece of wire soldered both to top and bottom of the via and your problem may go away. if not, continue tracing. eventually you will fix it. I have done exactly the above multiple times and every time it was a bad via caused by leaky caps. Every time it was a connection between Alice, Paula and the DRAM. Some had additional sound faults and they were bad connections between the TL084/LF347 op amp and the surrounding 22uF SMD capacitors.
If you suspect a connection on the video DAC, start by tracing all of the pins using the site I mentioned above. More than likely there is a broken track under the socket. If you don't find a fault with the video DAC connections migrate the tracing to the surrounding parts.
The key is before you make it worse by pulling good chips, trace all connections so you know the circuit is good.

[max232]

Probably not the case in this instance but are the solder joints good on the underside of the power connector.

I had one 1200 were the screen faded/flickered sometimes going black then coming back on.

Someting being triggered when a floppy was inserted.

The firm click as the floppy was being inserted was moving the very fine cracks in the solder on 3 of the large pins under the power socket.

[RobSmithDev]

So a little update as to my progress

Its a Philippians board so was completely missing the 1.2v reference. Soldered in the correct part and after finding a poorly soldered resistor on the reverse I now have a stable 1.2v ref.

So after a long wait (booting - im about to try with an internal fdd) the fading issue has gone away. But its now black and white coming out of the A520, I know its PAL and it gives colour on the A500+ so something is still wrong here. And still no output from the onboard composite output suggesting that video encoder chip is faulty, although that can wait for now (unless its causing the above issue somehow)

Time to get my scope out (unless anyone has any pointers?) - I will be trying the above suggestions also. Not gonna be beaten by this board.

[MigaTech]

Looking at your clear image your Video DAC chip is seated in its own socket, which is cool by the way. This means you can check function without de-soldering. Anyways on other rev boards there is a wire jumping the pins on that DAC, could be a thought or nothing at all. But just in case you wasn't aware. From memory though I think it does cause display problem if you remove the wire from a DAC that needs it. As your board isn't stock it could of been miss fitted when the PLCC socket was mounted.

[RobSmithDev]

Looks like on the 1D4 board the above mentioned wire isnt required on the Video DAC (it seems to be physically connected on the motherboard, so obviously corrected in this revision). Anyway, turned it on today and momentarily had a color picture! Which then dropped back to black and white.

Whilst doing some trouble shooting on the board, specifically what was going on with the CXA1145 encoder I noticed that it wasn't receiving a signal on Pin 10 (CS_IN), tracking back to U23, I noticed there was a problem with its pin 3 which goes off to Lisa. I started probing at this pin on Lisa (26) and the colour came back. Now some really weird stuff happened. When I let go it went back to black and white, and if I pressed slightly harder it stayed colour when I let go. Repeating this a few times either disk animation froze or the picture went black!

Whilst I don't know if the above pin is actually directly the issue it really does point to some dodgy connections or perhaps broken tracks/vias around and relating to Lisa.

I guess my next step is testing each one. While schematics are great, I thank you so much @fordav1 for the link to http://beta.amigapcb.org/ its amazing!

@dlfrsilver no the PSU hasn't been recapped, after getting the scope on this and seeing a perfect signal I'll leave this one for now, but its still on my todo list.

@MigaTech Its been very useful. The videodac does appear to be working great (when its working), and when its not, its not receiving the input signals.

[fordav1]

the CXA1145 is near the SMD capacitor area and tracks to/from it pass near the caps around the power connector area. If you get an intermittent connection on the video RGB I would suspect some prior capacitor leaking damage in that area. Trace all the connections from the CXA to where they go, and/or trace LISA pins. If LISA is involved you may also have track damage under the IDE connector too. Use pcbexplorer and multimeter and you will eventually find the broken via.

[RobSmithDev]

Thanks for all the input! It’s very useful to get hints as to where to look!

So made a small breakthrough. Was looking at Lisa socket under magnifation (using my phone) - we’ll did t look quite right but everything tested ok. Cleaned her up, the socket and the DAC chip with alcohol and massive improvements! The system is now a lot more stable. Pressing anywhere on the board now makes no changes and I get colour output. This does still flicker to black and white but now looks more likely dodgy joints around the RGB socket. I also discovered some pins on the CXA1145 that aren’t making electrical contact with the board so that’s not gonna help. Gonna re-solder them and trace out as above.
I did finally get it to boot a disk, but he sound is VERY quiet and muffled - haven’t looked into why yet but I have a few guessses already.

[MigaTech]

Good to know you are making progress. It's all about perseverance with the Amiga. As for the sound issue, replace the caps, most likely the problem. You are almost there!

[Chucky]

No composite and bad sound.. yes capissue.. BUT just replacing caps will not do it.
leakage have damaged traces around it. so you need to do trace-repairs.

[MigaTech]

Originally Posted by Chucky

No composite and bad sound.. yes capissue.. BUT just replacing caps will not do it.
leakage have damaged traces around it. so you need to do trace-repairs.

Not always the case but yes it would be worth checking the continuity of the surrounding traces.

[Daedalus]

Originally Posted by RobSmithDev

I also discovered some pins on the CXA1145 that aren’t making electrical contact with the board so that’s not gonna help. Gonna re-solder them and trace out as above.

Yep, the chroma signal to/from that chip will cause a loss of colour if not connected. You're best off removing the chip completely along with the other components nearby and cleaning them up, as the electrolyte leakage in the area will continue to corrode things under parts if you don't clean it up.

I did finally get it to boot a disk, but he sound is VERY quiet and muffled - haven’t looked into why yet but I have a few guessses already.

Yep, as others have said - capacitors. But at this stage there's likely a lot of leaked electrolyte on the board. Even if the traces haven't been damaged, there's still probably a lot of cleaning up to do. Be sure to do it, because if you leave it there, the electrolyte will eventually eat its way through those traces.

[RobSmithDev]

@Daedalus - looks like someone has already had this chip off once which probably explains why its been badly soldered back on again. But i'll have another go and clean it up.

Ironically its not a capacitor issue with the audio, was probing around the board and spotted some very odd voltages feeding U15. Tracing around the +12v was completely missing. Continuing I found that R301 was missing which is part of the power supply to the chip! I have no idea what the previous owner of this board was doing, but explains the distorted signal. I temporary shorted it with a pencil lead (yeah I know its not 10R but its something) and the sound came back loud and perfect!

Also noticed C322 was missing which is part of the audio filter circuit!

FYI: Although done badly, I could tell the board had been recapped before I got it by the mess they had made!

Aside from soldering in the new SMD parts (when they come) i've only the composite video output to fix.
I'm going to remove the tv modulator anyway and hack something else in, either an S-Video output or squeeze in a composite->HDMI converter adapter, but that's just an idea right now. Also have it hooked up with an RGB to VGA adapter and the picture is great (a little bit of noise down one side, but that could be the lack of shielding right now)

Thanks for all the help so far

[Daedalus]

Yep, I've seen those resistors fail before and cause very low and distorted audio. One going missing is a different story though! Anyway, it's good that you're sorting out all the issues. It'll be very satisfying when it's done

[RobSmithDev]

Whilst waiting for the replacement parts to arrive I de-soldered the modulator and surrounding support circuitry. This leaves that area a little less cluttered. I also had a go at fixing the pin on the CXA1145 but looks like a mod-wire is needed here, I have a feeling the pad is damaged.

Whilst doing this I had a look at the composite out, and noticed I was getting the signal from Pin 20 on the CXA1145, it just wan't getting to the back. Traced around, something is wrong with how C237 connects to the motherboard, so that'll be another mod-wire or re-solder job, either way, with a big of wiggling I got an output which is excellent!

Nearly there, just a bit of tinkering, cleaning and replacing parts and i'll have a fully functional A1200!

The next part is the whole Retrobrite process, but I'm going to try something a little different. I havent had a whole lot of success with this process, but watching [ Show youtube player ] (8-bit guy) he did some experiments so i'm going to try the Ozone/UV method. I think his experiments weren't as good as they could have been because Ozone breaks down too quickly to be a fair comparison, but i'll take some before and after photos so you can see what happened...

Gotta finish fixing up the miggy first though