A1200 repair guide.

[Marchie]

Hi Guys,

I have a non-working 1200 mobo to resurrect, and as it's my first A1200 repair I'm at a loss for where to start. It's a REV 2B, I get a power-LED when I power up and that's about it.

It looks a bit worse for wear at the back of the board, so I'm suspecting possible water damage akin to being left in front of an open window during rainfall or some such.

No caps with obvious buldging, leaking or dull solder joints underneath. Are there any typical problem areas to look at first? Do I just go to AmigaPCBexplorer and start testing traces with my multimeter?

Any advice more constructive than 'recap everything right now' will be much appreciated!

Thanx!

[solarmon]

Post some detailed pictures of your motherboard. Let others advise on the state of it.

For boot up issues, visual clues are the behaviour of the following during boot up:

* Screen (colours and changes)
* Power/Status LEDs (brightness and flashes)
* Caps Lock LED (number of flashes)

So provide as much info as possible for the above.

Then you can start to narrow down on where to start looking in the motherboard with a multimeter and logic probe.

[solarmon]

I forgot to mention about checking the power supply and take readings during load with the A1200 connected and on.

[solarmon]

The Amiga can also be prevented from booting up because of various power good and reset signals - so these needs to be checked to see where it is being prevented from booting up.

You can see where these are by looking at the schematics:

https://www.amigawiki.org/dnl/schematics/A1200_R2.pdf

I have summarised this (or shown it in a different way) in to this diagram, mainly for my own benefit and understanding, but it might also help others:

(This is specifically for the A1200 rev 2B)

[Marchie]

This is all really helpful. Thank you solarmon.

Right now I get nothing on screen (no flickers, no colours, nothing), power LED (don't have a floppy connected at present, so no floppy LED), no caps-lock, the keyboard is untested though, I have a known-good keyboard but haven't had the chance to pull it out of my other system to test yet.

In the meantime I've attached some hires pics of the board, if you see anything obvious let me know.

[solarmon]

What video output method you using to connect to the screen? The only thing I can think of or see, and since you said the rear panel doesn't look good, then that could mean the ports and associated pins are corroded.

Clean the port pins - especially the video ones. Your picture is quite dark and blurry, but it looks like the pins on the 23-pin video port are quite corroded. Try also differrent video output - like composite, or RF.

If there is a problem with video output, but the A1200 is actually booting, then the floppy and keyboard might indicate that it has booted up (and you just can't see it).

Does the power LED come on and then goes dim?

[solarmon]

It might be just the lighting, but the board looks like it needs a very good cleanup - there does seem to be a covering of green corrosion on a lot of the components, especially those by the rear ports.

Provide some better, clearer, and closer pictures of the motherboard and ports.

you should also check the bottom side of the motherboard too - you'll need to remove the hex nuts fixing the ports to the bottom shield.

[solarmon]

If you haven't already done so, remove the kickstart chips using appropriate tools and/or methods, and inspect the sockets and pins. Clean the sockets and pins using isopropyl alcohol with a clean toothbrush, and/or preferably with a wire/fiberglass brush pen.

[paul1981]

Regarding the video, there is a black screen and a blacker than black screen when there is no video at all. Which is it?

[Marchie]

Thanks for all the help guys,

Removed the ROMs, sockets look pretty clean to me (pics attached) wiped but haven't gone the full toothbrush yet. Tried a different set of ROMs, still nothing. Have also taken some closer photos of the rear ports.

Attached a floppy drive, I get solid power and floppy LED, no dimming, etc, just straight on, no drive clicks, so pretty sure it's not booting at all.

Screen gives a white line on first power, then black screen (NOT blacker-then-black). RGB and mono/line-out both the same.

I will finally get day off work tomorrow to study your diagram solarmon and maybe troubleshoot this properly (if I can figure out where to start!).

[solarmon]

Thanks for the update and for the pictures.

It doesn't look as bad as I had originally thought.

What is the history of this A1200? The caps pads actually look quite shiny - so I suspect it has had a recap?

I noticed one of the IDE pins is bent - so be careful of that.

You mentioned the Floppy LED is solid? It should only come on when there is floppy activity. If it stays solid, normally that means it is not connected correctly - data and power cable. Just make sure that both data and power cables are connected properly - i.e. pin 1 to pin 1 at each end.

Black screen normally means an issue with the CPU. EDIT: or traces going to/from it.

Checking the reset signals might help identify something.

I would then suggest you get a Diagrom and use the serial port (assuming that it is working) and see what it spits out.

[Marchie]

Yeah I don't know if it was re-capped and/or if that was the reason for it no longer working. I'm not seeing any tell-tale signs of a bodgy recap job of the sort that would do more harm than good.

Is it common for the CPU itself to fail? (at least MC68EC020's are cheap and plentiful, unlike the custom chips!)

I'm using Amiga PCB Explorer to test all the traces going in and out of the CPU one by one, up to pin 47 - so far all good.

DIAGROM is a couple of weeks away yet, so doing what I can until then.

[solarmon]

The traces themselves might be OK. There could also be issues with the active and passive components.

Obviously there is a lot to check and you need to try to narrow it down a bit. Checking the reset signals might help with that. Check traces and components especially around the SMD caps.

And definitely diagrom will help. Although, there are cases where even diagrom fails to boot...

[Marchie]

All the traces from CPU tested good, so I had another look at the under side of the board and found a couple of smudgy looking solder joints (all of which are caps). I'd value a second opinion before I start pulling caps though, the smudging is brown-ish which usually means flux rather than corrosion right? (maybe a bad recap job?)

Pics attached, I figure I don't need to tell you where to look!

[solarmon]

Yes, those are the through hole caps (C811 and C408). The legs on the large ground plane are a bitch to remove - so it looks like the re-capper used a load of flux but did not clean it up afterwards.

Those through hole and SMD/SMT caps shouldn't stop the A1200 from booting. In fact, it is said (I have not tried) that a working Amiga would boot up without those caps installed.

I would check the power-ok/reset/halt signals. I find using a logic probe helps to make this process easier and quicker. (You'll need to hook it on to a ground and 5V point on the A1200)

[Marchie]

Ok, so I guess that's not my problem after all. (Is it possible someone tried to remove the caps but couldn't?)

Ok, going into unknown territory with testing signals now...

I have a logic probe (well kit that I've yet to build). How do I use it? Any good guides out there on how to test reset signals?

[solarmon]

What is the logic probe kit you have - specifically what functions does it have?

I used the following Youtube videos when I started to use logic probes:

[ Show youtube player ]
[ Show youtube player ]

The first gotcha I came across was that I thought that I could power it off a 5v battery! I

Then realised that I had to connect it to logic +5V and ground - i.e. of the board that you are probing - after reading these guides:

https://www.electronics-notes.com/ar...ogic-probe.php
https://www.electronics-notes.com/ar...guidelines.php

Note, the guides above doesn't mention about the speaker function of the logic probe, which is very useful to have.

When looking at the schematic, the pins/traces names are significant in how they are written. If it has a leading (or trailing) "_", or leading "/", or sometimes it has a bar across the top of it, this means that it is "active low", otherwise it will be "active high". Here is a good explanation. especially because it uses 'Reset' in context:

https://www.quora.com/What-is-the-me...d-logic-design

Also useful to know and to use that terminology, the term 'assert' and 'de-assert' of a pin or signal:

https://electronics.stackexchange.co...o-assert-a-pin

I put together my diagram above, to summarise and help me visualise better the various power ok, reset and halt signals. It is shown in the schematics, but they are split across multiple pages.

I hope that is a good start for you. I've still a lot to learn myself so I'm hoping others will correct me if I'm wrong with anything.

[Marchie]

Originally Posted by solarmon

What is the logic probe kit you have - specifically what functions does it have?

This one:
https://www.jaycar.com.au/logic-probe-kit/p/KD6100

It's probably garbage, but I'm hoping it can at least test a couple of circuits for me (if that's what logic probes do?).

And thanks for the help, this is going to be a fun exercise.

[Marchie]

Working!

You were right Solarmon - the reset line was being held down. There is a transistor at U49, which (apparently) holds the system in rest until the voltage stabilises, under which there is a resistor, once we removed it, the system works perfectly (I think R629).

So much fun bringing dead systems to life again.

[Unkown]

Originally Posted by Marchie

Working!

You were right Solarmon - the reset line was being held down. There is a transistor at U49, which (apparently) holds the system in rest until the voltage stabilises, under which there is a resistor, once we removed it, the system works perfectly (I think R629).

So much fun bringing dead systems to life again.

Congradulations Happy Days Another A1200 ALIVE & WELL!