Two special Amiga 4000s: More Jops Repairs

In my previous post in this series, I managed to diagnose and repair three very strange issues that were causing issues with the video output. Now it is time to continue the repair to the rest of the motherboard.

Bad ICs and traces

I tested both U177 and U891 which I had removed much earlier on in case they were causing bus issues. U177 handles the address select for the RTC and U891 handles 8bits of the data bus for the Fast RAM. Both are very close to the battery, so they often get damaged.

In this case, they were both bad. I have plenty of spares in-stock because they are a common failure point in Amiga 4000s. New ones were soldered on.

I also noticed two traces were bad from U177 to the RTC.

To resolve this, I desoldered the resistor array (RP176) between U177 and the RTC, attached patch wires to U177 and fed them into the holes of the resistor array. I then soldered the resistor array back. This makes the patch wires nearly invisible.

The other common failures I hadn’t tested for yet are the IDE and power LED traces, they have traces and vias right next to the battery and can get eaten. In this case, the power LED is fine, but the IDE LED trace was broken along this route. I ran a patch wire under the board through the vias to fix this.

Motherboard blitz

It is at this point I did a lot of things in one big blitz. Including:

• Recap the entire motherboard and CPU card
• Solder a new socket for Buster (and clean Buster’s pins)
• Removed J975 (corroded and never used)
• Replace the audio connectors
• Replace the SIMM sockets
• Replace the joystick / mouse ports
• Swap DiagROM for the original Kickstart ROMs
• Clear solder from the RTC battery holes
• Popped in the original 2x 1MB SIMMs for the Fast RAM that came with the board

It was a lot of work, but this is the end result.

It still needs ultrasonic cleaning and replacement solder mask, but it is already starting to look better.

Testing

Now for the moment of truth… I connected a Gotek drive with Amiga Test Kit, flipped the switch to see what would happen, and…

Success! Now things start to become a bit easier. I could only get this far if the entire chipset was working.

Both the RTC was working (I set the current date/time to test and watched it count away) and all the RAM.

In addition, every other test passed too.

RTC battery

Just like the Amiga 2000 I restored recently, I’ve decided to go for a Li-SOCL₂ battery. This is a non-rechargable 3.6v lithium battery in AA form-factor that is designed for low-drain applications and is intended to last 10 years. I’ve soldered a diode into one of the wires of the batter holder to stop charging.

The wires have dupont connectors, so the battery can be easily removed and replaced with something else in the future.

Clean and solder mask

The final part of the motherboard restoration is the ultrasonic clean and then replacing the solder mask. I used to use clear nail varnish for this, but this time I decided to use UV cured solder mask. I’m not the best at spreading this, but I covered the important parts.

Of course, the seal on the plunger for the UV mask tube failed, sprayed it onto my hands and I first noticed after I wiped my face. Note to self next time: wear gloves, this stuff does not come off easily.

Finally, this is what the assembled motherboard looks like.

Rust removal

A few parts were quite rusty when I received the machine. In particular, the area where the joystick/mouse ports were. The ports have a plate on them which was very rusted. So, as an experiment, I soaked it in HG Rust Remover diluted in distilled water for about 12 hours.

I then used Brasso metal polish to clean up any remaining tarnish. This is the end result.

It isn’t perfect, but it is so much better than before I started.

Next up the case. I painted on neat rust remover solution onto the affected area.

After leaving it overnight, cleaning it up, using a wire brush to remove some tarnish and applying some Brasso, it is starting to look better.

There are other small parts of the case that need treatment as well, but this was the worst of it.

PSU testing?

I removed the PSU from the case and heard something rattling inside. I opened it up to see what it was. I’m glad I did…

The rattling piece was a bit of plastic, which is probably OK. The PSU is very dusty inside, I could clean that up, but… That mains wiring, that is bad. If it got hot for whatever reason, those joints could desolder themselves, and you have a flying live power lead in the PSU. At the absolute least, these connections should be covered in heat shrink. But really, spade connectors should be used.

I’m not going to test this PSU, it is likely going to require a lot of work. Instead, I’m going to use my ATX PSU in this machine.

Next time

The motherboard is fully repaired, and I now have a solution to clean up any remaining corrosion on the case. So, these are the remaining items:

• Finish any remaining case rust removal
• Repair or replace the floppy drive
• Something special I’ll reveal when everything else is done

In the new year, Jools will be coming back to me as well to be finished off, and there will be an update post on that.