A3000 repair

[James1095]

So after not doing much Amiga related for a while, I found myself in possession of a nice looking Amiga 3000. Unfortunately the battery leaked and did some damage but I caught it before it got as bad as most of the pictures I've found online. Unfortunately it acts essentially dead. I've verified the power rails all look good and gotten as far as observing a roughly 1-second burst of activity on the buses and ROM enables when power is applied or I manually assert the _FAIL signal but then it goes dead. The clocks are still active but the address and data buses are static. I'm not real familiar with the Amiga architecture but my suspicion at this point is that it's starting to load the ROM and then crashing. Are there any common issues that can cause this? I was under the impression that the area around the battery is primarily video circuitry so I haven't yet desoldered any of the ICs in that area to inspect closely for hidden damage but it cleaned up pretty well and I don't see any rotted out traces. Does the Amiga have any sort of built in diagnostic like the POST codes on a PC? The machine was found in a hoarder house that was being cleaned out so I have no knowledge of it's previous status.

[James1095]

Well that schematic is certainly a lot nicer than the one I was squinting at yesterday. Tonight I pulled the hex inverter at U477, it was pretty nasty but actually tested good out of circuit. I found an open trace between that and the phase pot on the back so I cleaned the area, installed a socket and repaired the trace. There was some corrosion in the socket for Denise so I replaced that too. The video circuitry seems a bit more lively now but the CPU is still not doing much. I removed the ZIP RAM that was installed and moved the extra chip RAM over into the fast RAM DIP sockets in attempt to eliminate RAM as a cause but that made no difference. Whatever is happening it's not getting very far, I can power cycle the machine or manually assert the _FAIL signal and it gives me less than a second of activity and then the CPU hard locks with no activity on any of the address or data lines, it's very consistent. I never get anything at all on the screen although I did get valid sync and a few random white bars at one point after I reset it. I think that's just a case of the lights are on but nobody's home. The video circuitry is running but with the CPU crashed there's nobody at the wheel.

[James1095]

I don't have any other Amiga parts to try so that's not an option unless somebody wants to loan me a CPU board for testing. I am capable of surface mount rework but I'd really prefer to avoid it unless I'm quite certain the CPU is bad, especially since I can't just go out and buy a 68030. I hadn't thought of the PALs failing but if the code is available my device programmer does support at least some of those, what part(s) are appropriate for replacing them?

[James1095]

I found a German site with the jdec files but it didn't say what GALs to use. I've worked with FPGAs and CPLDs but PALs and GALs are a bit before my time so I'm not all that familiar with them. Does anyone have a list of specific part numbers that are known to work?

I don't suppose there's anyone in the greater Seattle area with a CPU card we could try in this thing? I'd be happy to bring the machine by sometime to mess with it. I just have nothing to test the parts in and no spare parts to swap for diagnostic purposes. With what Amiga parts tend to cost, I don't want to just blindly throw money at it and hope something fixes the problem.

[James1095]

Cool, that thread has a list of part numbers that will work. I'll make sure my programmer supports them and then I'll buy some blank GALs, looks like they're still cheap.

[James1095]

So while I'm waiting for the new GALs to get here, I found the Amiga diag ROM which looks very promising. http://www.diagrom.com/

I found a place that has some 27C400-150 EPROMs, will those be fast enough to work in a 25MHz A3000 or do I really need the 105ns ones the author of the test ROM is using? Another potential option is to make an adapter for some 4Mb PLCC32 flash chips I have but I've got too many irons in the fire already.

[James1095]

Well I got the new GAL16V8D's and programmed them with the code I found for the A3000 PALs. Unfortunately it made no difference, although they do run quite a lot cooler than the old PALs did. Looks like I need to wait for the EPROMs to come so I can make some test ROMs. Hopefully that will provide some clues of what to check next.

[James1095]

Well I finally got the EPROMs and programmed them successfully, unfortunately there's still enough wrong with the system that not much happens. It doesn't lock up now, I see continuous activity on the address and data lines but I'm not getting anything intelligible out of the serial port and nothing on the display. Checking with a logic probe I can see the tx pin is pulsing but I've not put a scope on it yet to see what's actually happening. At this point there are a few more things I want to try, going over the battery area with a fine tooth comb and testing all the vias, replace a couple of TTL ICs that had some minor corrosion which I now have replacements for. If that doesn't give me signs of life then I think it's time to send the motherboard out to someone who has more resources and experience with these. I'm capable of replacing the CPU chip if I can confirm that it's bad, but I don't want to perform the potentially risky operation only to find that I'm no better off. Everything I do in search of the existing problem has the potential to create new ones.

In contrast to the standstill here I did manage to partially resurrect an A2000 that belongs to someone else though it crashes before fully booting up. It had battery corrosion and the CPU socket was shot, it would only show a white screen until I replaced the socket and cleaned it up and now it shows the kickstart screen just fine and starts booting. I plan to have another go at it when he has time to come over again.

[James1095]

Did you fix it?  I see an A3000 for sale in the Seattle area on eBay now  

http://www.ebay.com/itm/Commodore-Amiga-3000-Working-With-New-Battery-/132010755276?hash=item1ebc7444cc%3Ag%3AxqcAAOSwA3dYM8UU&nma=true&si=jlGN0%252FWePa9fZt6sK%252F%252BPq7euEs0%253D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557

No, I'm afraid that's not mine. When I do get it working this one is a keeper, unless a nicer Amiga falls into my lap like this one did. It's going to be a prized addition to my collection of vintage computers.

[James1095]

After setting this aside for a while I pulled it out again last night and had another look. I was originally going to hook up a logic analyzer but I found that the ROM sockets are too close together to get a test clip on both at once, and a QFP-132 test clip for the CPU is $200 for a used one. Accelerator slot is an option but even the mating connector for that is $30 and I'd have to build a harness for it.

Anyhow I decided to poke around with my scope instead and found some ugly waveforms on most of the data bus lines. I need to get out a pad of paper and make a list of which ones look like this and maybe that will narrow things down a bit. This is with the Amiga diag ROMs that are supposed to do something on even a rather broken machine and they don't even get far enough to spit out anything meaningful from the serial port. Can anyone who has scoped the bus in one of these confirm that the spikes are abnormal? They're certainly not valid TTL levels but it occurred to me that they may be occurring when the nothing is using the bus.

Is there anything on the data bus besides the CPU, ROM and DMAC? I was looking over the schematic and didn't see much that directly connects. A few bits here and there but not the whole bus.

[James1095]

Odd that you can't see the waveform, I attached it as a standard png file. What I'm seeing is some ~2.5V spikes intermingled with normal looking 0-5V transitions.

Uhg, I *loath* facebook, but I might have to see if I can figure out my password and log in. I haven't used it in years.

[James1095]

Maybe if I convert it to a .jpg that will work, I'll give that a go.

Yeah I realize it's very difficult to troubleshoot remotely, I've been on the other end of that effort many times. That's why I was thinking the logic analyzer might offer some good insight combined with the knowledge of precisely what the test ROM code is doing.

[James1095]

I think that particular waveform came from pin 20 of the lower ROM. I did some poking around the other night and almost all of the data bus lines looked similar so it may be normal. I'm really running up against a wall trying to fix this thing, I'm tempted to try to find someone semi-local to work on it, shipping to and from New Zealand would cost a small fortune just for the shipping.

[James1095]

Well shoot, I thought I was onto something there, oh well, thanks for checking on that for me. I'll try to pull it out again tonight and have a look at the overlay line.

$35 is not as much as I had assumed it would be, international shipping has gotten hugely more expensive in recent years, sending stuff to the UK costs at least 4x what it did 10 years ago. Still $35 over, assume roughly the same to get it back and that's $70 in postage alone and doesn't count paying you for the work. It's not that I can't afford it, but I've got a mountain of projects competing for my money and time. Also I'd really like to figure this out, sending it out feels a lot like giving up.

[James1095]

Ok that gives me some ideas on where to look, I haven't had a chance to get it out again but I'll report back with the results when I do. So this is with the original kickstart ROMs fitted rather than the diag ROMs?