SuperMac Spectrum/24 Series III Display Artifact Issue

[MacOSMonkey]

Good job. It could have been bad solder, debris or electrolytic corrosion. Unless you thoroughly clean the hole (and the pins -- to remove any oxidation/surface issues, there could be residual problems (and it might depend on the degree of via-trace dendrite invasion, etc.). If there is debris or corrosion at the trace end, then it might not matter how many times you solder it. If the traces were not resistive (perhaps implying little discontinuity or dendritic corrosion), then the heating impact may have been timing-related and adding a DIP socket may have added a very slight propagation delay (maybe ~30-40ps? -- it might not need much), plus an impedance and trace characteristic change -- possible reflections, reduced pin contact area -- maybe changed the signal propagation. You could scope a soldered vs. socketed part to see what the signals look like and if there is any significant difference. It could have also been something related to the package and wire bond connections. Using a DIP socket usually applies additional inward flex strain to the pins -- more than a PCB through-hole footprint would. Anyway -- glad it's working! And, it sounds like there is enough clearance for mounting the daughtercard with the DIP socket added -- so...great!

 

[croissantking]

I think the problem will come back soon

 

[Phipli]

Easily probed resistance for various traces, pins, etc, while the card was running, but did not find anything of consequence.

You don't want to be measuring resistance on something while it is powered, you might damage it or your meter, but will get weird readings regardless. Multimeters are not designed to measure resistance in powered circuits!

 

[jmacz]

You don't want to be measuring resistance on something while it is powered, you might damage it or your meter, but will get weird readings regardless. Multimeters are not designed to measure resistance in powered circuits!

Hmm, I'm actually not sure what I was writing there... I think I was trying to say more easily probed the voltage since the card was flat on my workbench, but I mentioned traces so maybe I meant to say "while the card was not running". Guess I was tired.. but no, did not test continuity nor the resistance while powered, only voltages.

I wish there was a permanent ability to edit posts here... kinda unfortunate that the edit option disappears after a little bit.

 

[jmacz]

And, it sounds like there is enough clearance for mounting the daughtercard with the DIP socket added -- so...great!

Yup, there was enough clearance luckily!

I think the problem will come back soon

LOL!

 

[jmacz]

I was able to get my hands on another Spectrum 24 Series III daughter card. This one actually did NOT have the labels on the ICs sanded off! Here it is for future reference.



10 of the 18 are Lattice GAL22V10s. 3 of the 18 look to be registers. And 3 of the 18 look to be SRAM. 2 of the 18 I can't tell as there are labels over them but are probably 2 more Lattice GAL22V10s.

I did try reading one of the Lattice GAL22V10s off my other daughter card (the one that had the issues) and unfortunately looks like the security fuse was set on it. It's returning high bits across the board. I think Bolle had a process to read these but probably beyond my skill level.

 

[MacOSMonkey]

Yep - GALs. That board was probably produced later after nobody cared about secrecy. You can compare the serial numbers to check. But, same hardware, later build.

 

[jmacz]

When I was mapping out the traces, I was completely confused by what I saw since the various chips were wired "differently". I had the wrong assumption that all of these were the same. This now explains a lot. Only 12 were GALs.

 

[MacOSMonkey]

Well...I told you they were GALs. Alas, I did my best. ^_^

 

[jmacz]

Well...I told you they were GALs. Alas, I did my best. ^_^

Yeah, and I did listen I just thought all 18 of the chips were GALs but looks like 6 of them are not, that was what I did not expect.

 

[MacOSMonkey]

No shame there - for either of us - hard to know anything about a 35-year-old de-identified build. But great that you now have a spare...and hopefully it works! You can retire the other one.

 

[jmacz]

No shame there - for either of us - hard to know anything about a 35-year-old de-identified build. But great that you now have a spare...and hopefully it works! You can retire the other one.

Haha, yes, the other one definitely works. But I was getting the urge to attempt to learn how to reverse engineer the GALs to be ready in case something else happens (knock on wood). But with the security fuse set, not sure if that's possible for me. Maybe someone with the proper skills.

 

[MacOSMonkey]

I don't think the reading trick is guaranteed -- but maybe Bolle could comment. .I think there's also a chance you might damage the part, whether or not it worked.