I have 2 Mac II (820-0228-A) motherboards that will chime, but no video. I have cleaned and recapped both boards, as well as fix the usual bad traces between the VIA chips and the batteries. I'm using an original video card and matching monitor. I'm even using a recapped power supply. I've looked over the board with a microscope and cant find any bad solder joints or traces. I'm really lost on what section of the board to check now.
Mac II motherboards driving me crazy. Chime but no video after a recap.
- Thread starter jzmacdaddy
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If you have access to a scope, check and see if the nubus clock is there, it should be 10MHz. Also, if all the clocks and voltages check out, I poke at all the pins on the SMD chips with a dental pick looking for loose pads. It's more common than you'd think. Finally, and I am not sure how much this applies to Mac II's, but it is also common to find broken traces right at the pads or under some SMD chips. If one is corroded, sometimes the only way to fix it is to pull it from the board and test every trace, then resolder it with nice fresh clean solder.
I've had a few Mac II's with no video. NUBUS circuitry on those machines takes up like half the board, and any broken trace anywhere will lead to lack of NUBUS and therefore no video.
I would start by carefully inspecting traces. Possible suspects would include ICs in the vicinity of leaky caps, such as B1, B2, C7, C8, and G8. Trace damage could be around these ICs, or underneath them. B2 is especially susceptible to trace damage.
I would also scope the NUBUS clocks as @dschnur suggested. I've had one Mac II with a dead 40MHz oscillator, which is easy to identify by scoping pins 2 and 18 on G8.
If everything above checks out, I would start scoping the bus transceivers C/D/E/F/G/7/8 - they should all be showing activity on inputs and outputs, as well as the resistor packs 3, 4, 7, 8, and 9. Lastly, you could check the signals on the nuchip and surrounding buffer ICs, and the NUBUS slots themselves.
P.S. If you don't have a scope, you can still check continuity/resistances/voltages on those ICs using schematics.
I would start by carefully inspecting traces. Possible suspects would include ICs in the vicinity of leaky caps, such as B1, B2, C7, C8, and G8. Trace damage could be around these ICs, or underneath them. B2 is especially susceptible to trace damage.
I would also scope the NUBUS clocks as @dschnur suggested. I've had one Mac II with a dead 40MHz oscillator, which is easy to identify by scoping pins 2 and 18 on G8.
If everything above checks out, I would start scoping the bus transceivers C/D/E/F/G/7/8 - they should all be showing activity on inputs and outputs, as well as the resistor packs 3, 4, 7, 8, and 9. Lastly, you could check the signals on the nuchip and surrounding buffer ICs, and the NUBUS slots themselves.
P.S. If you don't have a scope, you can still check continuity/resistances/voltages on those ICs using schematics.
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Thanks for the advice. I pulled all the transceivers and tested them with the Retro Chip Tester. All of them were good. No bad traces under the chips. I have not scoped any clocks yet, but I'll try that.
The one board I have was remarkably clean, even before I cleaned it. The leaky caps caused very little damage to surrounding components. I have traced everything from the nubus slot pins back to the chips they connect to (Bomarc's schematics), and everything toned out. All the address and data lines check out too. I have to believe it's an oscillator or some other chip. Luckily, I have a Mac II board that would work if you push on it in a certain place (around UC7), but I could never find the crack in the board. As much as I don't want to do it, I may swap every nubus related chip and resistor pack from that board to the clean one.
Success! It was the 40Mhz oscillator. It was outputting a strange waveform on pin 2 of UG8. I removed UG8 thinking it was the cause, but scoping out pad #2 on UG8 showed the same odd waveform. After swapping the oscillator from my donor board, I got a normal 40Mhz waveform. re-soldering UG8 and firing up the motherboard got me video. Been running it for a few hours and it seems pretty solid.
Congratulations!! That makes at least two of us that had the 40MHz oscillator go bad. I wonder if it's a common problem with the Mac II.
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