After I mounted the mainboard back into the case, and verified that the correct voltages were still on the transformer output, I attached one drive unit and switched the device on. LEDs came on, but drive LED didn't turn off! Okay, that is not good, but we can work with that! Motor spins up when closing the latch -- that is excellent!
...to be continued...
Notices by root42 (root42@chaos.social), page 2
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:45 JST root42
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:43 JST root42
So I went to fetch the oscilloscope to start probing. Suddenly -- POOF! Smoke, and the lights in my workspace went out. The mains filter blew -- while the machine was switched off! The mains filters on PETs and CBM Floppy Drives are between live and neutral BEFORE the power switch, so ALWAYS connected!
I quickly pulled the plug (not needed thanks to my fuse panel) and inspected the damage... -
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:42 JST root42
The fuse is still intact, as the machine wasn't turned on. Also the two huge capacitors seem to be fine, but probably need a bit of reforming (not due to this event, but because they are decades old).
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:41 JST root42
As a quick sketch: What Commodore did was to put the filter in front of the fuse and switch. Had it put the filter behind the fuse, the fuse would have blown, and no smoke... Or at least LESS smoke.
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:40 JST root42
And here ist the culprit. A mains filter to avoid RF feeding back into the grid. It used to be mandatory back in the day, but isn't used quite that often anymore. This specific type of filter is very hard to find nowadays, and IIRC my other CBM machines don't have one either. So I guess I will skip this and wire up N and L as shown in the schematics.
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:39 JST root42
Let us have a look at the mainboard first. Bench power supply is set to 8V and 2A and feeding into Pin 3 of P4 (+8VDC) and Pin 9 (GND). The board draws 1.8A. So far so good. Nothing gets super hot.
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:38 JST root42
Also good news: RESET goes low, then high and stays high on the first 6502.
However: the clock signal is 4 MHz!
The CPUs are supposed to run at 1MHz. However IIRC the clocks are supposed to be phase shifted so that the 6502s take turns in accessing the memory, as the 4KB are shared.
So something is iffy with the clock generation. -
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:37 JST root42
The schematics are there but not REALLY helpful as some of the numbers are totally splotchy! Will have to measure each step. My hope is that some logic chip is borked and the clock is not divided down to 1MHz.
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:36 JST root42
I got a hint that the 74LS74 flip flop divides the clock twice. First to 2MHz then to 1Mhz. But both outputs look misshapen and measure at 4MHz.
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:35 JST root42
Out with the old, in with the new.
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:34 JST root42
Oh wow. Suddenly we have 2Mhz and 1MHz signals!
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:33 JST root42
We have life! The machine boots!
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:31 JST root42
Next step is now to reactivate the power supply. I removed the mains filter and rewired all the connections using fresh solder and WAGO clips. I also cleaned the power cord. Afterwards I tested that GND is connected to all metal parts of the case, N and L have good continuity and low resistance and that the power switch works and properly opens and closes. I think this should be safe to turn on!
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:30 JST root42
The voltage without load looks good. I think it's safe to reinstall the mainboard and power it up. If it comes online, we can attach the drive mechanisms and cross our fingers! How are your bets?
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:29 JST root42
The board is in, and first power on test is successful.
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:28 JST root42
Drive unit 0 is attached and motor is spinning when closing the latch.
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:27 JST root42
Sadly, we still have issues. The HEADER command does nothing at all. Drive doesn't even go into error mode. No spinning and no head movement. Error channel lists error 10, no error string. That error isn't document in the manual...
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:25 JST root42
The DIRECTORY command does move the heads and spins the disk, but ends in an error LED lighting up. Error status in BASIC is still 10 / empty.
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:24 JST root42
Well, if you plug the IEEE-488 plug all the way in, things suddenly start to work! @8bitgames 's Hibernated works, and PETSCII Robots, too!
I think this was a resounding success! From literal trash to treasure. I still have to do something about the rust on the top cover, as well as perhaps replacing the regulators with @elosha's low noise switching regulators, to protect the mainboard from future overvoltage events, and make it run much cooler. But at the start I never thought to get this far! -
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root42 (root42@chaos.social)'s status on Sunday, 26-May-2024 01:50:23 JST root42
Drive 1 has some startup issues. Drive 0 seems fine though. I think either the motor or one of the bearings is still gummed up. After a few tries it starts working just fine. I mean, come one! If you spent the last thirty years in a damp basement you would need a couple of minutes to get up to speed right?
I can HEAR how it is turning too slow!