With the vic-ii removed...does the floppy work?

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finepie
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Re: With the vic-ii removed...does the floppy work?

Post by finepie »

Thought I'd post an update to all the masses following this riveting story.

Well, I've replaced ICs u25, u26, u27, u28, u13, u14, and u16. And I only broke one trace along the way! I'm quite proud of myself.
I also replaced all the 6 bigger capacitors while I was at it.
But, basically no real change, just some minor variation in the garble pattern.
However! I replaced the VIC-II, and now I have an actual image...sorta. Clearly something more is STILL broken. I don't what the heck happened to this poor board. Now the dead test actually outputs something, though.

Using my amazing skill at reading hieroglyphics, I deduce it's happy up to the color ram (so that's on OK on zero+stack page, screen ram, ram tests 1+2, and PLA). Not sure how much I can trust any of that, though. Exciting screen caps below!

So, that leaves about half a dozen ICs and all the RAM. I don't want to add up how much I've spent so far :oops:



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Re: With the vic-ii removed...does the floppy work?

Post by eslapion »

I overlooked a lot of posts from you guys and I've vanished for a little while but I felt I had to give you guys my conclusion about the initial question.

No, the C64 will not work without the VIC-II and you won't be able to do anything, including access to the floppy.

The VIC-II is responsible for refreshing the RAM and generating the CPU clock but you could always replace the DRAM with a Saruman-64 module and use a few flip-flops to convert the dot clock into the CPU clock which the VIC-II does with discreet logic ICs. It still wouldn't work.

That's because the VIC-II is also responsible for generating the RAS/CAS RAM access signals and timing thereof. If the CPU can't access the RAM then nothing works.

The VIC-II is also responsible for generating the AEC signal which specifies if the CPU has access to the data bus. Without it, once more, nothing works.
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finepie
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Re: With the vic-ii removed...does the floppy work?

Post by finepie »

Thanks for all the info, this whole thing has been very educational.
And I'm determined to get my c64 back to life - this thread WILL have a happy ending one way or another!
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Re: With the vic-ii removed...does the floppy work?

Post by rmzalbar »

I've been gone for a REMOVED too. Really hard time for me recently, including having to move from a house into a tiny apartment, which is very bad for hobbies. Yeah.. color ram, char ROM, logic, data/address paths.. probably more than one of those things.
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Re: With the vic-ii removed...does the floppy work?

Post by finepie »

Latest development: a sad blow to my morale.

I replaced the color ram and all the ram chips (with saruman), thinking this HAD to be it now. But I've made things worse. Maybe 80% of power-ons I get a black screen, 20% a screen with border and just a jumble of characters all over the screen. Dead test same result.
I checked for shorts and continuity on every pin on the mentioned chips, twice.

Sigh.

QUICK UPDATE: PHEW! It turned out to be bad solder that I only spotted on the third try, because it was soldered on one side of the board but didn't make a connection to the other.

So, that's a relief and the board is back to, well, where I was before the RAM replacement, unfortunately. What have I not replaced so far? U8, U15, U20, U31. And the three ROMS, CIAs, and CPU. Time for a bit of a think.
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Re: With the vic-ii removed...does the floppy work?

Post by Gyro Gearloose »

The C64 motherboard uses plated through holes, that means when the PCB is first manufactured, the holes are plated with copper inside which conducts to both sides: a healthy PCB would still work with solder just on one side of a device pin.

This suggests that either you damaged the board while pulling out the old chip, or your solder joint was bad the first time. Did you see strange brown/orange residues on pins of removed parts?

I would try gently pressing the PCB while powered up and seeing what effect this has. A good PCB wouldn't change behavior with a simple finger press here and there.

There might be a broken trace or via somewhere?
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finepie
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Re: With the vic-ii removed...does the floppy work?

Post by finepie »

As you guessed, this latest issue was indeed another bad trace. I had checked this one twice before as well, so maybe I was just putting pressure on it just the right way to make a connection, but on a third check I spotted it, this time on the color ram. Probably thanks to me when I replaced it.

Now with that fixed, I actually have an ALMOST working C64! I was pretty thrilled to watch the dead and diagnostic tests fly through without crashing or any visual puke on a nice crisp display. FINALLY.

So, what's remaining busted is maybe the CIAs. Diagnostic test reports them both as bad, and keys q, space, 1, 2, and backspace don't work, which are all on PA7 which is CIA1 pin 9. Indeed, voltage doesn't dip when I press any of the keys on PA7 on any of the pins, as it does for all other keys. Other than that, I think everything is working.

What's a little confusing though is that even if I swap the CIAs, that issue remains. So can it be that both CIAs have broken in the same way? I don't know if that's bizarrely unlikely or totally plausible. I guess it's not the keyboard itself though, since even with no keyboard attached the diagnostic test still reports both CIA as bad.
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Re: With the vic-ii removed...does the floppy work?

Post by banman »

Hi finepie,


Good to hear you are making good progress.

Sometimes these things require perseverance like you have shown.

Your keyboard problem may be caused by dirty or worn contacts where the key meets the PCB.

Could you swap out the keyboard for another one? Alternatively you might try to remove the keyboard's PCB and use some test jumper wire to make the connections. It might be a broken track on the keyboard PCB connecting a cluster of keys.

I had a few keyboards that had keys that wouldn't make the proper contact. Some of the keyboards had keys busted off at the stalk. Evidently in frustration by the last user not being able to type text.
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Re: With the vic-ii removed...does the floppy work?

Post by banman »

Hi finepie.

To illustrate a point.

I fixed a neighbour's remote control for a TV as a favour a while back. I couldn't work out what was going on
Maybe 3 to 4 keys would not work at all. Everything else was ok. Only by chance did I get some action when I pressed some random keys simultaneously.

How odd I thought. I even removed the decoder chip to see anything unusual. Nothing.

Only by very careful probing and the use of a magnifying lens on my camera did I notice that one of the non masked areas of the PCB board had corroded away. A test point to be more accurate.

Maybe it was marginal at the factory to begin with. I'm not sure at all. The owner said it had worked long ago. It took ages to work it out.

That one line made a matrix with several different keys on the controller.
By pressing other working keys did a new network of paths become available.

I fixed it by blobbing a big bit of solder over the area untill there was continuity.

I think the C64 keyboard PCB board has a similar system of connects.
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Re: With the vic-ii removed...does the floppy work?

Post by rmzalbar »

I had a similar issue on one of my boards that was fixed when I replaced my CIA sockets. The originals were single-wipe, felt pretty loose and apparently not making very good contact after several swaps of CIAs for testing purposes.. and I didn't rule out that perhaps i had an unseen broken solder connection to the trace. The machine-pin kind are great for reliability, so long as your CIAs don't have solder on their leads (if they do, cleaning off with desoldering braid and flux is sufficient.)
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