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

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

Post by Gyro Gearloose »

Well, not really. An oscilloscope is "nice to have" but as you can see, in digital logic troubleshooting, all you really need to look for is "activity" vs. "stuck at 0 or 1". Some signals should be stuck, but most should be doing "something".

There is a tool called a logic probe and all it is is two or three LEDs and a piezo buzzer with some electronics. It is entirely handheld and you just poke around the board and you can see right there what is going on. But a DMM can still tell you a lot.

Here's a 128 being troubleshooted in an episode of 8 Bit Guy.



It is kind of strange that the 64 made a picture and then not.

How are you connected? I'm not too sure what a SCART plug does. Does this connect to the video connector of the 64, or the RF port? Is there a way to see if you get a picture from the RF output? Did you try another cable?

Anyway, you can check all sorts of things with a DMM. And as the video shows, you could probably just spend half an hour really visually inspecting the board (both sides!) for corrosion. It is not obvious what the source of corrosion would be, but anything could have happened over decades.

You can also check the troubleshooting charts for the C64. You might have bad ROMs.

I think the 8701 is outputting active clock signals since you're reading a (averaged by the DMM) intermediate voltage. Can you check if the VIC-II is outputting its clock to the CPU with the same test? You can certainly poke around with your DMM. Check all data lines. Are they doing something or stuck at <.2V or >~3-4V?

You can certainly poke around with your DMM. There are many things you can check at home. Have you taken the bottom shield off the board?


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

Post by finepie »

SCART is an old european standard plug that can carry composite, s-video, and some other stuff. It's a physical form factor, not a signal standard. I'm plugged in to the video port, the cable itself is this: https://www.ebay.com/itm/Commodore-C64- ... SwwR5dVFYO

I don't have any way (currently) to test the RF or another cable, but I kind of doubt it's that since I used it for years, and get nothing from tape or floppy, and I've also seen a couple similar accounts of my "works briefly after being taken out of storage" situation.

I'm getting 1.2v out of the vic-ii clock pin, so that's normal I guess.
However, pin 8 (IRQ) is about 5v, and 14 (color) is 5.8v.
Likewise, cpu pin 3 (IRQ) is ~5v. CIA chips IRQ is also ~5v.
All the others seem to be in the normal range you mention.

I don't know if those are wrong but at least for the IRQ, this video claims it's wrong:

I believe the bottom shield was a US-only thing, mine only has the lame silvery cardboard top "shield".
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Re: With the vic-ii removed...does the floppy work?

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Progress! of sorts...

I realized that since I replaced the PLA, I hadn't re-checked some basic things.
Now, with the SID removed, I get an image! Sadly, it's a garbled mess:
(This old test LCD screen is cracked, so it's just the blue garble we care about).
I checked a couple pictorial fault guides and it vaguely resembles some CIA faults, but whether I have the CIAs in or not, I get the same thing. Apparently, with both CIA removed I should be getting a blank bordered image.

So, as best I can figure:
1. Original PLA was broken (because I didn't get this at all before, SID or no SID).
2. The SID is likely kaput as well, since having it in results in the black screen. But I guess I don't know for sure. But anyway:
3. Something else is still at fault because without the SID I get the garbled image. And it's not the CIAs, the PSU or power in general, probably not the new PLA, and probably not the clock but that's not certain.
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Re: With the vic-ii removed...does the floppy work?

Post by Gyro Gearloose »

Indeed nothing is certain but there is a lot you can still do. Did you check to really make sure the PLA is inserted properly? It's not off by one pin or not seated properly?

Display sure looks weird. Your IRQ lines should be close to 5V. When you see a signal name in a schematic with a line over it, it means it is "active low". A zero means the function is asserted. You want the IRQ to be mostly high. The NMI as well.

Does the display look the same every time when you power up? Try leaving it off for 10 seconds, then turn it on. Do that several times. Does it look the same?
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Re: With the vic-ii removed...does the floppy work?

Post by finepie »

I just checked continuity of every pin on the PLA to their destinations, so there's definitely no broken traces or contacts as far as the PLA is concerned.

The image is always the same. I tried what you suggested, plus gently tapping or wiggling components to see if anything causes an on-screen reaction. Also by the way there's no reaction to any keys, including run/stop restore, the screen is just that same wavy garble all the time.

Maybe I should get a dead test cartridge, they're pretty cheap.
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Re: With the vic-ii removed...does the floppy work?

Post by Gyro Gearloose »

I think you need also to buy the cable harnesses to make a complete test with such a cartridge.

In any case, can you look into the expansion port connector? Maybe something fell in there?
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Re: With the vic-ii removed...does the floppy work?

Post by Gyro Gearloose »

Oh and the fact it's wavy kind of scares me. Can you check your 5V and 12V DC lines on the motherboard, but with your DMM in the AC range? If you have a normal DC supply you might read a few 100mV of AC noise, depending on the frequency response of your DMM. This represents the noise and ripple on the DC supplies, which have noise on them in normal operation. If you read something in the volts of AC, that means there is something wrong with your DC supplies, that read an good average value but too much noise on it.
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Re: With the vic-ii removed...does the floppy work?

Post by eslapion »

Gyro Gearloose wrote: Thu Jan 14, 2021 7:24 pm My friend, the VIC-II doesn't only produce the display, it also refreshes the DRAM. So if there's no VIC-II, there's no way the 64, any model, can work. Essentially it would have no RAM.

Even worse: the VIC-II also generates the clock for the CPU, so it doesn't matter if there's no RAM, there's no clock! You couldn't even get the thing to jump to the RESET vector to execute code in ROM.

The drive is a separate computer, so whether the drive works or not has nothing to do with the 64.
I suppose it would be an interesting experiment to replace the DRAM with a Saruman-64 module so no refresh is required then use a simple 74XX161 or 74xx163 counter to divide the dot clock by 8, as the VIC-II does, to generate the CPU clock and see if the datasette will work or any blindly typed commands could get the disk drive to do something.

If the kernal is JiffyDOS then technically you could blindly type '@$' and the drive should start seeking the directory.
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Re: With the vic-ii removed...does the floppy work?

Post by finepie »

I get 0.001v of AC on 5v line and 0.004 on 12, so I guess that's within the range of normal?
Can't detect any shorts or junk in the cartridge slot or anywhere else...

I've ordered a dead test for starters, hoping that reveals something.
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Re: With the vic-ii removed...does the floppy work?

Post by Gyro Gearloose »

Perfectly good reading there too, means there is very little noise on the DC as far as that kind of test can show. It means that there's no low frequency ripple that could be caused by a bad power supply.

Well I guess I can ask more questions: did this 64 ever work correctly? Has anyone else tried to repair it? What is the part number of the VIC-II and what is the frequency written on the crystal?

ROMs do fail too sometimes. The fact that removing the SID changed things is a clue. There might be something preventing data lines from working correctly, like a wonky ROM forcing a data line to some bad value. It is weird that the 64 seems to be booting up a little bit since the normal border color is there.

Do the same "digital level" DMM test on all 8 data lines. There should be various different DC levels, if there's one stuck near 0 volts or greater than say 4 volts that's not right. If they're all stuck that means the CPU froze. In any case, you can do the tests faster than you can read my unnecessarily long explanations...

I'd also look for any small MOS branded ICs, especially U13 and U25 near the eight DRAM chips, on the right of them on the PCB IIRC, but on the left on the schematic.

FYI on some really old electronics like my 1960s stuff, service manuals very often included various normal voltage readings at critical points of the schematic, sometimes even showing waveforms. Typically for TVs. The company was called Sams, they made these books called Photofacts and Computerfacts.



This is not for your 64 model, but it might be worth a look, you got this far after all!
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