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

Disk drives, Monitors, SuperCPU etc.
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finepie
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Re: With the vic-ii removed...does the floppy work?

Post by finepie »

Thanks for that SAMS link! I had heard of that book before but didn't find it. I've gone through pretty much all the voltages listed in the schematics. The most extreme discrepancies found are:

VIC-II:
pin 8 is 0.1v, should be 4.9 P according to the SAMS schematic (IRQ)
pin 9 is 0.42, should be 4.9 P (ROW4, whatever that means)

CPU: pin 3 is 0.1, should be 4.9 P (IRQ)
CIA u1: pin 21 is 0.1, should be 4.9 P (IRQ)
(these connect to each other, continuity is ok)

User port B is 5.09v, should be P
Keyb pins 13-19 are ~5.07v, should be P

There are many other variations but basically anything that should be L is ~0, H is ~4-5, and P is somewhere in betweenish.

Personally I can't tell much of anything from that.

There are some fun ("fun") tests later in the book to check readings while pressing certain keys and so on. I'll do that while I'm waiting for the dead test to arrive.
Gyro Gearloose wrote: Sat Jan 23, 2021 8:44 pm 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?
The c64 was in more or less regular (monthlyish) use ~10 years ago (as the office MULE competition machine :-)). I pulled it out of storage about 3 years ago which is when it worked briefly before dying. I did some checks at the time, just pulling out various chips to see the reaction as per some very basic troubleshooting guide, and you know, now that I think of it, I might've done the stupid mistake of accidentally swapping the pla and sid at the time. That could explain why they were both dead. So, perhaps my original issue was the garbled screen, and I just broke it further. Can't be sure but it kind of makes sense and rings some tiny bells in my brain. But, anyway, PLA is fixed and SID is removed, so I guess I'm back at my garbled screen. No one else has tried anything on the c64 and it looks quite pristine overall, no signs of post-factory soldering or tinkering as far as I can tell.


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

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Oh and the VIC-II is 6569R3 3984, and I don't seem to find the frequency written anywhere. None of the RAM or logic chips are MOS (cia, roms, cpu, vic, clock are the only MOS chips).
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Re: With the vic-ii removed...does the floppy work?

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Did the remaining checks (a logic probe would help a lot...maybe I should get one):
u3 pin 20 3.7, should be H
On all the RAM, u9-12 and u21-25 pin 3 is 4.9, should be P.
u14 pin 3 0.2, should be P
u14 6 0.26, should be P
u14 9 0.26 should be P
u14 12 0.27 should be P
u27 pin 1 1.8, should be H (AEC)
u27 pin 2 5.1, should be P (DMA)
u28 6 and 12 5.0, should be sb P (COL6)

Fun one, the book has some tests to detect reaction to key presses on the CIA pins. All of them fail (no reaction on CIA to any key presses) except test 6 (CIA u1 pin 13 reacts to key presses of ZCBM..). So that's pretty neat I guess, not that I know what it means!
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Re: With the vic-ii removed...does the floppy work?

Post by Gyro Gearloose »

Well I think you're moving along nicely.

The fact that the IRQ signal is constantly low is bad.

https://en.wikipedia.org/wiki/Interrupt ... processors

Basically as long as the signal is 0, the 6510 will react to the IRQ signal and never have time to do anything else.

The chips that can generate an IRQ are the VIC-II and one of the 6526, IIRC.

What I would try next is to remove both the VIC-II and the 6526 connected to the IRQ line, U1 on this schematic but I don't know which one it is on your model of board.

The idea is to try to look for a change in the IRQ level. What keeps the IRQ line at 1 is a resistor that connects to 5V, this is called a "pull up" resistor.

Basically measure IRQ voltage at the 6510 with the two chips removed. If it's now closer to 5V that's a clue. If it's still near 0V, the 6510 itself may be damaged.

BTW are you observing ESD safety while tooling around? You need to discharge your static electricity into ground somehow so you don't zap 64 chips.

I still need to digest your other measurements.
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Re: With the vic-ii removed...does the floppy work?

Post by finepie »

Well, that is interesting.
So, following the schematic the IRQ is connected on VIC pin 8, CPU pin 3, CIA U1 pin 21, cartridge pin 4, and RP4 and R51.
If I remove any one of the three chips - CIA U1, CPU, or VIC - the IRQ goes up to 5v.
I swapped the two CIA's around, nothing changes.
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Re: With the vic-ii removed...does the floppy work?

Post by Gyro Gearloose »

OK so I guess that means the 64 tries to boot up, something generates an IRQ, and it's not being handled properly and the line stays low. Software problem? Bad ROMs for some reason?

I think this means it's more likely the bigger chips are probably OK.

I've never troubleshooted a computer like this before... It'll be interesting to find out what the problem is.
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Re: With the vic-ii removed...does the floppy work?

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Small update: dead test arrived and, well, there's garbled action on-screen that goes through patterns. So, "something" is doing stuff...but I'll need to research more to understand what I may be able to deduce from this. If anything.

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

Post by Gyro Gearloose »

I'd look into the support chips, specifically the two 74LS257 (U13 and U25) that help the DRAM to work.

What do these chips, U13 and U25, have written on them? If it's a MOS number like 7708 those are known to fail.

Also, what is the brand of your DRAM chips? If the manufacturer logo looks like MT, those are known to fail as well.

To quickly test U13 and U25 with a DMM, check pins 4,7,9, and 12 for activity. If any of these pins appear stuck at 0 or 1, that's bad.
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Re: With the vic-ii removed...does the floppy work?

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The U13/U25 are both Mitsubishi and none of the pins are stuck. I checked them all against the SAMS book, nothing seems out of the ordinary as far as a DMM can tell...
The RAM is all Hitachi.
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Re: With the vic-ii removed...does the floppy work?

Post by Gyro Gearloose »

OK. Getting more and more interesting. How about checking for shorts? With the power off and all cables removed, check the data bus pins for unusually low readings to ground, repeat for address bus.

Put DMM in resistance mode, put black lead somewhere on the ground, then test all the data and address pins of the 6510, each pin should be similar to each other in some sort of highish resistance. If one stands out from the rest, report back ...
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