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

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

Post by finepie »

On the c64 (250425), if the VIC-II is removed, should blindly trying to load off the floppy result in disk activity? I couldn't find a clear answer on this.
I'm trying to debug a black screen...


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

Post by Gyro Gearloose »

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

Post by finepie »

Thanks for the info. Some troubleshooting forum I found made it sound like you could still make the floppy react even without the VIC chip, guess that's not the case then.
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Re: With the vic-ii removed...does the floppy work?

Post by Gyro Gearloose »

You have to be more precise, "react" could mean anything. If the 64 has an added hardware reset switch, you can press it and an attached 1541 will reset as well, but this is entirely a low-level hardware function that will work as long as there is power. A missing VIC-II won't change that.

The only chip the 64 can work without is the SID. You also lose paddle inputs but how many games use that? GEOS with the 1351 mouse would stop working, that's about it.

If you want to troubleshoot a black screen 64, you have to go step by step.

You really should check voltages first, since the Commodore power brick is notoriously unreliable and can fry a 64 badly.

The next most likely culprit is the PLA chip.

What tools do you have and do you have experience with electronics?
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Re: With the vic-ii removed...does the floppy work?

Post by finepie »

I'm pretty newb at this but wanted to see if I can get anywhere with just the various guides online and a multimeter. Chips are cheap, working c64s not so much...
The full details and what I've done so far:
- power led comes on, floppy spins, but i get a black screen. It worked right after being pulled out of storage but went to black (no borders, not "no signal", just black) pretty quick.
- checked the PSU, its good. checked voltages on various places around the board based on repair guides and youtube videos, its always within described limits. VRs, 7406, logic chips..
- SID, CPU, VIC, CIAs, PLA, roms, and clock are all socketed, tried removing/booting with them removed one by one, doesnt help. I get nothing (no signal at all) without the vic, clock, or cpu, the rest just give the same black screen.
- SID and VIC get pretty hot, as in you can touch them but its uncomfortable after a while. CPU is mildly warm. No other chip has any finger-detectable temp change.
- PLA has been replaced with PLAnkton. I took a stab in the dark replacing it, but it seemed a very common issue and the plankton is cheap. But no change.
- capacitors not bulging or leaking, board is quite clean and doesn't appear to have had anything done to it ever.

I know that without a spare working c64 and/or an oscilloscope I can't get much further than this, but if there's anything more to check and make any educated guess, well, as mentioned, chips are pretty cheap.
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Re: With the vic-ii removed...does the floppy work?

Post by Gyro Gearloose »

Chips aren't cheap if you look at the prices of VIC-IIs and SIDs.

What about the fuse? Did you connect anything to this 64 between taking it out of storage and it failing?
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Re: With the vic-ii removed...does the floppy work?

Post by finepie »

Well it's relative; I meant a chip or two is way cheaper than a new c64.

Fuse is fine, I checked that. When I took it out of storage, I checked the PSU before plugging anything in, saw it was good (and in fact a friend had borrowed it recently and used it for a couple weeks on his c64 without issue), plugged in the psu and screen, and it worked for a few seconds and then went to the black screen. I can't recall exactly if I powered it off in between or if it went to black screen from the working state while powered on.
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Re: With the vic-ii removed...does the floppy work?

Post by Gyro Gearloose »

A further thought occurs. The MOS branded support chips also fail regularly. Your version of the motherboard uses an 8701, as seen here

https://www.breadbox64.com/c64-hardware ... 425-rev-a/

Any of these smaller ICs with the MOS logo should be on the list of suspects. If your 8701 stops working, you could get weirdness.The 8701 replaces the discrete component oscillator/PLL of the original 64 motherboard. Luckily this chip also has a replacement.

http://www.melon64.com/forum/viewtopic. ... hilit=tolb

These chips fail because of some sort of manufacturing defect, so there's nothing you can do to predict or prevent it. They just fail, that's all. I have a Plus/4 that had two MOS branded ICs instead of generic 74LS parts, one of which failed. Luckily that was an easy replacement.

https://myoldcomputer.nl/technical-info ... ic/8701-2/

You could just check pins 6 and 8 with your meter in voltage mode. If you read something close to zero or ~4ish volts the outputs are stuck and aren't working anymore. If you read some sort of intermediate voltage it means chances are the outputs are toggling and the meter simply reads an average voltage.

BTW you in PAL or NTSC land? Are you sure you have the right 64 for your region? What kind of display are you hooked up to?
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Re: With the vic-ii removed...does the floppy work?

Post by Gyro Gearloose »

I sort of ignored my own advice: step by step.

My usual process is verifying power supply, reset, then the clock circuit.

The reset circuit keeps the 64 in reset for about half a second after you power it up, in order to give time for the power supply voltages to stabilize and the clock to stabilize.

The signal is generated by a 556 chip physically somewhere to the left of the DRAMs on the board.

http://personalpages.tds.net/~rcarlsen/ ... 9-2of2.gif

The 556 is U20. The reset signal comes out of pin 9. It's inverted by one inverter inside U8. Check with your meter in voltage mode on pin 8 of U8.

Put the meter in manual range to read 5 volts. Hook it up. With the power off you should see .... zero. Duh. Now keep your eye on the meter and flip the power on. You should observe a slight delay between seeing zero and a reading of about 5 volts.

If you see any other reading, like a continuous zero (or something less than .2 volts) you have a problem there.
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Re: With the vic-ii removed...does the floppy work?

Post by finepie »

For the 8701, I'm getting 1.93 out of pin 6 and 1.37 out of pin 8.
The reset test works as described, after a power on there's a little delay then it jumps to 5 volts.
Also 5v out of pin 14 on 7406 (u22) and 74LS14 (u23), pin 40 on the cpu, and everywhere else that I can find to check the voltage.

I'm all PAL out here, and it's a PAL c64, connected to a TV by scart. Since it worked briefly and the floppy doesn't respond I think I've ruled out TV issues.

Sadly, I guess that's pretty much the end of the road until I can find an oscilloscope or spare c64... right?
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