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VIC-20 with no video out

D

DrAM19

Experienced Member
Joined
Mar 5, 2021
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76
Location
Near Washington, D.C., USA
Hello Folks,

I recently picked up a new VIC-20 that needs some work. When the VIC is powered on, my 1702 has a black screen. This is a 324003 rev board, so its an early model with the 2114 RAM.

I verified I have 5v and a good clock. I also reseated all socketed chips with some deoxit. I checked the 6502, the 6560, and the KERNAL ROM at UE12 in a working later model VIC, and it continued to work. I tried the good chips from the other VIC in the broken one, but that did not help.

Using an oscilloscope, I checked the reset pin on the 6502 and saw normal behavior. At the data lines at the 6502; there is a very consistent pulsing pattern on all of them. The address lines at the 6502 are all pulled high and there is no activity.

At seemingly random times, when I power it on, there will be activity on the address lines. Normal and consistent activity. However if I powercycle the system, it goes back to the high and inactive behavior. I cannot find a cause for this or repeat it at will. And of course, the random times when this behavior occurred, I was not hooked up to or near the 1702 to see if there was some kind of video out. It happened most often when I was removing a chip here or there, like a ROM or VIA. But again, it was not consistent or repeatable.

My next inclination is to desolder and socket the 65245s at UD8 and UE8, so I can isolate the RAM from the CPU. Maybe a stuck ROM is making the CPU lock up.

What's held me back from desoldering the 65245s is that even address lines CA 13, 14, 15 are stuck high and do not go to there; they only go to the 74LS138 deMUXes at UC5 and UC6.

Does anyone have another idea or theory about why those address lines are all stuck high? Or maybe the address line behavior is a red herring, and I should look elsewhere.

Thanks!
 
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My first suggestion was going to ask if you know the video cable you're using is a known good cable, but the flat address lines would confirm a black/blank screen on the monitor.

From previous repairs I've seen, the most common failure seems to be one of the ROM chips. If you have an eprom programmer see if you can read them back and check the contents. A NOP generator will probably be a useful tool as well to help check the address lines.
 
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I did a recent thread on resurrecting a VIC - but I think it was a more modern VIC than yours (perhaps).

Check CPU pin 7 for activity. This pin (SYNC) pulses HIGH each time the 6502 CPU fetches and executes an instruction.

You could also try a NOP generator (if pin 7 is not pulsing) to see if that causes the CPU to be happy. This will permit you to test the CPU address bus, address bus buffers and address decoding logic (the 138s).

Happy to help out if you want to go this way...

Dave
 
Black screen meaning no signal (no power on flash at all) or black screen meaning dark raster?
If the latter, then yes you probably just have bad ROM or RAM. The ROMs go bad in those pretty often.
If it's the BASIC ROM, then some game cartridges may still work unless it's shorted.
If it's the character ROM, the machine will boot to border colors so it's not that (unless it's shorted).
If it's the Kernal ROM, nothing will work.
 
Thanks for the replies eight088, Dave, and rittwage, I appreciate the thoughts.

The video cable is good, I tested it on a working VIC.

Pin 7 has a bit of activity right after the reset line goes high, and then it locks up the same as the others. I double-checked and see the same type of behavior on the address lines. A few pulses right after reset then they lock up. A NOP generator is a good idea. I'll have to buy or build one, since I do not have one right now. I might have the components to make one in my box, not sure. Do you have any advice on how to best take advantage of the NOP? I've never used one for troubleshooting so I know I will be quite inefficient at first. This would go in place of the Kernal ROM?

Sorry about being unclear on the behavior. There is a power on flash but the screen stays black.

I tried all 3 ROMs in my Retro Chip Tester to see the CRC32, and the return is consistent for all 3. I have to fish out my minipro later to dump and hash the ROMs. I also know the Kernal ROM is good because I could try that in a known working VIC and it worked fine.

What cartridges can I try with the Character and BASIC ROMs removed, that will work? I'll see if I own any of them.
 
The NOP tester is generally a 40-pin "socket sandwich" that plugs into the CPU socket - and the 6502 CPU plugs into that.

If you have a spare 6502 CPU - you can bend the eight data bus pins out and strap them to 0V (via hard wires) and +5V (via 1k resistors) to simulate a NOP instruction = $EA = 1110 1010.

There is plenty of information on the Commodore PET threads. There is one that I have posted on at length only within the last few weeks!

Obviously the IC references are different - but (if you can point me at the schematics for your particular machine) I will be able to help.

You can also purchase a unit from @bitfixer and @Nivag Swerdna that incorporates a NOP generator. They are specifically designed for a PET, but the NOP part should be OK for your application.

Dave
 
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Thanks Dave.

Yeah the schematic you last posted from myoldcomputer is correct. I was working off the TM from Zimmers, pg 4+ http://www.zimmers.net/anonftp/pub/...dly_Computer_Technical_Manual_(PN_990440).pdf. It is NTSC.

I can try to bend some pins to get the $EA. Will probably bend pins on a spare socket (with thin pins so I don't damage the socket in the board). I was looking at this NOP design http://www.6502.org/mini-projects/nop-gen/nop-gen.htm Are the 4148s not necessary, and the 47 ohm to ground?

I did get a ROMulan PET RAMulator from Nivag to fix my PET (and of course it did, bad ROMs and RAM). Wasn't sure if I could use that board in my VIC. And of course I have not used the NOP functionality of it before.

I'll to check the recent PET threads to see what to look for when the NOP is running.
 
While probing around, I noticed weird behavior on BD0 going into the 65245 at UF8. It was holding at an intermediate state around 2-3 volts sometimes. This made me think there could be bad RAM in the UD row of 2114s.

Without a better idea, I started desoldering one at a time, with the intention of pulling and testing a chip, installing a socket, and then going onto the next one. Conveniently, the first 2114 I desoldered, a HM472114P-3 at UD2, was not readable by my chip tester, so I replaced it with a TMS2114-20NL in a socket. Now the machine reliably boots to BASIC with 3583 Free.

Now, the keyboard is not being read about 99% of the time. I took a different VIC keyboard and it worked so I will disassemble the keyboard and give it a good cleaning. A bit of mechanical work with a pink eraser and then some IPA is my plan. Then I will put the machine through some more tests. Hopefully smooth sailing under clear skies ahead.

Dave and Rittwage: Thank you for the advice!
 
Well done. Yes, the intermediate voltage is a guve-away.

Were you using the NOP generator for this test by any chance?

Dave
 
Hi Dave.

I did not get around to building a simple NOP, or figuring out the NOP function in the ROM/RAM board in my PET. I started fiddling with a NOP from loose components, but took a break and played with the oscilloscope a bit. The intermediate voltage behavior on BD0 was visible shortly after the reset line on the 6502 was released after power up, and before things locked up.

I did put in an order for a NOP from bitfixer, when the backorder clears. Once I get it, I'll install it in a working system to get comfortable with what it shows me.

I can also report that the keyboard works flawlessly now.

Thanks!!
 
Excellent.

Yes, the NOP generator is extremely useful.

Let me know when it arrives and I will walk you through the operation of it.

Dave
 
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