What freaks me out about this is that I've not done any 6502 programming in over 35 years, yet I could follow your commentary quite easily and understood exactly what was happening. On the other hand I can't remember what I ate for tea last night!!!

I loved my Vic 20. I had the base model with 5K RAM. I made good use of the cassette interface to get more out of the limited RAM by loading in parts of the program and data from tape as needed. It cost about $99 at the time I got mine and I was super-impressed by how nice the keyboard was, compared to the similarly-priced ZX-81.

Few things are as rewarding as learning and understanding more of what you care about. Your channel is the gift that keeps on giving!

It's amazing how you always go deep to understand the problem and not settling for just making it work again, great!

Hi Noel, as always a nice video. I have a tip for you. I noticed that you are using a glass fiber contact cleaner to clean the board contacts. Glass fiber is pretty abrasive and actually damages the protective gold layer. Next time you have to clean similar contacts try a simple pencil eraser, you will be surprised with the result.

Noel repairs in nutshell: "aaaaand, nothing." ;)

I like how you show the chip diagram and where you put your probe and the scope all in the same window and explain everything you are doing it right!

After many years, I finally got a Vic-20 (my first computer back in the '80s). It had the same symptoms you described in your video, and you know what? The kernal ROM was faulty! 😀 I just burnt a new kernal, and it's alive again. Now, I see the composite output quality is really poor... I'll follow your suggestions in your video about improving composite output quality. Gracias por tus excelentes vídeos, Noel!

Great job, I love VIC-20, my first home computer in 80's. Regards, Pasquale IW0HEX

About two years ago I pulled a VIC-20 out of a dumpster. It didn't boot, but I had another one that worked, so I just swapped socketed chips over until the working one stopped working. Well, the first chip was the VIC-1 which was OK, the second chip was the Kernal and that didn't work, so I put the known good one in the faulty VIC-20 and there was nothing else bad with it, so I was happy, made one of these crappy socket adapter constructs, burned a new one and now I have two VIC-20s. But all I ever do with it is sing along to ALL MECHANICAL CREATURES UNDER HUMILIATION COMBINE TOGETHER AND FIGHT FOR ROBOTIC LIBERATION. Also I just found out if you share a 1541 disk drive between a C16 and a VIC-20, the C16 doesn't mind at all, but the VIC-20 really doesn't like it. So the C16 has to be unplugged if I want to use the VIC-20.

Yet another VIC20 saved and another excellent video!

Whatever Noel is repairing or doing, I always learn something. Oh, how I love this channel !!!!

Well done Noel! Great to also see easy repairs. A knowledgeable support network is always a bonus.

Hi Noel. Interesting video. I always find that the best route for initial diagnostics is voltages, clocks, reset line, interrupts. Loading a reset vector (and possibly an initial stack pointer) on startup is an extremely common sequence for any CPU. The VIC-20 is obviously also looking for a cartridge "signature" during its startup sequence - another very common technique. I thought it was very strange how you spelled Kernel for the EPROM until something in the depths of my memory dragged up this particular Commodore-ism! Keep the videos coming 👍😁

Nice Noël, I will finish of my video for over two weeks and then sit back and relax and watch your awesome VIC20 come back to live!

Nice tip with with resistor bridge thing.

Great explanation on fault finding on how the starts sequence works, please more of the same. Thank you.

I absolutely love these commodore repair/diagnostic videos! You do such an amazing job. Thank you

The Vic 20 was my 1st ever computer, followed by the commodore 64 which supported sprites from memory. I remember buying an assembler cartridge so I could program the 6502 in machine code. It was hard going for a while, but it helped me understand how the CPU operated, calling internal routines to switch out the ROMs and use the memory underneath them for storage etc. It also helped me understand computer architecture with address and data bus and the use for interrupt routines to name few. One thing that stood out is it had a tape drive which could take several minutes to find a game on it, the the same time again to load it. Great memories.

A 6502 will disconnect itself from the bus and wait for reset as soon as it executes a so called "crash" code. This is why the CPU did seam dead: The bad ROM did result in random data on the bus, which could make the CPU either execute a crash code or end up in an endless loop. As soon as I saw you suspect the CPU I was expecting to be diagnosed fine in the end: It is perfectly normal behaviour for a 6502.

Personally I would use turned pin headers for adapters that have to plug into IC sockets, as the square pin type can easily damage the socket preventing an actual IC from working reliably in them. One "feature" of some versions of the 6502 that is not commonly appreciated is that if it reads certain undefined instruction opcodes it can cause the processor to "jam", giving the impression the CPU is faulty. If your kernal or basic ROM is faulty, the processor can be given one of these jam instructions. I'm not sure if this is true of the 6510 variants used in the C64, but the original 6502 fitted to the PET and VIC 20 does appear to suffer this "feature".

Thanks for another interesting watch Noel. Always feel like I'm learning something new. I've recently started getting more into electronics from building mods and basic fixes for old machines and these videos are great for ideas on what to look for.

The only thing bad about this video is that I have a meeting in 5 minutes and can't finish watching it now :)

The mark of the true professional is the ability to make the difficult stuff look easy. That, and the willingness to share one's successes and errors to the benefit of others. Bravo! The breadboard trick is a good one. Would it be equally handy to put the female header pins on an old (failed) chip or chip socket to hold them straight during soldering? Also, since the female header pins look to be of good quality, could you not have just removed the old socket from the board, soldered the adapter directly to it, and let the adapter serve as a new chip socket? (Lower profile for the whole thing that way, too.)

Hi Noel. Great video!

Been watching your videos for a few weeks now and totally loving it. Keep up the great work.

Great video! Thank you for sharing your work flow on diagnostics. The way the kernel ROM is kinda like how Color Basic, Extended Basic works on the TRS Color Computers. Then the cart ROM has two ways to be used on the Color Computer. This can also be a rabbit hole to go down. :O

Cool! These videos give me a bit more confidence that fixing these problems are indeed doable. Unfortunately I don’t have an eprom programmer though ..

Assumptions... Very informative (as usual). Thanks very much for this!

That surprised me that it was a ROM, I thought it was one of the 6522 chips. It has been a lot of years since I repaired any Commodore computers, Nice find. Lee

With the C64: Commodore didn't want to first build a game console that could only be started via a plug-in card. But the project was canceled and the function was found in the later C64. Therefore the plug-in card of the C64 can switch off all the kernel ROM and operate its own kernel. With the C20, plug-in cards were always an extension of the existing system.

TIL that the VIC-20 has a different cartridge signature string than the C64 (CBM80). Neat.

With the MOS 6502, there isn’t any pin where one can completely disable/float the 6502 processor pins. Therefore it is not possible to make a diagnostic card that with its own processor can overtake and drive the bus. Also signals A14+A15 are not routed through to the expansion slot which adds another layer of difficulty. Just wondering if one is putting a larger capacity ROM in the kernal position, why not have a dip switch so one could swap between 2 different kernels like Jiffy DOS and PAL.

Always good to see a Vic-20 saved from the bin. Good job as always. I may have considered removing the socket from the motherboard and soldered that adapter board directly in. It may be too tight for that socket and over time it may fail again.

Interesting and well presented, Noel....thank you.

I still have that Zaks book... Brilliant. I started out with the Ohio Superboard II before I designed a 2k memory mod and a 64 character mod... made all the better because of CEGMON.

Because it is a shared bus, I think that any chip on the bus has the potential to stop all bus activity. I also had a VIC20 which produced video but just all black video. It was caused by a faulty RAM chip, and came back to life after changing that IC. Interestingly, the VIC 'dead test' on the 'Penultimate cartridge' worked and correctly detected the faulty RAM.

Thank you Noel, as always a fascinating and interesting video! Learned a lot again. Best wishes, Michael

Another lovely video. Thank you.

Noel: Always put the suspected dead part, like this CPU, into a working computer to prove it is faulty. Also Noel: Imma put this nice working ROM chip into the dead board and see if that fixes it... 😂 Just teasing. Keep up the awesome work! You're doing a great job saving those Commodores from landfill.

That was interesting repair. Now we know, that VIC-20 can't work without kernel ROM.. What code remains was in that bad ROM?

I just had to replace a Kernal ROM on my Plus/4. Looks like they really tend to fail quite often. Lovely video, thanks for the deep dive in the start up sequence. 😊

5:45 Programming the 6502 book by Rodney Zaks. Thanks.

You used the breadboard to align the male pin headers on the EPROM adaptor, but aligned the female headers manually. If you put male headers into the female ones, then you can flip the board over and use a breadboard to align the female headers too.

Thanks for great content !!!

Nice video, keep it up, thanks :)

Good Video, good way to show the screen and to show the pinout of the chip and on which pins you are working, great for beginners to follow! One small thing: the adapter pins that go in the socket of the ROM seem very big (thicker than the ROMs original pins), they will probably ruin the Socket, or is this just fine?

Excelente video!! Saludos desde España!

another one saved. >> where did you get the vic-20 diag kit?

You wondered if the VIC diagnostics cartridge needed any functioning RAM to work. it’s actually a very well thought out system and includes 1K of RAM on the cartridge, though that doesn’t replace the video RAM it’s enough for the cartridge to run it’s diagnostics.

I used to repair C64's way back when. We had a test cartridge with LED array that could tell you what chips were good or not. All it needed was the video chip and CPU to run, as well as none of the address or data lines stuck. This was especially useful when all chips were socketed as I could pull them all except the two mentioned and start adding chips back in. But what about the kernel ROM you ask? I would suspect the C64 had circuitry to disable the kernel ROM and let RAM or expansion port take it over.

To check CPU activity I prefer to use the lowest address bit (A0 in your case) tends to be a very busy signal.

How did you photograph the thumbnail for this video? Back lit, but legible IC markings.

TI-99/4A also requires a working ROM before you can use the cartridge port!

Hi Noel! Where can I get the VIC 20 test cartridge with the clear shell?

Will the Winbond W27C512 work in these situations. The datasheet states it is electrically erasable.

Could you have a “system test” ROM to replace the kernel, sorry, kernal ROM that has a reset vector that just jumps to the address at A000 since that seems to be the only bit of the kernal needed.

Hi, I like your video and my vic20 does exactly like the one in your video. You wouldnt have any Kernal -06 avail for sale or the Eepprom?

Great video !! On my BBC Master 128, I had hard time finding why it was sometimes not working after power up. I found out that one of the ROM (It has 5 roms) was faulty wich is weird thinking that it is programmed once and there's no possibility to erase anything..

Replace the RF 4-pin choke too. They are still made.

You're going to need to get yourself a Coleco ADAM and look at all it's quirks

Did you try the old ROM again? Perhaps it just needed a re-seat and some de-oxide.

Were you able to close the case with this modification? I saw a different video where someone had done a VIC 20 mod and the case wouldn't close.

I'm curious about the Dragon cartridge. Do these adapters work with the 2532s in the Dragon cartridges?

Curious to know what could brick the kernal rom, like I doubt that it came from the factory that way.

Interesting to see that it was assembled in Hong Kong and not China 🙂

Even if a diagnostics cart for a system does not need the system's ROMs. A faulty system ROM can still corrupt the data bus. A faulty IC may not respect its chip select signal and hence corrupt the bus.

I've seen bad ROMs keep a C64 from dead testing, too.

Sounds like it would be a good idea to make a replacement Kernel ROM diagnostics tester. Put that in to test the system.

I just repaired a C64 with a bad Kernal ROM! Shame the original ROMs aren't widely available.

It has some AMD chips? (Two chips on lower left corner of the board).

I'm sure the FCC appreciates your removal of the emf shield.

Does this mean that the C64 diagnostic cartridge somehow replaces the kernal ROM in the memory map?

8:45 Why you use this chip tester? Did a retro chip tester from Stephan's 8bit-museum can test a 6502 CPU's ?

Just curious, was the old ROM corrupted or just dead?

It would be interesting to compare the contents of the bad ROM with a good one. I wonder if they are masked ROMs or if they are OTP EPROMS...? (OTP=One Time Programmable, in plastic because it's a lot cheaper than a ceramic windowed package). If the whole ROM is bad, then it has clearly had its day. But it's also possible that only some bits have become corrupted, and if it was OTP EPROM then they most likely would have gone from a 0 to a 1 (the initial erased state is always all ones). In such a case it might be possible to reprogram the chip with a correct version of its original data using a suitable EPROM programmer. Note that you can only program a 1 to a 0 in this kind of device, so the original data would be required (not an alternative version with some differences), and the corruption must be from a 0 to a 1, and not the other way round. (Just thinking out loud - hope it helps.)

So even though you can bypass the ROM in the C64 (or can you? I honestly don't know how that works and after this video I'm wondering) I've seen C64 repairs where a bad ROM prevented the dead test from working. So you never know.

kernAl?

"clearly I haven't been in this before" LMAO I BEG people to throw away the cardboard ones all the time. I'm not happy about the metal ones either, but I'm like that's on you. But the cardboard ones are harmful IMO.

20:20 Technically it's KERNEL not KERNAL I do understand that Robert Russell MADE AN HONEST MISTAKE IN SPELLING in his white-papers and then the program writers of the VIC-20 took it as gospel that the spelling was KERNAL (not knowing the spelling mistake that was made) and in turn they just adopted the name into VIC 20 but at the end of the day , it is a mistake. Carrying on a Mistake does not make it correct LOL When i learned computers as a young lad it was KERNEL Back in the days when we had M.C.P. (Master Control Program) so yeah , Technically it's "KERNEL" if it was me i'd be reprinting the label

Psst... You spelt Kernel wrong on your label! 🤫😏

Your "music" is annoying as hell.

"Just burn" The github instructions state if burning one ROM image burn to $E000 (for a 512 EPROM or EEPROM). Is that what you did here? @NoelsRetroLab