Let's troubleshoot and fix this Commodore PET

#commoore #rgb2hdmi #repair #diagnostics #romulator

Fractional free RAM no more! Let's dig into figuring out what is causing the PET to misbehave so I can figure out exactly which chip is bad to get this great machine up and running again.

--- Video Links

Part 2: This part!

RGB2HDMI profiles for the PET with CRTC: (these will be included in the RGB2HDMI release)

Romulator RAM/ROM FPGA based Replacement: (VIC-20 video)

Commodore PET 2001 Series:

Zimmer's archive of PET schematics:

Commodore PET Diagnostic clip on ROM:

RGB2HDMI Github repo:

Buy a RGB2HDMI:

Adrian's Digital Basement ][ (Second Channel)

Support the channel on Patreon:

-- Tools

Deoxit D5:

O-Ring Pick Set: (I use these to lift chips off boards)

Elenco Electronics LP-560 Logic Probe:

Hakko FR301 Desoldering Iron:

Rigol DS1054Z Four Channel Oscilloscope:

Head Worn Magnifying Goggles / Dual Lens Flip-In Head Magnifier:

TL866II Plus Chip Tester and EPROM programmer: (The MiniPro)

TS100 Soldering Iron:

EEVBlog 121GW Multimeter:

DSLogic Basic Logic Analyzer:

Magnetic Screw Holder:

Universal ZIP sockets: (clones, used on my ZIF-64 test machine)

RetroTink 2X Upconverter: (to hook up something like a C64 to HDMI)

Plato (Clone) Side Cutters: (order five)

Heat Sinks:

Little squeezy bottles: (available elsewhere too)

--- Links

My GitHub repository:

Commodore Computer Club / Vancouver, WA – Portland, OR – PDX Commodore Users Group

--- Instructional videos

My video on damage-free chip removal:

--- Music

Intro music and other tracks by:
Nathan Divino
@itsnathandivino

[tomwidauer]

I always wanted a PET. I cut my teeth on a 3008 in '79 or '80, either way, a very long time ago, and there is a fair bit of nostalgia in play here because of it. But thinking about it, it wasn't a particularly good computer and I am not sure nostalgia would justify the exorbitant cost of those things today. I do remember the first time I walked into the computer room at school. I was utterly disappointed. To me, a computer looked like something out of Star Trek, blinking lights, moving tape reels and all that. What I saw were nine boxes with a screen attached to them. All up, we had seven 3008s, a 3016 and the crown jewel and hardest to get to, a 3032 with a dual floppy drive and a printer. After a week, I started writing BASIC games and felt like a superstar 😀

[pieroc]

One thing i really like about your videos is the fact that you show your viewers that no matter how good can someone be at repairing stuff there's always mistakes happening at human level and that is completely fine, instead of discouraging about the repair you just laugh about it and keep going.
Great video
Keep the good work

[ranieriphoto]

One of your best episodes yet Adrian! I love how you used the classic “deduction and reasoning” approach to troubleshooting vs the “swap parts and pray” technique to fix this classic computer. Lots of valuable lessons to be learned…

[ethandicks]

In order to use the DIAG ROM pin to jump to the machine language monitor successfully, you have to reset the stack pointer before you enter other commands.

Specifically, the ";" at the front of the register dump is _also_ the MLM command to load registers.

I used to have a reset button and a DIAG ROM button on my PET for when I'd crash it writing machine language programs. I'd hold the DIAG ROM button then tap the reset button then release when I saw the monitor printout. The reason for doing it is because this drops you to the monitor prompt _before_ it tests/wipes main RAM.

Notice how the SP is 01. When you jump to the monitor in normal usage, you see a much larger number like $F8. If you don't fix the SP, it can't even do one JSR without the stack wrapping. You have to use the cursor keys and move up and right and edit $01 to $F8 and then press enter. On a working machine, it will reset the stack pointer and leave you in the monitor so you can inspect/alter RAM etc. You can't just jump to BASIC because zero page isn't fully initialized yet. It at least lets you nose around and see why your program crashed.

[evensgrey]

I'm amazed that the machine ran as much as it did with a flaky bit in each byte of RAM.

[shanesrandoms]

Love that moment you realised your mistake of the vertical position 🤣 it's one of those moments you can't get mad, you just have to laugh at yourself 🤣🤣
Awesome video as always Adrian. Keep up all your awesome work 👍

[dmaifred]

I loved this. In the eighties my elementary school in North Vancouver (Burrard View, long since bulldozed) had one of these The first and only computer at the time. Spent my lunch hours being antisocial ioading basic games with a tape. Def enjoyed first programming with basic. Thanks to Mrs Stewart (d?) For your forward thinking and the soon to arrive apple IIc and uncountable hrs playing Evolution and Logo. Pretty sure I still have a floppy disk notcher in Canada to make them double sided. :). If you were lucky you could borrow one of the three computers over the summer break. My first actual computer was a Ti994a. Loved it. Had to spend days with my Dad typing in then debugging code from a magazine . No disc. Just printed text to re type and save on the Sony tape drive attached with a headphone cable. Then on to C128, 286, 386 etc etc .... Never as much fun..

[Scruit]

When I saw FF flip to F7 I knew it was the right path - it's the same bit that as when AA flipped to A2.

[Dukefazon]

From time to time you amaze me with your knowledge and the way you are thinking. Amazing job!

[pipschannel]

That's some next level troubleshooting you did there Adrian!
I fully agree with your statement in the outro: Thorough Diagnosis is the most important step of any repair ever. It's all about zooming in and understanding what's really going on under the hood before you order an entirely new engine ;-) It's also empowering to actually use one's diagnostic mindset to find the truth, get to know failure modes and learn new things by discovering them. If everyone would have this mindset the world would be a better place ;-)

[rogerharper]

Thanks Adrian, this really takes me back! In the 80’s I owned a succession of Pets and one summer I wrote an accounting system on one for a friend who wanted to automate his law office. You could do a lot with these machines with a little work

[MarcelHuguenin]

As a non-regular viewer but 1979 Commodore CBM3016/32 user (as thy were named in The Netherlands) this was a lot of fun to watch!

[DavidWonn]

Glad to see you now have it 100.01066% fixed! ;-)

[Carstuff]

I do love seeing computer history being kept alive, with modern solutions :)

[horusfalcon]

Heh. Any moment now I expect to hear, "It works! It freakin' works!" This was fun to watch. Your troubleshooting path shows in-depth knowledge, and does pinpoint the problem so that minimal components are swapped to correct the problem. Well done!

[TheEmbeddedHobbyist]

Great to see it up and running, a pet is not just for Christmas with some loving fixes it could last a life time 😀

[timbald]

Your channel is strangely addictive! Thanks

[dgrond]

Your best video yet. All the things you did for diagnostics i used before but not in this way. The way you used it was epic and is proof of your skills and understanding the way a computer works !

[StevenIngram]

I really like how you used the ROMulator as an alternative to the "parts cannon." It was very effective. :)

[beauslim]

It is interesting that you mention your troubleshooting philosophy. When someone thinks about what might cause a problem, then tests it in one spot, thinks some more, tests another spot, I call this the "engineering" approach because I've seen a lot of engineers use this method. I have worked my entire career in IT, but my formal education is in biology. In biology you don't generally have a single test point, so you have to choose some test that eliminates a bunch of variables (preferrably half). Then you choose another test that halves those variables. You quickly come to one variable. It is essentially a "shotgun" approach because you kind of don't care what the cause is until the end. I find this approach highly effective and fast when debugging circuits.