The Dead Test does a very limited RAM test, because the C64 is in Ultimax mode. In Ultimax mode only 4KB of the C64's RAM is available and this is what the Dead Test tests. (This is documented.) The Diagnostics Cart does a full RAM test. Therefore don't assume all of the RAM is good if the Dead Test passes.
I recall the IBM PC diagnostics only testing the first 256 KB because that's all the original 5150 shipped with, and that could cause similar issues, especially because anything above that would be in another bank or even on an expansion card and thus could not be assumed to be good just because the machine boots or passes basic diagnostics.
@@XLV750RD01 With D0 bit fully faulty, the dead test would flash the screen rather than boot up with the tests. So the D0 RAM chip was partially working for sure, the fault was in memory cells with higher addresses.
@@XLV750RD01 Dude, I know the C64 from left to right and top to bottom. Carefully read again what I write, it is exactly what was going on in that machine.
I noticed a pattern: it is always the last chip you swap that fixes the issue. Why don't you just start with the last one? It would save you some time ;P
I've always said that one day, when looking for something, I'm not going to stop looking when I find it, just so I can say it wasn't in the last place I looked.
@@awilliams1701 My guess is that it's about originality. In the 1980s, the RF shield was required to bring emissions down to FCC standards. Of course, it's no longer required, but it's an original part.
@@SpearM3064 Well yeah that's why it's there, but it's not required anymore and the FCC doesn't give a crap about these machines. So the question isn't why is/was it there. The question is why do people care about it going into the trash can where it belongs?
@@SpearM3064 I am not an C64 enthusiast nor capable of fixing them and I understand the originality sentiment. That said: Adrian at least tries to revive these machine's while others don't give a toss (pun intended).
Here's a tip for testing the 74LS chips. If you have a TL866-PlusII eeprom programmer and use the Xgpro software, under Device->Logic IC Test there's a logic tester that can not only detect the IC, but verify that all the pins are working properly.
The risk is that those chips can be marginal - test just fine in the TL866 but don't in circuit. Same with DRAMs. Had that happen WAY too many times to trust chip testers.
@@neilbradley Yeah that stupid tester in the TL866 is flaky. I've had it lie to me several times. I think it might be due to the fact that it's not really using the chip in the same way that the motherboard is. In other words, it's maybe flipping the CE and doing a few tests, and that's it, whereas that CE line might be left on in circuit, and the chip might get warm over time and have a thermal issue because of that.
@@Calphool222 Yes, exactly. Each system that uses the chip will have different loading and timing characteristics - something the TL866 (I assume) does not.
You'r "off by one" aside was the really useful observation. This must demonstrate a bit about your failure and HOW dead-test is checking RAM. There is something interesting there that I suspect the Dead-test author could learn from that bad RAM chip. I would send it to them. (too late it's in the bin, but I knew it would be)
For those schematics that span two image files, just load them into a graphics program and merge the images, and then save the resulting complete schematic. If the original scans weren’t perfectly square you might need to tweak one or the other with some rotation or skewing to make things line up right.
My desoldering gun gets blocked just the same as yours, what I do to reduce the blockages is when I suck the solder off the components I continue to hold the trigger On for same time after and sometimes pointing the nozzle upwards with the trigger On, this gives it time to clear the gun,,, keep up the good work 👍
One thought I had when you were checking the outputs from that chip where the signal from 2 of the pins looked odd - compare it with the output from the known working Zif c64. If they are the same on that one, then that could be ruled out as the problem.
Fun video! A couple days ago I watched this video as the first Adrian's Digital Basement video I've seen. I was surprised to see there was such an inexpensive scope, not that I keep up with these things. I bought a C64 new in 1984 and when it stopped working in the early 90's I just boxed it up and bought a C128D to replace it. After watching this I got a little excited and started thinking Hey I could buy this little scope and break out the old C64 and see if I can fix it. But now after watching the repair-a-thon videos 1 thru 3 I realize it takes a lot more. Adrian repairs these C64's mostly by narrowing down which chip is likely bad and then swaps chips to find the bad ones. So you really need to have these extra chips on hand not to mention the sockets, dead test cartridge, and soldering and desoldering iron. And if the PLA is bad another video suggests it might cost about another 60 bucks to manufacture a substitute for that. So all-in-all not really worth it just to try to fix a single C64 unfortunately. :(
Loved the part you got confused with the three suspected chips in you ZIF motherboard just because some poor chip legs contact. That's the worst "I'm lost" moment you can get... 😁
I have been told that the C64 was the most successful home computer ever made. While I don't know how many were made it does seem that sometimes between Adrian or Jan Beta have at some point half of them have been repaired on RU-vid.
I waited so long to hear your catchphrase, "would you look at that? It's WORKING!!!" only to be hugely disappointed. I'm now doing a sad 8-bit dance shuffle to the Monty on the Run high score theme. "Would you look at Black? He's SHIRKING!!!!" :-) :-)
thank you for doing this video. this is an excellent video for troubleshooting and where things go wrong and the steps you are doing in a good positive way. thanks for your time doing this, it helps us all out
I suspected a ram chip from the get go, but since I never did any C64 repairs, I figured I'd watch and see. The period characters on the test screen is why I thought a ram chip was bad.
Aaaaaaaaaah, a great video to watch with a cup of coffee after dinner with a cookie. 41 minutes of entertainment and fun. Love to see another repairathon!
Just a quick tip since you have the wonderful HP 547A current tracer. I have luck with it sometimes detecting bad RAM without any de-soldering or test programs. If all RAM chips are the same type/manufacturer/batch, you can use the 547A to "feel" the center of each RAM chip, setting the sensitivity so that it juuust lights up and compare the chips back and forth. It takes little while to get the feel for it, but if one chip makes the 547A light up easier than the others it is often bad (as in more over all current flowing through it). I've found this method to be more reliable than trying to compare chip temperatures by hand. Other chips can sometimes be detected in a similar way. Love your videos. Keep up the good work.
If you're like me and don't have a solder sucker, there's a neat trick to desoldering an IC with solder wick. On the underside of the board, of course, start there and get as much solder off as you can. Then flip to the top side and if there's enough room, run the wick against the legs on the IC to suck up any more that may have been left behind on the top side. Typically when I do it on 14 or 16 pin ICs, there's always at least 1 pin that doesn't completely desolder, but with a little patience, it'll clean up and the IC will just lift right out.
Adrian, i like your channel a lot. I am still flabbergasted by the way you are not using a ESD wristband, and tossing away the shielding. After fixxing i would put everything back to be a as close to the real machine. CIA's are vulnerable to static discharges known when touching the joystick ports. I like the way of your aproach with the hot air not to lift any traces. I would use the TL866-II to check the logic chips for faultiness.
Oh boy, I learned that a few years ago. Full Diag Cart is my goto UNLESS it won't run at all. Too bad Dead Test failed me in the past so I don't rely on it for RAM Tests. -Mark.
As someone who worked in configuration management...manufacturing will say that a part is a part is a part, regardless of the revision. You're lucky that you get a revision on anything. If a change doesn't affect form, fit, function, interchangeability, etc, you won't get a new part number either.
Great video, Adrian! You know there is always going to be some creep that is sitting there shouting "It's the RAM allocated to the screen. One of the chips is bad...." - I have to admit, that it was me this time and boy, was that a very lucky guess as I'd be crying into my pillow if it had been something else lol! ;o)
For testing 74-series TTL and 40-series CMOS chips, you can use the MiniPro 866. Under device, select the "Logic IC" and then select or search for the one you need to test. Works for all gate-type chips (not counters, for example).
The bad SID and control port readings are in deed caused by a failed paddle test. I had the exact same reading when I replaced an 8580 with a 6581 and forgot to change the caps for the filters and the paddles (I didn't neglect to change the zener for the supply voltage, so the chip itself was working fine). So the test seems to be quite thorough.
Dunno why i find these vidoes fun to watch when i dont even have a c64. I had when i was younger, but i think remember ours died because the fuse was going always. And i think it was imposible to get new ones in my town at that time.
U26 and U16 are the bus interface to the color RAM, and they explain why the color RAM is "special" and the VIC doesn't use DRAM for the color in text mode. U26 latches the A0-A7 of the color RAM address from the VIC when AEC and RAS are low (because that is when the VIC has the low byte on the MUX bus. A8/A9 are available as discrete outputs, this is important later... ) U16 disconnects the color RAM from the CPU data bus when AEC is low, so only D0-D3 from the color RAM is on D8-D11 at the VIC. This is also really important. It is a genius design that allows the VIC to fetch the color and character codes at the same time via a 12 bit data bus. The C64's memory actually runs at 2 MHz, with the CPU getting it half the time and the VIC fetching the character patterns or bitmap the other half of the time. Every 8 scan lines, the VIC halts the CPU for an entire scan and also fetches the character pointers (or color data for bitmaps) from screen memory: A10-A13 are set to the screen base register and A0-A9 increment over that row's addresses... but those lines also go to the 1k*4 color RAM so the matching color data appears on the VIC's D8-D11.
Always love all your videos, but especially anything Commodore! Words to live by: "Run every diagnostic you can, first." :) I can't wait for you to hit 100K subscribers! At least I won't have to wait very long. :)
Thanks Adrian for your video. Dead test cartridges don't always tell the truth. I have a commodore where the cartridge tells me that everything works (with harness) but when I run the load command to load floppy it hangs and does not give up control. I tested several disk peripherals (1541 and pi1541) and I have the same thing. The cable is good too. The u8 (7406) is good too. I tear out what little hair I have left. :)
After years of dreaming an accessible and affordable desoldering gun, I could finally get one (wasn't too cheap but cheaper than a professional one with a station). This week I used it to desolder one of the analog stick in a PlayStation 4 controller. It has 3+3 legs for 2 potentionmeters, 4 legs for a button and 4 structural points of the whole analog stick housing. It used to be a very difficult process but now that I have this desoldering gun I'm so happy and I can do jobs like these much easier and makes repairing stuff fun again.
Haven't watched any of it YET (and I will!), and I tend to criticize dialogue/monologue: you say (regularly) "let's get right to it!" But then you roll the retro (icky smeary video the type I haven't seen since 1994) intro. You see, you don't get *right* to it. I know this is going to be a great episode.
Great work, Adrian! RAM issues may be really hard to diagnose. The worst case I meet time to time is when the most of available tests pass, everything seems nice and working but real programs glitch occasionally and randomly. This lead me to start making own tests for the machines I repair. I found kinda good heuristics which reveals such hidden problems: fill RAM with increasing bytes, check, cyclically shift the contents of RAM byte-wise, repeat from "check". Usually a couple of iterations is enough to reproduce the glitch.
If it were me I would get out the heat gun and freeze spray and see if I can turn it from working to non-working or vice-versa by heating/cooling specific chips. Very good general tool for debugging as often a fault in a chip will fix itself at high or low temperature (usually low).
A bad machine can't always test itself. If you use 1 to mean failed, and bit 0 is stuck 0 at that address, it becomes OK. On a modern machine, the internal caches have ECC, so the BIOS has a chance of correctly testing RAM, while configured for cache-as-ram. If the cache is bad, it will most likely MCE (machine-check exception).
Great video!! 100K coming soon I hope!! And your skills with troubleshooting C64’s are just fine. Test equipment can sometimes lead you down a path that takes you away from the actual problem. The old saying (from when I was in the Navy) is: trust your test equipment, until you can’t. Working on aircraft fire control computers ... sometimes our TE would lie to us and make the job harder. But, it happens, and you did a superlative job working through it. Maybe the dead test module needs an upgrade? Really liked this one, Adrian!! Great job!!
Normally I watch these videos and can’t see what’s wrong, but this time I saw the screen and thought it was a RAM issue, only for Adrian to say the RAM is fine. Oh well, wrong again. Very surprised to see I was actually right!
Hi again, also in the manual for the diagnostic they talk about 2 capacitors C48 and C93 have to be 1800 pF they sometimes have a different value that also makes the port test fail.
The IC puller is probably not good for large IC soldered on to PCBs. Saw that Parfractive guy snap a 40 pin in half when he didn't desolder a center pin properly - lol -. For the RF shield I was thinking a large 100W hardware iron with the copper tip hammered flat to a chisel shape, so it can heats and get under the tab on one motion.
Late to the party, just wanted to mention that this a common issue with software RAM testers. If they don't find errors, it just means they found no errors. Not that there aren't any. It's not the fault of the dead test cartridge, just a limitation of the technology. Whether a memory cell can keep its value depends on many factors (values in neighboring cells, access patterns and timings, temperature, voltage fluctuations, etc). "Real" RAM testers are big machines that can simulate all those different conditions and cost 5-6 figures. Cheap software solutions can't do that. It's the same with programs like Memtest86 on the PC. It's also why these tests loop and don't run just once. The idea is that you let them run for a while (12-24h) to cover a wider range of randomly fluctuating factors, some of which will hopefully trigger the problem at least once. Software tests still have value, however, because if they *do* find errors, then there's definitely a problem.
For everything is a first thing to happen. So now you know that the death test does not always work well at one test, so next time test both or more versions of the death test. It makes you only a better troubleshooter with extra experience. I loved the video again so thumbs up!
At 33:34 you can see the display momentarily shift to correctly say "BAD" and shift other characters into different states. This may have helped in diagnosis!
The board number in the documentation may be the latest PCB version, but there weren’t any circuit changes. It may be little hard to see changes like internal power or ground planes, changed vias and the like. It is sloppy documentation IMHO but for the time it was common.
@Adrian - in answer to your question @12:58 "Why don't the schematics at Zimmers.net note the board revision for those schematics", it's because the schematic scans don't have board revisions written on them, and over 20+ years of maintaining the site, no one has ever mentioned it to me. Well, now I know. Expect a reference there in 10 minutes.
"There was a period of a few years in there when the chips were less reliable". I wonder if that period coincided with when they were turning out machines hand over fist lol.
Nice, I love a good repair!. My "Luck" on fleaBay has been terrible. I keep buying "Dead" machines only to discover they work fine or have a bad power supply. I need a Fix Fix!
Perhaps the drill should be to make sure it at least responds to the dead test cartridge first, and as soon as you confirm a understandable display, then switch to the full diagnostic cartridge for analysis. As for the shield, you may want to save a sheet or two of that insulation, it’s called fish paper and often used for insulating barriers around energized parts.
Yup, for a little constructive criticism, I think that this should be mentioned first and foremost - you have to get your hands on some schematics and/or troubleshooting guides. To be fair, that depends on the machine though, because a popular computer like the C64 will have tons of troubleshooting material available whereas other rarer machines may not.
@@LeftoverBeefcake It's fantastic that Adrian leaves all his problem solving in his videos, and has schematics to hand! It's like watching a mirror image of 90% of his audience fumbling and trying different things. We all do it, and that's why it is such a great educational tool.
Your mention of schematic version vs PCB version difficulty brings up my question about no available 'scope images taken at various key test points. After all this time, with all of the people repairing the huge number of broken C64s, I'd have thought someone would have taken some photos to do that by now. Maybe they have and I just don't know it. Don't own a C64 myself.
hey adrian here is a test for your tool belt you may or may not have compressed air/freeze spray, spray down the various chips to see if anything changes when cold
also i have a small request... can you make a video of a cold boot of a c64, working normally playing donkey kong arcade just for the music? im trying to find a copy on youtube and all i can find are segments of your videos or compilations of dos music!(hey... im not complaining, im probably going to blast this mix today at high volume)
Amen, I’d like to have one as well. I’ve wanted a c64 since I was a kid parents wouldn’t get one because Dad said computers are a fad (because of game crash).
@@disposablebasterd I'm going the cheapskate way myself. A c64 DTV modded to allow you to use 1541 disk drives on it, PS/2 keyboard, and joystick ports.
I’ve considered it myself but honestly I’ve wanted a c64 since I was a small rat child and never could convince my parents to buy one for me, because like I’ve said Dad believed computers were a fad that would be gone by the end of the 80’s
@@disposablebasterd My family got us the C64 for gaming, and had an IBM for their work/bills/etc. Aunt and Uncle had an Apple II and a Mac. Only computers I didn't get to use that I'd like to try are the atari 8 bit machines. Getting back into c64 with modding this C64 DTV though. The DTV costed me something like 20-ish bucks. Then after the mod I'll be able to hook up a commodore disk drive to it. Cant use cartridges or Tapes, but here in the US, the Disk drive is enough for me.
