Lincoln City C64 Testing and Repairs - Part 2

hi there and welcome back to Adrienne's digital basement this is part 2 of my Commodore 64's from Lincoln City or a pair series this will be the final part where we take a look at the third board and then revisit the first board to try to fix that strange 65 26 fault that it's having alright let's get right to it [Music] all right we're back to board number three this is the one that when we put ded test in we're getting flashing that's indicating that this RAM chip is bad this is the board that has nothing in sockets except for the Vic to and I've already gone ahead and removed the RF shield off the bottom to prepare for desoldering of chips so I'm still getting two flashes on the dead test cartridge and I'm gonna warm up the desoldering iron I'm gonna remove this Ram chip right here this is the one that indicated it was bad according to the dead sets documentation and while I'm at it I'm gonna remove the PLA and I'm gonna remove the sid chip the pla cuz there's a good chance that this might be faulty as well so it's good to have that in a socket and i'm gonna remove this sid because if this works it might be useful to use it on a different machine anyways so i was looking at the back of this board to see if there was any rework done and this boards in really good shape generally but looking at an angle i can tell that one chip has been reworked and it's this Ram chip right here they did a really good job they cleaned up all the flux but the soldering is just not the same as every single other chip on the board that ship definitely looks suspect so when we flip the board over which chip is it that gottsy soldered this one right here I noticed the pen mark on it that's the one that was indicated as bad and check out the brand MT micron technologies all the rest of the RAM chips are any C's but this is an empty no I didn't notice this initially but if I had seen that this was an empty Ram chip and the dead test cartridge is telling us that that chip is bad I am almost for sure certain that this chip is a problem but we're gonna find out when yank it off so I just wanted to point that out before I do it and through the magic of editing i've removed those three chips the bad Ram chip well what I think is bad in the PLA and the CID chip this motherboard was really easy to desolder these three chips some other boards are a lot easier than others not really sure what the reason for that is but almost 100% of the solder came out a little bit of hot air is all I needed and those chips just popped right out so I have the sockets installed and this board even though we haven't even tested yet I've cleaned up all the off the back of board so it looks all nice and factory fresh all over again although Commodore would never use these types of sockets that would imply spending a few extra cents and you know they did everything they could to cut as many cents off as possible okay so I have a new Ram chip here let's pop this into the socket all right PLA is in extra Ram chip is in I'm gonna pull out the debt test cartridge we are gonna see if this thing just boots straight into basic are we ready this is the moment of truth oh really they're nearly there there could be a problem with the PLA but there could also be a problem with the character ROM a blue screen with no text when you boot into basic really could indicate a character ROM problem so we can get a good idea what's going on with the dead test cartridge because the character ROM is not used when you use dead tests so let's pop this into the computer just see what happens I do have to say that the blue screen is a great sign with this replaced Ram chip on here that MT ran chip okay there we go all right so it came up and you notice the character is all look good and that's again because it's not using the onboard character ROM so this character Rama may well be faulty so my have to suck that out let's let this run through a full diagnostic okay it passed all of the tests sound test thus it's not even installed but at least from a dead test perspective all of the chips are good we have a good set of RAM and probably a really well good working PLA I'm gonna reconfigure this from dead tests to the regular diag test and I have these little dip switches on here and I just switch it to two and five right there and this is the standard die cartridge this one requires the ROM chips to be working so when we turn this on probably gonna have a blank screen with no characters well we actually have working characters as well well let's let this run through see what this gives an error on all right now it's saying the basic ROM is bad so I guess it's not the character ROM because we're getting letters but basic is bad so I guess it's just crashing and instead of booting up properly into basic more magical editing and I have a socket installed and this is the basic that I've removed from the computer so if we turn this on now we should just get a blue screen which is exactly what we were getting before okay and there's no cartridge in here 901 2 2 601 this is the basic ROM light removed there's really only one basic ROM that was ever used on all the Commodore 64's it's all the same version 2 it's basically the pet even has a better version I think the pet goes up to version 4 they kind of lock this one at version 2 so I have a replacement basic ROM here let's pop this in turn it on and we have basic ok so that clinches it this ROM is bad big old X so I want to run the Diagnostics using the diagnostic harness just to make sure all these chips are good but the user port and the tape port look absolutely horrible so it's time to try something that viewers have been recommending for a while and it's brass oh this is a metal polish and I'm gonna try to polish these metal contacts to see if this thing works well for me never used it before so I'm going to squirt someone a cotton swab here I put way too much on well that's amazing look at that that's made that really nice and shiny let's try on this cassette port which is in pretty sad shape ok I'm shocked that's that's made a giant difference Wow let's attack the bottom here alright that's shocking those are shiny and they look amazing now I'll just clean this up I have some isopropyl alcohol on this paper towel and I'll just wipe this clean so I get off any Brasso residue that might be still on these connectors the cassette port has seen better days there's a little bit of corrosion still on there but I don't think there's too much I'm gonna be able to do about that I don't think it's gonna cause any kind of issues anyways all right the diag cart is in I have the harness connected everything is plugged in keyboard loopback is on I have the original CID back in I have the speaker hooked up so will hear the audio output and let's turn this on and see what happens we have everything working all OK's the CID is sounding a little off but it was at least producing sound and not a lot of weird pops had a few extraneous little blips and noises and the sound on the noise Channel was a little weird as well but it might sound fine in games so let's give that a try now because this machine appears to be fully functional now alright easy flash three is connected along with the keyboard and a game controller so let's turn this on we get the easy flash three menu let's try my good old standby Donkey Kong arcade [Music] [Applause] [Music] [Music] all right there we go all right I turn the volume down this is obviously working fantastically the SID sounds absolutely fine playing the Donkey Kong intro there so there have been times when I've had a Sid that is working okay with the diagnostic cartridge seems fine there but when you play an actual game you get glitchyness and other strange sounds that's not the case with this one it's working great so let's get out the old pen and start marking up the chicks so both 6520 sixes worked perfectly this basic ROM that I installed is working fine kernel ROM is great character ROM is good CPU is working perfectly this PLA works perfectly this Sid is working seemingly perfectly and the Vic I've already marked a check mark on it that is working great and this little Ram chip that we've installed that is working perfectly and now as well so in summary board number three bad empty Ram chip as if anyone is surprised and a bad basic ROM otherwise we're looking good okay I wanted to revisit the board number one this is the first one we looked at that pretty much worked except for that one error where it said interrupts when I was using the diagnostic cartridge and the test harness so I did is the diagnostic cartridge said you two might be problematic so I've gone ahead and socketed that chip and let's run this test again just to make sure that error is still happening and then we'll swap this chip with another known good one see if that error goes away I've also brass out these two connectors so they look nice and shiny now all right let's see what happens when we turn this thing on currently has one of the poppy bad Sid chips in there so when this runs the sound test we'll hear lots of popping as opposed to the tones we should be hearing all right we're still getting bad on the interrupts test with you two being the problem is turn to speaker up same problem with the SID you just barely hear it and lots of popping noises okay let's swap this chip you two with a known good one see if that problem goes away this chip came out of my zip machine the one I leave in there permanently and it does work properly although originally I pulled this out of a 128 thinking it was not working correctly but this was before I had the diagnostic test harness and I think there might have been a different fault with that machine and this wish this chip was actually fine so has a check mark but I have an M on here for marginal alright we're still getting bad on the interrupts you two so it's definitely not a problem with this sixty five twenty six chip so I'm glad I didn't throw this away or cut the legs off it it's actually a good chip so there's some other fault on here I'm gonna have to figure out what exactly interrupt is on this diagnostic test and see if we can find out where that fault lies okay let's look at the schematics to try to troubleshoot this board so right here is you tube which is the chip that it's saying is bad I have obviously socketed it and it's still having a problem now it's seen interrupts is bad and I RQ is pin 21 right here that is the interrupt line on that you to chip it's hooked up to the nmi signal which is the non-maskable interrupt that signal goes up here all the way to the sixty five ten CPU it's also pulled up to five volts right here through this three point 3k resistor pac-3 but when we look down at the bottom this does go to the cartridge port but in this case it's not being used but you'll see this little bit of electronics right here there's a seventy four oh six which is you eight and here is the triple five or five five six timer and what this is doing is if we follow this trace up here is this is going to the restore key so on the 64 when you push restore it actually generates a non-maskable interrupt and that is buffered by this electronics right here so the five five six ensures that your quick press is held for a little bit longer this is like a little bit of a debounce circuitry and then this is a buffer and it goes up to the CPU and it sends the non-maskable interrupt when you hit run stop and restore together kind of does a soft reset of your 64 but those are the only things that are hooked up to that line now I made a little cheat sheet here too show me all the components that are involved and the signal so the u8 u xx u 2 u 7 and resistor PAC 3 I want to just double check that everything was looking good now I've gone ahead and done that and all the signals have correct continuity so when taking a look at the 7400 6 which is this chip right here pin 6 is indeed connected to the interrupt line on the 65 26 and it also is going to the non-maskable interrupt on the CPU and actually on this board holding run stop and restore correctly resets this machine so that circuitry is working correctly here's the manual for the diagnostic cartridge and as you see we're in section 5.15 interrupt test and this talks about what this diagnostic is actually doing it indicates here the diagnostic instructs the 65 26 at both u 1 and u 2 to generate and IR Q and a non-maskable interrupt that is picked up by the CPU and if it's not then it is shown as bad it also does something with the alarms and the timers and then it generates non-maskable interrupt and if there's a problem there it also gets a bad and then the final section here I don't exactly know what this means the data line interrupt is tested by outputting on the 65 26 CIA u2 and waiting for an interrupt to occur on the 65 26 CIA you - sorry u1 and u2 data is an output on a 65 26 u2 and a check is done if our interrupt to occur on u1 if the interrupt occurs the interrupt is shown is okay I'm a diagnostic and if it's not sensed it's bad so what I did off cameras I used my oscilloscope to check the interrupt lines on both of these chips and the interrupt line on the CPU and everything looks absolutely perfect I'm seeing pulses of the interrupt lines coming out of both of these CIA's and it's shown on the pins on the CPU and just to reiterate I've replaced the CPU with a known good one and that also has no effect on that diagnostic test I also took my zip 64 and connected the diagnostic test harness to it ran the diagnostic and everything passes that test perfectly and I used the oscilloscope to check all the signals while it's running the test in that obsession and it looks exactly the same as this board the only thing I am thinking is that maybe there's a bug or a slight error in the diagnostic and it's saying that you too is bad when in fact you one is faulty so I hate doing this because I like to understand what the fault is on the machine before I just remove chips but I'm really at a loss for the fault one I'm having right now with this so it's time to heat up the desoldering iron because I'm taking this 65 26 out I'll put it in a socket so I can test this in another machine and we'll see if this changes the fault behavior at all okay I remove the chip and it's right here I'm gonna have the Zips machine all comes up to the test harness let's put this one into the same position that it came out of on this machine so right here into you one plug the keyboard test loopback back in and let's turn this on I just ran the Diagnostics a second ago before I switch the chip and everything was testing perfect on this machine no failures whatsoever oh boy interrupts tested okay I'm completely at a loss all right well I'm gonna reinstall this back in the board of course with pin headers so it's removable in the future well three Commodore 64 boards that are all clean and working really well now this one here had the most faults it had two bad RAM chips and a half bad 65 26 that was preventing it from working and it had a bad SID so lots going on here but now it does work fine now that I've changed out the RAM I am missing a 65 26 though so I am actually out of spares anyways this one here which originally had all the chips soldered on it has less faults it had a working Sid which is thumbs up to that but it had a bad Mt Ram chip which I've replacing works and had a bad basic problem that prevented it from booting so this machine is now restored and finally that leaves us with board number one which only had a bad sit originally but otherwise worked and it has that weird fault where the interrupts show bad from the 6255 but I've been playing games on it that's why I have my SDI EC plugged in here and everything seems to be working absolutely fine on this board I can't find any problems with actual software operation so if you have an idea what might be the failure I would love to hear it down in the comment section because I am totally at a loss the first time I've ever worked on a 64 that I couldn't completely fix it so there you go I guess even Adrian get stumped once in a while all right there you have it this is the end of the repair series if you enjoyed it you know what to do you can hit a thumbs up if you didn't you know what to do as well you can hit that thumbs down button subscribe for more videos and I definitely love to hear your comments and suggestions in the comment section below especially if you have a hunch on what that fault is with this 64 board which is the one that has that strange interrupts fault I just can't figure it out because this machine seems to work perfectly in normal use but yet Diagnostics show an error so anyways if you know what it is put your comment down below I'd love to hear it thank you very much everyone thanks for watching goodbye [Music]

Channel: Adrian's Digital Basement

Views: 34,328

Rating: 4.9803853 out of 5

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Id: n8Ijlus6xvQ

Length: 18min 39sec (1119 seconds)

Published: Sat Feb 15 2020