Commodore 64 - Part 2: Can You Trust the “Dead Test?”

well hello everyone it's uh time to take a little more of a look at this commodor 64 that we saw in the part one video so where we left off was I was going to kind of do some general checking of the system and do some voltage checks and things um just to kind of make sure that everything looks looks good to go here um I did get some comments from the last video about this power supply and folks did mention that yes that is a serviceable supply um it is still something that can be worked on and uh you put something in line for over voltage and this would work just fine so maybe I'll get back to this and kind of go through it and uh get this up to snuff but this isn't what I'm going to be using today so what I'll be using today is this aftermarket Supply it's a modern power supply with adequate over voltage protection and we'll use it even though I could work on the other one and get it going we're just going to go ahead and run with this one so where I thought I would start is um I'm going to do a couple of things today just before I get going just to eliminate some common problems and kind of in in sequence they are I'm going to go ahead and pull the S and just set it aside for now uh I don't want anything to potentially happen to that if I do have some kind of a voltage issue here so I may also go ahead and pull a few of the other socketed chips too and then just kind of check the power rails make sure that everything looks right so that I don't make anything worse than it is should this machine have issues and we haven't even really turn this one on yet so um this is still a bit of a mystery box for us you might remember it's a a very early revision and so uh I know that there are some things about this that are not going to be super but uh we'll see where we're at most of the pin legs look fairly shiny so that's a good thing for all right let's take a stab at um looking at a few things here I want to check the power switch when it's in the on position here and looks like we have a ground side here all right I am going to go ahead and hook up power and then we'll see what we see on the incoming side of this so we have 4.96 2 which is good and 12.1 AC which I think is fine um I that's a no load condition so I would expect that would would pull down substantially so all right we'll check voltages a couple other places and we'll uh pick it up here off the cassette Port interface and we're going to be looking for DC there 4955 that should be fine we should pick up AC off of off of here okay when I was testing around on this board I found right away that I had an issue um related to uh no AC on the user Port the the 9vt AC that's supposed to be there so of course first thing I checked was the fuse and guess what we had a blown fuse so um that's the first thing that we've run into and I think that uh is a good place to start so with a new fuse we'll plug that in and kind of check around again and and see what we get okay that looks quite a bit better so we're actually seeing about 12 volts ac there but again I kind of expect that um given that uh there's no load on this system so I think we'll we'll be okay there um now let me kind of Pop around the board and check out some other voltages okay we have 12 volts and 5 volts or 4.96 on the Sid socket so that's a pretty good indicator that things are good there okay we're picking up 4.96 667 at uh the 6510 socket so I I think we're in good shape here I'm not not too worried about the voltages we're seeing on here so I'm going to repopulate the board and then we're going to plug it in and see what happens all right with everything populated but the Sid I'm going to go ahead and and we're going to power this up and see what we see okay Moment of Truth okay we are picking up an ntsc signal but it is a black screen so we are putting out video but we are not seeing anything now one of the things that I don't have up to this point is I don't have a uh dead test cartridge or a uh Diagnostics cartridge for the 64 I do have a ROM for the 128 but uh I think that's something that I need to remedy so seeing now that that this is nonfunction um I think it would be good to have a dead test cart so I'm going to take a few minutes and I do have the bits and pieces to do that so I going to put one of those together okay I've got my two boards together uh the red one's going to be my dead test the green is going to be my uh regular diagnostics test and now it is time to go ahead and burn the ROMs for these all right here's our broken 64 and we'll get it set up with the dead test cart because we already know that this has a black screen okay right away we are getting a flash pattern and I believe that is a code one according to the documentation that code points to u12 which is this Ram chip right here however um it really could could be u12 could be u12 and other RAM chips could also be some of the other supporting chips on the data bus so um it really could still point to anything but we have a likely culprit in u12 so at least we have something we do have a flash code at least we know that we have a working um processor and we have a working BC chipper we wouldn't get this far so um one of the things we could do just for fun is to pull the other chips on the board that are not necessary for the dead test and then see if anything changes just on the off chance that it isn't the ram but it's something else on the data or address bus that's that's messing things up but um I doubt it but we can check all right now we're just down to the chips that are necessary to run the dead test let's see if we get the same result it appears we do so we're right back to our single flash um so we're sticking with our current uh Diagnostics looking at u12 and probably the best thing to do is to go ahead and pull that chip put a socket in and uh put in another ramp chip and see what happens okay here's something interesting just for fun I decided to put in just the restra Diagnostics cartridge and see what we get there and initially we power up the system and we get a a nice black screen but if I go ahead and reset we can get the Diagnostics running now it has has a pretty ugly looking screen but it is running so this is kind of interesting and the other thing that's interesting is it passes its Ram test um that's sort of interesting wow that looks terrible but what's interesting is our dead Test shows something with u12 um but when this runs it does not so that's [Music] interesting okay just for fun I pulled the the revision five Vic 2 chip out and put in a revision 8 I'm not 100% certain if the revision five chip or revision 8 chip is in fact good it's just something that's kind of in the part stash so I just wanted to switch that and see if there was any change no it looks like the behavior is similar let's see if the re yep the Reet although the video looks pretty much it looks quite a bit better so okay that's that's interesting that's a little bit of a gain um unfortunately this is the um Vic chip that really goes with this system date wise so uh but it really just looks terrible um so it's interesting that the system doesn't boot at all but when I give it a re a manual reset it boots um is that something that could have to do with the reset circuit um is there just kind of a timing thing on boot up where um I don't know I'm babbling what do you think this is really interesting so we have a system that that looks good on the diagnostic um the only thing we have to do is just give it a reset to get it running okay here's the deal so looking at this system I just thought you know what I'm going to go ahead and rig up just a uh just a cartridge slot um reset button since the reset button on the diagnostic seems to get this thing kicked into gear I thought well um let me go ahead and do that because I know that doing a reset via the cartridge port completely overrides anything trying to stop the reset function so I could do something on the user Port but I'm going to plug this in to this port and see what happens because this is just weird I'm starting to think there's not anything substantially wrong with this other than maybe something with a reset circuit and so that's what I'm going to have to look into and yep we have a black screen if I do a reset H all right so interesting okay so I need to look at some schematics and figure out how the reset works and we may just have an issue with that um the other thing I might do is pull the CPU and uh just make sure that we've got good Connections in all the pins especially the reset pin and maybe there's no chip replacement in the future of this other than that original Vic just H it just looks awful okay let's go ahead and try to capture the reset so I am on currently on the reset pin on the processor and if we power this thing up it immediately goes High um then of course when we do a hard reset now we get the system booting all right so I think we've pretty well narrowed this down to a problem with the reset circuit or at least there is some reason why the CPU never gets a reset signal on Startup and so we were able to plug in a little cart with a reset button on it and prove out that the system is functioning we've run the Diagnostics everything looks good good with this revision of the motherboard the 326 298 revision a um the reset circuitry does not match the schematics in for example the programmer's reference guide when I look up the schematics for this board online I can find them but there's a factory bodge here that is not shown on the um the schematic and there is a missing resistor a 1K resistor in this circuit as well well and in doing a little research online I found that a lot of people have had issues with the reset circuit on this early board and uh cartridges that were plugged in that sent reset signals um you know multi- carts and things like that would crash the system cause other problems um and so a lot of people have modified this area of these boards and I think I'm going to do the same I I don't really know that my reset problem is specifically uh these two chips there's a um open collector uh version of an inverter chip here and then we have a 556 timer also here and I think what I'm going to do is I'm going to pull at least the timer off and install a socket because I'm going to make a little bit of a mod where I'm going to keep this bodge wire uh but I'm going to pull up pin um nine on this chip pull it up out of the socket so that it's not in circuit and then I'm going to restore the 1kohm resistor to the r36 position this is the mod um that most people use that makes these old boards um more compatible with later types of cartridges or multi carts and uh I think I'll do that anyway so while I have this out I'll have a socket in there and if it does happen to be the 5.56 that's at issue I do have more of these um I'll plug a clean one in uh and then um I also wonder a little bit about the 7406 uh but if it turns out to not be an actual component in the reset circuitry I guess I'm going to have to run around here on the board and find out what exactly is keeping the reset signal high all the time okay uh so this is pretty much just with replacing the 5.56 and I want to see if there's any difference at all and it does not appear so behavior is still the same so we have a new 5.56 and we're still getting no reset um so I am going to go ahead and do the 7406 and uh check then and see what's going on and if not that then we have a lot more searching to do okay all right well the uh experiment on the ic's and replacing those ic's that are part of the um reset that circuit does not seem to have done as expected which hopefully was going to be to correct the problem but uh doesn't appear so um one of the things that I'm going to do is I am going to just quickly do the mod that's talked about a lot for this particular board and and see but I don't I think that this should still reset on its own um even without that modification I mean they did sell this model and uh I'm sure that that all of them wer having this this issue so part of the mod is to pick up pin nine um out of the socket so I'm just going to kind of straighten that one outward and put it back in and then to uh install the resistor that's shown on all of the um diagrams or the schematics and that would be r36 and that would be a 1kohm resistor and this is is a mod that I'll leave in place even um if this does not correct our problem and we have to search elsewhere because this will be beneficial in the future for this machine um so that its cartridge Port is a little bit more useful um with special carts that need to be able to reset uh the system yep we're in the same state okay um I did go ahead and pull the cap out of there that's being used on the 556 at least one of them um doesn't measure anything near what it's uh supposed to be so I'm going to go ahead and replace that um that's one of two that are in the circuit um but let's just try it at this point and see what happens that was it okay okay what have I learned from this so what it turned out to be I did go ahead and replace that uh one capacitor in the timing circuit when I pulled this little guy out um he measured just literally like 0.009 or whatever Pico farads supposed to be a 10 microfarad cap so I replaced that and um Lo and beh hold that fixed our issue so um I'm reassembling this some of you are going to say why are you putting that heat shield back or the uh heat shield the uh RF Shield back on this and again just authenticity I'm just kind of putting this thing back together the way that it was and uh that's what I plan to do um keep it as authentic as possible I've replaced the original chips back in the sockets I installed I did leave my mod in place uh for the reset circuit but other than that we've got a pretty much authentic original um 326 298 board uh with all of its flaws with its crummy vicu chip and uh terrible image quality and and uh all the flaws and bodges um contained there in on this board so okay everything's back together and yeah not cleaned up yet but um that's something that can wait this thing's a little kind of on the brown side so maybe in the nice summer sun I'll get a chance to sunbrite this or retrobrite a little bit and get that looking good again but let's go ahead and run our final Diagnostic and again I do apologize for how bad this looks um this is not just my video capture this Vic 2 chip in this is really this bad um it looks a little better in the in the normal um boot with the light blue on dark blue it's not quite so bad but uh yeah this is this is just hideous but between my my cheap HDMI converter and um and the condition of this Vic 2 chip itself doesn't look super great there we go um I guess I can check them out a little more and I can boot a game cartridge in this and see what happens and I think maybe I will do that I don't promise to be any good at this so you're not going to judge my game playing skills [Music] here [Music] oh I am terrible at this I used to play this back in the day at the arcade and with that system it was much better but now there I've lost and something you can really see here is just how terrible that Vic 2 chip is so if I were actually going to be using this system uh that for sure U would have to be changed out for a newer revision and I do have a few of those around here um but I think while this is just stored I have other um Commodore 64s that I will be using um kind of more as my daily drivers and this just kind of a neat example of a of an early system so I think I'll keep it the way it is for the most part it works our black screen is gone and at the end of the day it all turned out to be this with that it's time to pack this Commodore 64 back in its box and until next time thanks for watching

Channel: Primal-Bits

Views: 92

Keywords: Retro-computer, Retrocomput, commodore64, computer history, Vintage computer

Id: ZBnN0HX29EQ

Length: 28min 18sec (1698 seconds)

Published: Sat Feb 10 2024