Repairing a completely dead IBM PC 5150 motherboard

well hello everyone and welcome back to Adrian's digital basement what you see on the bench here is an IBM PC 5150 and what you see right here is a motherboard from one of these machines now this motherboard was one I featured on the second Channel video recently where I had several motherboards I took a look at and this one was not working not working at all I did some rudimentary troubleshooting in that video which I'll link to of course down in the description below but I never got it working so in this video we're going to take a deep dive look at this motherboard to try to figure out exactly what's wrong with it I do want to add one thing though that unlike the Commodore 64 I'm not super familiar with the circuit design of the original IBM PC motherboard so there's going to be quite a bit of figuring out and learning on my part as we go through this video so strap on your oscilloscopes and let's get right to it [Music] all right so the PC motherboard let's try to dig into this and see if I can get it working first things first though I need to test out my capture setup just to make sure that I don't have a black screen on the capture setup that is leading me down the wrong path with this motherboard so on the bench we have my 46 test bed motherboard here let's just get this thing hooked up we're going to use a CGA card because ultimately 4 or the original IBM PC even though you can use a VGA card definitely recommended to do troubleshooting with a CGA card because it's uh well pretty much what that thing was designed for along with the monochrome card so with the card in the 46 motherboard I have it hooked up right here to the RGB to HDMI which is how we're going to be capturing this video and let's give this thing a power up a soft Flash and there it is I just want to make sure without a doubt the capture setup is working and that looks absolutely perfect with this CGA card here so we're good to go if you're curious about the CGA card that I'm using here this is a VTEC card so it's out of the VTEC line of computers and yes recently I've had a lot of conversations on the channel about VTEC what I like about this card is it offers 100 full CGA compatibility but also it doesn't offer any of the snow that you get on CGA cards so any software package that accesses the card's video memory directly doesn't result in any snow which means in DOS if you do dir and they're scrolling with the BIOS routines you don't get that flashing which you get with the original IBM CGA carts there's another version of this car that I actually do have and it has the composite video jacks right here it has two because one is monochrome and one has color output so one of them always has the color burst disabled which can be useful if you're trying to use it with a TV set in addition there's a toggle switch right here that actually switches between MDA and CGA so this is a dual purpose card that can do both so that's really one of the this is why I keep this particular card around for testing because of that Dual Purpose functionality and it's just a good fully compatible CGA cart okay so the PC 5150 motherboard if you haven't seen the first video where I first showed off this motherboard just a recapper it's a 256k motherboard so 64 to 256k that means it uses normal 44 or 4 164 chips it doesn't use 4116 like the very earliest version of this board that means all the dram that's on here is 5 volts only you don't have to worry about the 12 volt and -5 volt rail like on the 4116 chips this motherboard when I got it was really dirty so I gave it a good scrubbing it was soap and water dried it off and well now it looks perfect in case you aren't totally familiar with the original 5150 motherboard it only has five Isa expansion slots unlike eight on the later XT and it also has two keyboard Jacks here what looked like keyboard Jacks and that's because one of them is actually for a cassette drive one of the Jacks takes the keyboard this one here closest to the edge the other one the cassette Drive which in basic which is built into the ROM bios on this lets you allows you to save and load programs without having a disk drive as far as I'm aware IBM did sell no disk drive versions of the original 5150 probably the very cheapest bare bone machine so all you could do is use the cassette drive to save and load programs if anyone did use this at all in fact I'm really curious if you had an original 5150 back in the day without any disk drives did you use the cassette drive on there to save and load programs was there anyone who actually did that or pretty much everyone splurged for at least one floppy drive so you could boot into dos or something like that now let's plug in the power of this motherboard I really didn't do much troubleshooting on this thing in the first video when I worked on it I just sort of tried to make sure that everything looked like it was set properly on the dip switches here that the CPU was good and I think this clock generator I see right here was missing so I repopulated that from a spare I had but the motherboard didn't appear to do anything so maybe it's mirac fixed itself and there won't be much of a video here but let's just see what happens there we go the motherboard is powered up right now I know we don't have any lights or anything on here but the power supply underneath here the fan is on so it hasn't shorted out and we're getting absolutely nothing on the CGA cart here just like it was doing before now if you have a 5150 that doesn't appear to be working and the system does power up like this the very first thing you need to do is double check the dip switch settings here minus zero degrees.net has a really good guide on how to do that so just Google for minus zero degrees and 5150 switches and it'll take you right to this page so there are two switch blocks on the motherboard as you can see right here switch one is the block closest to the center of the motherboard that's this one right here and that describes these switches right here all right first switch no floppy drives so switch one should be on which it is switch two should be on for no math co-processor which is right here there's an empty socket installed RAM on the motherboard so we have two switches here it's currently set to off and off which is Banks zero one two three populated that is correct that's actually what it matches this particular motherboard it says here in the 5150 these switches do not enable and disable the ram see no one blow and important if you have the 1027 82 dated Motherboard BIOS see Note 2 below note 1. IBM 5150 all the banks are permanently enabled switches three four only inform the bios of the population however see no two below and Note 2 says there are software bugs in this version of the bios so all four Banks must be populated and the switch is set appropriately it says here on the extra information page that if you have that buggy bias and you don't have all four Banks the system will still turn on you just have it reporting erroneously the amount of memory you have I just want to make sure this is not a problem that just kept the system from working at all all right back to the switch settings so the next one that is probably going to be causing us a problem right now is switch 5 and 6 here currently is set to On and On so card has a bios so like a VGA or EGA card because that's what I was testing with last time I tried this motherboard we want to have this set for five on and six off which is CGA at 80 columns so there we go and then switches seven and eight are entirely due to the number of floppy drives and yes this machine does support up to four floppy drives so two internal and two external but I think this doesn't really matter how you set these if you have the original switch this one right here set to on which is no floppy drives all right the next switch block which is this one right here this sets the total amount of conventional memory that this particular motherboard has now remember the motherboard itself can take 256k of ram but you can add an Isa card that has additional memory on it and bring this system all the way up to well I think this particular one with a 256k on motherboard can go all the way to 640k here we go this system currently should have 256k so we should have one zero zero one one that's on off off on on and look at the motherboard here we have on off off on on and the other ones don't matter so I currently have them all set to off that means that the switches were all set perfectly except for the fact I had it set for VGA and not CGA so that can actually cause a no display situation because of the mismatched video setting on this switch here so I just turned the system back on and we still have absolutely nothing happening and that's with the switch set correctly now all right so down to some troubleshooting first thing we want to do is we're going to take a look at some of the signals on the CPU here to help facilitate that I'm going to use this little clip thing here which I can just clip right onto the CPU you don't need to have one of these it just means it's a little bit easier to use my oscilloscope Probe on the CPU without having to kind of get behind on these pins back here which aren't so easy when I have the camera pointing down at the bench sort of blocking my vision okay so for checking the signals on the 8088 very first thing we're going to need to do is bring up the pin out of it here and let's start poking around all right first off I'm on pin 40 right here which is VCC turning that on right here you can see that we're at 5.1 volts or so so we're looking good there if you watch my videos regularly on troubleshooting you'll know there's a few things that you need to check while troubleshooting one of these machines or any machine for that matter if it doesn't show signs of Life what are the first things you need to do check voltage well we did we get in five volts you also need to check that there's a clock that's valid and you need to check that there is a reset signal that is valid here's the clock right here on pin 19 so that's that pin and if we turn on the computer and zooming in right there 4.77 megahertz so that definitely means we're getting a good clock now I am curious this clock what I think is a clock generator right here on the motherboard if we pop that out think the clock is going to go away all right with that IC out we turn this on yep sure enough we have absolutely no clock signal so without this little IC in there we weren't going to get any kind of working system all right it's back in the motherboards just make sure there it is 4.77 megahertz move that out of the way so we're looking good that clock signal is nice and stable this little IC right here this is made by AMD it's an 82-84 and you'll find this on any of the original pcxt or original PC motherboards even the Clone ones are going to have this chip it might have two of them in fact if it has a turbo mode often they did that by adding a second one but yeah you'll find it somewhere on the motherboard it's not always in this position but it has that same part number all right voltage is good clock is good let's do reset which is pin 21 right at the bottom here when I turn this on I'm going to have a look at the data sheet but it should start out low and then go high let's see what happens okay it goes from high to low let's double check that that is the way this works I think it will be we actually notice how a test right here has a line on the top of it or the read write pin here has a line on the top of it that means that those are active low now on the 6502 the reset line is active low which means that when that signal goes down to ground it puts the processor in reset so when you first turn on a computer like on a Commodore 64 and you look at the reset line it starts out around ground and then after a few hundred milliseconds or whatever like that it jumps back up to five volts and the processor starts executing because there's no line over here this means it is active High the computer when you turn it on the signal should be high and then it should go low to disable the reset and start execution of the CPU and we do that and that's exactly what we see it starts high and then it goes low which means the processor should be out of reset right now and executing code so let's take a look at the address and the data lines now now you notice on the 8088 it says ad0123 through seven those are the address and the data bus lines zero through 7 they are multiplexed which means the processor uses every other cycle to either send data on the data bus or receive data on the data bus or send address line signals to all the other peripherals through those lines now the rest of these address lines up here are not Multiplex which means they're single function well these ones are over here these are Multiplex as well S3 S4 S5 and S6 but these ones up here are not which is why it just says a12 for instance so on the CPU here I am now on pin 16 which means when I turn this on if the processor is running code or trying to run code we should see plenty of activity here on address line zero and we're not it's just sitting there kind of doing nothing let's check out the next line here same thing let's go up to these address lines at the top that's just high so this processor even though it's not in reset right now so there's reset it's definitely not running anything all right let's look at the data sheets some more because I'm actually not super familiar with troubleshooting PCS like this so I don't actually know what you have to do to start the 8088 executing code now look at this so for the address pluses zero through seven is what we're looking at it floats to three state off during interrupt acknowledge and local bus hold acknowledge looking back up here there's this hold pin and hold acknowledge 31 I think maybe this is a way for the CPU to be disabled by the motherboard so let's take a look at what the hold signal does maybe that's somehow being held at a state that's disabling the processor and that's why this thing is just doing nothing pin 31 is hold that's an input and 30 is an output and that's hold acknowledge hold indicates that another Master is requesting local bus hold to be acknowledged hold must be active High processor receiving the hold request will issue the hlda or hold acknowledge high as an acknowledgment in the middle of T4 or TI clock cycle simultaneous with the issuance of the hold acknowledged the processor will float the local bus and control lines after hold is deselected as being low so if we see hold as high remember it's active high so if the hold is low that means the processor is not being told to basically go into a hold State now before I finish reading this the one of the reasons why this line is very useful on the 80 Ada's because if something else on the system wants to be a bus master or do dma or direct memory access then basically it needs to disable the processor as that other IC needs to take control of the address bus and access the memory this is something that the 6502 the regular 6502 does not have built in so to facilitate dma on a 6502 by disabling the address bus you need to have extra logic now the Apple II does that for instance extra chips on there and I'm sure other systems like the BBC micro does as well but the 8088 has this built in and this hold line is directed or controlled by another peripheral on the motherboard to disable the processor while a bus mastering or dma access is happening scope probe is on pin 31 the hold signal the input signal to the processor if this signal when I turn this on with the computer is currently off if I turn it on and it is high that means that something on the motherboard is telling the processor to disable itself and there it is it's high so absolutely something on the motherboard is telling the computer hey disconnect your data bus and if I move this over to the output pin which is pin 30 holding knowledge it's also high and if you remember from this data sheet here hold is active High and a processor that's being held will turn on the hold acknowledge signal which is also high and that is exactly what we're seeing so that is why this computer is not executing any code right now if I put in a diagnostic ROM on here or work on the Ram or whatever that doesn't matter nothing is actually going to happen on this computer until we solve the issue with that hold signal being high all the time now we're very lucky that there are schematics for this particular motherboard so all we need to do is let's reference that we have the 64 to 26k version so let's take a look at the document here from the technical reference from IBM all right well here we are in the schematics and I guess I'm misunderstanding this because there's pin 31 right there it's actually tied to 5 volts directly so it's kind of expected that that's going to be high and that was right here pin 31 but it also has this RQ GTO right here and that's what's labeled right here RQ GTO and down here pin 30 which I thought was the holding knowledge was RQ GTI so back to the data sheet let me try to figure out what these other signals are then all right here it is RQ GTO so these are alternative functions I guess for these Quest grants these are used by other local bus Masters to force the process release the logo boss okay so it has the same function each pin is bi-directional so their I slash o all right with RQ GTO having higher priority than RQ GT1 RQ GT has internal pull-up resistor so maybe left unconnected I think the key part I was missing was right here the following pin functions are descriptions for the 88 in minimum mode only the pin function which are unique to minimum motor described okay and down here it says it's in maximum mode so that means the mm the m n slash MX is at Ground so there are two modes of operation so obviously the 5150 probably every other pz clone runs this chip in maximum mode which offers more capability if we look at the m n m x pin which here it is pin 33 when MX for maximum is low that means that the Chip is going to be in maximum mode and I am on that pin right here and we turn on the system and obviously it's at Ground I'm sure if we look at the schematics that's just tied to ground on the motherboard that just keeps this processor in that mode all the time all right so back to these pins that means that pin 31 obviously is going to be at 5 volts all the time because that's what we see on the schematics but that means that pin 30 does go off somewhere else in motherboard but notice here it's pulled up to 5 volts through a 4.7 K hook so hooked up to pin 30 when we turn this on it just goes high right away at one time I turned it off and on and I saw it sort of float in the middle and then went high but it's high right away so okay let's change the trigger mode to Falling what I want to do is I'm going to turn this thing off and on and we want to see if we see this thing actually let's just clear this away there we go let's see if we see it being pulled low even briefly no nothing is pulling it low uh whatsoever let me just start looking at all the various signals on the chip and try to figure out which one of them might indicate why the processor is not running all right so all the address lines that I looked at so far are floating now there are these ones here which are Multiplex this is address line 16 17 18 and 19 and it's multiplexed with these extra signals here which allow for some signaling to come out of the processor but I bet you these lines are also tri-stated and then we're on 37 which I assume is address line 17 S4 it's currently at 4 volts so it's currently high and then S3 which was this one was low so code or none what does that mean let's turn the processor off and on or turn the computer off and on oh there was a little activity there did you see that a little something a little something something not that time though so we were seeing one and zero code or none the information indicates which segment register is presently being used for data accessing okay well I don't quite know what that means let's just move on okay pin 32 Rd read strobe indicates the processor is performing a memory or i o read cycle now I'm pin 32 here and it is just low so let's turn this off and on so look at that we do have a little a little Spike of some type of activity there it almost meant that it tried to do a read so since this has a line over that means it's active low which means the processor is currently trying to read memory next up is a ready signal ready is an acknowledgment from the address memory Iowa device that will complete the data transfer the ready signal from the memory or i o is synchronized to the 8084 clock generator to form ready the signal is active High and the 8088 ready input is not synchronized correct operation is not guaranteed if the setup and hold times are not met all right and I'm on the ready pin and it is currently low so when I turned off the computer there was a spike there just for a split second but now it's gone away so this is low all the time I wonder what that's supposed to look like while the system is operating so this is this is coming in from uh the 80 84 chip right here and it's low all the time does that indicate a problem next up we're on pin 18 interrupt request which is a high active High signal and it's just currently sitting there at low so that's fine I guess there's a test signal which is currently High here's the test signal and is pulled up to 5 volts by 4.7 K resistor and it's also tied to this busy line and this is the math co-processor socket pin 17 is non-maskable interrupt it is active high it's currently just low power cycling the computer doesn't change down here in the maximum section S2 one and zero these are outputs and it looks like it has various modes that the processor can be in so let's take a look at what these are this is pins 26 27 and 28. all right we're on S2 which is currently low S1 is currently low and finally s0 is high so we have 0 0 1 read i o Port read i o port power cycle the computer this is double check it's currently in that mode and it is I guess at least it's not in a halted state or something like that so one thing that's interesting here is if the processor is currently trying to read from the data bus then that makes sense that address lines zero through seven are Tri-State because it's looking for something to actually return some type of data either zeros or one back into the processor on those lines but what's perplexing to me is that the processor is just stuck in this state here where it's waiting for a read and is there some type of signal that comes back to the processor to say hey okay read from the data bus like I'm the processor just Waits until whatever on the data bus is put there and then something tells it to you know continue executing code which of the signals is on here that tells the processor to continue executing code and to read from the data bus and then go on to do the next thing is it the ready signal here ready is the acknowledgment from the address memory or i o device that it will complete the data transfer seems like that might be the one right there so the processor might just sit there and wait because this is an input here until this signal is sent to the chip and the chip will then sample the data bus and then continue and move on from there so back on the schematics let's take a look at this ready signal and let's see where this goes well it goes through this block right here and it does end up right here at the 8284 so that's that clock generator chip that I know I've changed out on this board it's this one right here now see this shaded area that it goes into that actually is like a bus of signals that goes Elsewhere on the machine and if we scroll down it's right here going to the other processor that's not installed the math coprocessor and that's it doesn't really go anywhere else so the only thing that's generating that ready signal is the 82 84. here's the data sheet for the 8284 and there is the output already and it's sort of made up of a bunch of other signals so that's a little complicated unfortunately that means there's all sorts of things that could be affecting this so that's the clock input from reset well we know reset works I'm assuming this generates the reset which we see is working well so it's this stuff right here async a enable a enable one ready these various signals here which go into the chip which could be problematic on this motherboard I'm assuming I'm assuming this is the problem here I don't know for sure to be honest now back on the schematics for the 5150 we have those various signals here that come into it so there's like ready one dma weight interesting ready to it's grounded address enable ready weight in fact these are the only two real relevant signals because Ready 2 is tied to ground address level 2 is tied to 5 volts so it's ready weight here and dma weight these two signals are what go into this chip and control that ready signal that comes out of it so let's go to page two see if we can take a look at those dma chips alright so one of the signals right here is dma weight and that comes out of this LS 175 which you know could well be bad um active low right this is an active low signal so if we look here at this chip right here dma weight active low pin four I'm currently on pin four and that is active low it is just sitting there it does start high and then go low so dma weight hmm my assumption here is that that should be pulsing maybe now the dma on the 5150 is is used to refresh the memory the derail so it's not like the dma is never used it is sometimes but um I don't think it should be active all the time right here and if we go to ready weight pin 3 which is currently low as well and that is active low and let's find that ready weight there it is ready weight comes out of that ship there if we turn the computer off and turn it back on it does have high and then go low so one of my biggest problems here is I don't really understand the architecture of the PC I mean I've been using PCS forever but I've never really delved down at the Block diagrams to see how all of these ICS interact with each other now here's the clock generator chip which generates the ready the clock and the reset I don't know why it says preset signal but I'm assuming that's reset that goes to the main processor and it says power but if we go down here this comes to the weight State logic which is doing I'm not 100 sure to be honest we do see the interrupt controller here we have a bus controller data buffers auxiliary processor non-maskable interrupt logic right here so there's the dma controller looks like the dma controller gets signal from this shaded area here which comes from iocs decoder I guess that's obviously just like a chip select function and it looks like this shaded area right here control bus makes its way up to here control line Lanes which I think goes into the way State logic so that's all those logic chips we're looking at that generate those signals that go into the clock generator looking at the dma section here it talks about how there are four dma channels and three of them are available for use but the fourth one is programmed to refresh the system's dynamic memory so dma is used to refresh all the dram on here it's not done by some other logic circuit this is done by programming a dma Channel of the timer counter device to periodically request a dummy dma transfer this action creates a memory read cycle which is available to refresh the dynamic storage both on the system board and in the expansion slots all dma data transfers except the refresh Channel take five microprocessor clocks of 210 nanoseconds so unfortunately IBM's technical documentation here doesn't really talk about what the ready signal should be doing and that logic that goes to it doesn't really say how the weight State functionality or whatever that dma stuff actually works I found this document online though if ready pin is placed at logic zero the microprocessor enters into the weight States and remains idle if ready pin is placed at logic 1 it has no effect on the operation of the microprocessor and just as a reminder that ready pin is definitely held low all the time on this machine so absolutely right there that's the problem why this computer is not executing code so it took me a while to get there but now at least we have an idea of where the problem is the problem is going to be with this logic right here or potentially some of the other inputs into this logic that controls or signals the 84 82 or 8284 right here and fact we saw that this pin right here pin four which is ready one was currently held low all the time which right here ready one bus ready these signals are indicators from a device located on the System bus that is available or data transfer has been received ready one and ready two are qualified by address enable one and address enable two respectively and pin three is currently low as is pin four I'm just going to double check by turning the computer off and on yeah it goes high and low we can see Ready 2 is tied to ground so we can assume that ready one being at ground right now as well is probably fine because that's probably just like a normal state for this chip to be in and then enable two is tied to VCC and we know enable one is currently held down at Ground as well which my assumption is if that were at 5 volts that would be normal operation of the system so let's go Trace back ready wait to see what chips are involved with that signal okay there's ready weight and that comes over here from u82 I'm going to start writing these down here which is a 74 s 74. let's keep following this over so there's a clock input here which comes from a whole bunch of things here through u64 74 ls20 which also is coming from these Gates right here but there is another signal here the D signal that's coming in right here this DAC signal here which I think is dma acknowledge there's also I O Channel ready right here which uh which makes its way here to the pr pin it's got a pull-up resistor so let's take a look at this 74 LS 74 and try to see the truth table on that to try to validate the inputs on this chip this is like what you kind of got to do you got to keep going backwards further and further to try to find the source of the problem so I think the output we're concerned about is the Q output which is right here q and it's currently low so there are two situations where the output can be low so I'm going to clip onto this chip and just validate that the inputs or at least the truth table here matches because currently this chip is not changing state which means that we can easily validate that these signals here going into it match the output that sort of validates that this chip is working I validated the 74 ls74 and the signals look good on there they match the data sheet the truth table sort of tells all the condition it's in right now is high high low this one right here and it's resulting in q0 I actually don't know what q0 means my assumption is that q0 is like the uninitialized state of this flip-flop and you need the clock inputs right here to actually go high to then toggle the outputs of this IC and I'm assuming yeah that's what q0 must mean now I'm currently on the clock input which is what we're looking at here on the oscilloscope and if we turn off the computer turn it off and on we never get anything on this at all nothing comes in here so my assumption is that this is the problem that this Line's not toggling like it should and that is causing um this particular flip-flop to never do its thing which never turns on the ready signal on this part of the motherboard here boy I should get this under the camera oops so we can uh see this a little better yeah so this chip is not outputting ready so the processor is never running I think because of the clock signal now when we look here at the schematics there's the clock it goes through a whole bunch of logic chips so it goes through this LS 20 here also this ls10 and which is connected down here to the output of this 175 which is fed through the CPU clock signal right here so the CPU clock feeds into the clock of u98 and I'm assuming this D1 output which is feeding into this should be doing stuff so let's take a look at these Gates um it's going to take me a while just to kind of plug this little um clip onto them and and check so I will report back when I find something that looks amiss we're on the ls 175 right here clock signal goes in on pin nine that's this right here and that's working fine but this is not getting the correct input signals so d0 here comes from another chip which we'll look at in a second uh the reset line which is hooked up here to the clear input if we scroll around here well it I follow the trace it is reset that is working fine uh clock is working fine oops that's the probe just fell off but d0 is not getting anything correct D1 I think looks correct but d0 is not so the output of this is what goes to this gate here anyways this all kind of goes into this clock input right here and that is not getting any pulses right because we know the clock input here on the 74 ls74 without the pulses isn't going to work it's not going to send the right signal to that chip to generate the ready signal for the processor um yeah and the other things here I don't think it's these this dma signal or these x i o r signals I took a look at all the inputs on here they're not just doing anything but my assumption is that this one here that gets the clock should be sending a signal into this u84 and it is not I think these are operating correctly but what's not operating well I think this chip is operating correctly too but it is not getting the correct input signals which means I need to follow this Trace here over to this IC this ls74 which is another flip-flop in fact is that the same one as u67 uh no it's a different one okay I also noticed that it's the hold signal here that goes to wherever something on page four so it's not the one I thought was going to the processor it's another one but that is also generated by this particular flip-flop it also has a clock signal which let's look at what this clock is looks like it's clock 88 whatever that is let's take a look and make sure that clock signal is working so there I am on pin three and we are getting some type of a signal here let's zoom in oops zoom in on this what are we getting 4.77 megahertz okay so correct clock is coming in to this chip let's take a look at these other signals this kind of TTL logic stuff is pretty complicated to follow uh for this reason okay so that just loops around uh this signal here is actually coming from the chip we were just looking at so that's not going to do anything so I think it's the clear signal we need to be concerned about which seems to come from this gate right here it's pulled up and then this hold request dma I wonder if that's coming from the dma controller and all of this is called caused by the dma controller causing shenanigans pin one clear is high the pr signal is low which is this state right here on the top clock doesn't matter and the output should be high and that does match we're getting high right there so I think the truth table here is completely matching so the reality is is that when this clear line is held like it is being held right here on the output of this gate that means I think this chip will not do anything okay looking over here at u52 I have the clip on it hold request dma pin two it's active low because it has a line over it and it is currently low hmm okay pin one should be just at 5 volts uh wait what it's not that's supposed to be pulled up as a resistor Network here pulled up to five volts and it's not that kind of indicates that this chip might be bad if this chip is damaged pin one is holding this uh pull up low so make sure I have good contact on the chip here yeah I'm on there what's going on let's see if this chip is hot it is not it is not warm at all let's look at the output on pin three that is at 3.4 volts I don't know this is all looking very weird I don't think this is this is working correctly this chip and that's going into the clear pin right there I'm suspecting there's something wrong with this IC so let's pull it out this will be the first chip I desolder on this board and let's see if that is our culprit alrighty there is the chip removed from the board pretty easy if you use lots of hot air on the back side it's easier to desolder these motherboards have a lot of copper in them they're just very well made so it Wicks away the heat so using hot air while you're usually desoldering iron really helps get those pins freed up first thing we got to look at is pin one here so when we turn this on it is now pulled up to five volts as it should pin two is also up at 5 volts so that is this dma signal that was like down around ground now of course the output is I don't know it's going to be floating because there is no output with this chip here because it's out of the board so I have another one let's pop this one in there it is and I have my clip on there so the chips in the board that's five volts that's pin one pin two as oh well that's different that is different pin three is the output and it's inverted which is what it should do and already well I mean that this chip right here for sure let's just go check out the CPU right now all right let's check out pin 22 on the CPU oh my God look at that so clearly the right signal is getting through the clock generator now so like so this motherboard actually can can execute code now well because of this bad chip all right that doesn't mean we're out of the water yet uh I think um might as well plug in the the CGA card who knows maybe this thing came to life and there's the input for the capture device and no we still do not have a working system unfortunately so we have another bad chip on this motherboard obviously something else is wrong now so we have some amount of activity which is a good sign but we're still not out of the water okay so let's look at these address lines again the ones that were always floating before so there we are let's turn on the system and okay so we still have no activity we still have no activity wait okay it's actually they are not floating but they're not changing either so the processor is still not running it's it's like it's trying to run and then it stops if we turn the system off and on yeah just just doesn't go anywhere at all all right but again if we go back to the ready pin right here we're actually getting some activity let's see what how long it takes to appear it should be blank or nothing until the system comes out of reset and yes okay and you saw there was a little bit of activity there so that's a good sign one problem is solved alrighty I think I'm gonna have to take a break here I've been working on this motherboard for a good number of hours it took a little while to decode that logic and find that bad chip so at least we have some progress the system is closer to trying to work but obviously it's still not actually running any code because if it were running bios code we'd be seeing activity there on those address lines the data address lines and we're not it's just sort of stuck so we'll have to wait for part two when I'll dig back into this motherboard and start the whole troubleshooting process again at the processor and try to figure out what next is wrong with this machine because clearly we have more faults so if you enjoyed this video I'd appreciate a thumbs up but if you didn't you know what to do huge thanks to my patrons their names are scrolling to the side of the screen they get Early Access videos and other behind the scenes stuff you can become a patreon at the link in the description below if you haven't yet subscribed to my channel and you're watching my videos regularly I'd really appreciate it if you do hit the Subscribe button I can see metrics and it looks like a lot of people who do watch are not subscribed and YouTube definitely seems to give more Impressions to videos on channels that have more subscribers it means that the exposure of the videos is just less but that is going to be it so stay healthy stay safe and I will see you next time bye foreign

Channel: Adrian's Digital Basement

Views: 41,769

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Id: W7LJDmuHOlw

Length: 42min 24sec (2544 seconds)

Published: Sat Jul 01 2023