My father payed $ 3000 US Dollars for that exact machine when I was a kid. At first he even didnt let me get close it, but with time I became the owner of it hehehe.

I admire the effort you put into repairing those old machines. They are without any practical use today. But they are kind of a historic artifact. Keeping them alive is a great service to history.

I worked at IBM in the early 80's and had one of the first 5150 machines (with 16K ram). Since IBM was charging about $400 for a single diskette drive I used the cassette to load and save programs in Basic. It was a rock solid 1500 baud interface connected to almost any cassette drive using standard audio tapes. It worked great.

34 years ago, when I was 14 I managed to get my first 100%-compatible PC (because by then we were speaking about the compatibility rate). It was an XT-compatible motherboard supporting 4.77 and 8 MHz which wouldn't work, nothing was happening. I let it heat for a maybe half an hour while measuring voltages everywhere, until I noticed an 74LS244 that was totally cold to the touch. That made no sense, so I concluded it had to be dead. I unplugged my power supply (a variable one which could already have fried half of the board), carefully desoldered the chip trying not to peel off tracks, cleaned the holes and soldered another one. I was delighted with a BIOS counting memory when replugging the PSU. I was very proud (and very happy). It has been my main PC for several years!

You're embarking on the exploration of a huge rabbit hole with the Intel and IBM data sheets. The 5150 is actually quite complicated, compared to the 8-bit systems of the time. I've read that 5150 manual forwards and backwards several times, and I still learn something new every time I mess with my 5150. This stuff is a blast to mess around with!

youre ability to read, decode, and understand the schematic diagrams is amazing. im trying to get better but it still makes my head spin

Q0 in the datasheet means "output unchanged". It's hard to put in words (especially when English is not my first language), but the described outputs are state "Q1" - basically the next state, given the current state and its inputs. If it says Q0, it's referring to the outputs as they currently are.

This is my favourite video to date I think. Even better than the field-found 64. Watching the learning process as you dig into the logic on an unfamiliar machine is incredibly satisfying. Thanks for sharing and thanks to whoever finds and shares the schematics so that this level of troubleshooting is possible. Looking forward to finding out who was the villain in part two.

"Strap on your oscilloscopes"? Even those of us with a ten tonne beast from the 1950s? Their poor vertebrae.

I bought an original with no floppy disks because the original drives were single-side only. Then I bought double-sided drives which were completely compatible since the original PC used almost entirely off-the-shelf components. Never used the cassette port.

I always love your debugging videos.. You're one of the very few channels that go down to block level diagrams to figure out what's going on. Keep it up please!

That is a great feeling finding a fault and fixing it! Watching your videos like this gave me the courage to troubleshoot an Apple II+ motherboard also with multiple faults. Going backwards one issue at a time got it working again. Can't wait for part 2!

At least it's now a step closer to being functional. Many people would probably have tossed it out as dead especially if they lack the electronics knowledge to poke around and figure out what is going on. I like how you're undaunted by the fact it's a machine you don't have a lot of experience with from a service and repair perspective.

Ha, brings back memories. I used to know these chips by heart '00, '04, '08, '74, '175, etc. Still, I don't think I possessed your patience and tenacity. Also, it would have been an eye opener back then to consider chips going bad as I had never experienced that it in the 1980'es; faults were almost always broken wires or shorted pins. I look forward to the next chapter in this adventure.

I was there when the 5150 was born. 1980, It made the most beautiful sounds.

I worked for a computer startup starting in 1980. My company had a contract with IBM to sell their PCs. I had to go to Boca Raton FL for IBM PC class. Just board swapping but the class and the area were great!

S3 and S4 can be used to give the segment registers DS, CS, SS and ES separate address spaces. The Altos 8600 is the only machine I know of that actually did this although only to permit some small bit of per-segment memory protection. The functionality was removed in the 80186.

Thank you for a nice video. To understand some of the logic you through you have to go back to how d-ram works: 1) The addresses are split in 2 parts row and column and these values are sent to the chip with RAS and CAS asserted respectively, in that order. 2) When the row is addressed the whole row is read from the actual memory cells and put into a row register. When the column address is given it just picks the right bits from the register, not the memory cells. As a side not this is why 'fast page' works as any subsequent read / write within the 256 bytes of the page (the row) are accessed directly from this register. 3) The important thing to note is that when the cycle is finished, the whole row register is written back to the memory cells and this is what actually performs the refresh. So, what is needed to perform a refresh ? cycle through all the row adresses and assert the RAS line within the mix refresh time. The maximum refresh time depends on the chip used but is usually around 60uS. Some 'low power' chips increase this to reduce power consumption. Now comes the question: how is this done in different systems ? - Some systems that have shared main and video memory rely on the sequential video reads to satisfy the refresh conditions (Amstrad CPC). - Some systems use 100% custom logic - The PC uses a mix of Timer / DMA and glue logic The designers of the PC opted for this solution because the chips 8259 Timer and 8237 DMA were going to be used anyway and contrary to IBM standard they wanted to only use off the shelf parts. In a working system the BIOS sets up both chips to perform a 'fake access' to all the rows in turn to satisfy the refresh conditions. I am not sure if the BIOS is involved in any of this (receives an NMI interrupt from the 8259 and programs the 8237) but the complex glue logic you looked at is a timing logic making sure that sufficient time is given for the refresh to take place. Also, READY is meant for slow devices to insert wait states and force the CPU to wait until READY is asserted. The complex READY circuitry in the 8284 is apparently there the synchronize the signal with the current sub-cycle of the CPU it cannot be toggled willy nilly. If the refresh is not happening correctly I would check if the 8259 timer is working correctly. The fact that you get a repeating pulse on READY could mean that is is working (DMA wait for refresh).

Onthe cassette drive thing, it amazes me to think that IBM PCs had such abilities to load & save data to cassette in the same way a speccy or a C64 could, for IBM, it seem a step backwards in technology, and yes, I know that tape backup devices were used commonly, but that's a different thing altogether...

One of the things I saw with these old IBM motherboards is broken traces near the memory chips. The board is not very well supported while inside the case, and a user adding memory could easily push the chips in with just a bit too much force. (And I mean seeing this back in the 80's.)

This video couldn't have come out at a better time, as I have a 5150 with the exact same symptoms on my bench right now! Great diagnostics, as always. Working backwards is so important. I absolutely feel that high when you found the bad LS00!

I used to work for computerland, and the only IBM PC's we sold without floppies, always came back within a day or two to buy floppy drives.

I appreciate the detail you go jnto talking about your troubleshooting process. There's a lot about circuit logic I don't understand, like why pull-up resistors work, or what the negative 5 and 12 volt rails mean. But as an IT professional, I applaud the time you take to start at the core problem and work back the why of the problem. Keep up the amazing videos.

Learn to love the "Single" Trigger mode on your oscilloscope for catching changes at power on.

That is some next-level troubleshooting! TTL logic fault-finding is always so fun :) (not really)

Q0 on the flipflop chip, just means that the chip's internal state is being held. so if Q was high before, it remains high, and if it was low before, it will remain low.

You always take a very systematic approach, and use Input/Conversion/Output as a basis for troubleshooting. That's how it should be. Well done!

Welcome to how my IT career began- oh so many years ago. Enjoy.

I finally subscribed! Your channel brings me back to when I was young, working on my dads IBM & PS2. My first personal computer was a AT&T 6300 and my first bought system was a 386DX with the separate co-processor. Keep up the great work!

Any PC I came across back then always had floppy drives, never saw one with a cassette player attached. I think the tape support in BASIC was pretty brain dead too. Getting a machine code program into memory required jumping through hoops.

Really impressed with the diagnostic working here - I love TTL fault finding though! :p

Bravo! Your debugging and diagnostic skills are impressive!

Great video Adrian! That was like an exciting crime thriller ;)

You do an amazing job bringing dead computers to life, I wish more dead systems could fall in your hands rather than landfills.

Amazing watching how much you learn as you grow. This one is a nail biter. Multiple faults can be a fun chase.

Wow, that was excellent hardware debugging and I love that you included to get confused by the CPU mode. What a brilliant episode!

I had a 5150 in the early 90s surplus from my high school without a working MFM HDD, for a short period before I got a working HDD I used the BASIC ROM and a cassette recorder from my TRS-80 model I.

This was a spectacular video. So well done. I think this, more than any other video of yours I've seen, really highlights the benefits of your plan of attack when diagnosing these old boards. Even though you don't know the PC/ISA architecture the way you do the Commodore 64's, you were still able to walk the schematic and find key faults without mass chip swapping. Well done.

Nice troubleshooting. Those were certainly simpler times. Long ago when I was a poor college student I spent a Saturday diagnosing my dead 486 motherboard and finally traced it to a bad diode. Fixing that was a proud moment!

This video was a lot of fun to watch, it really feels good when you are able to locate the broken link in the chain that keeps things from working. The fact that it still doesn't work sucks big time, but hey, it's a fantastic opportunity to record another video in the near future.

Amazing video, I really enjoyed the deep dive in the logic of that signal and can very much relate to the feeling when proper signal appears on the oscilloscope! Can't wait to see part 2!

I just wanted to say, your videos are the best companion to my work days. I love the tech but unfortunately its not in my reality. But having your voice in the background doing stuff you clearly love is such a great ASMR for me to focus on working. And the occasional DK danceparty is just a bless. Hope this is ok to say lol. Thank you!

Great video. Love the methodical debugging.

This was a suspenseful video! Can't wait for the second part :)

Brilliant fault finding so far, looking forward to the next instalment.

I didn't have this particular machine, the first computer I remember having at the home was a Heathkit Zenith computer. From what I've been able to find online, the Z-148 seems to be the closest match to my hazy memories. (Most of what I remember is a "store demo" program that came with it, but I haven't been able to find that at all, because I remember too little about it for Google to pull anything up.) But I'm always interested in seeing how these early computers functioned, so I love seeing these in-depth repair videos. Stay healthy, stay safe!

I loved this video 😂 I spent most of COVID lockdown designing and building my own "minimum mode" 8088 PC based on Sergey K. Xi8088 and I spent ages studying the 5150 and PCjr tech ref manuals (also a min mode machine) - tho I never really got my head round the DMA wait state logic, so great to see you tackle it too. Well done fining that IC I'm sure you must be getting closer. My guess next might be the tristate buffers for demuxing the data addr busses 😊

Very enjoyable. I really admire your diagnostic ability.

I heard in another video that the BASIC in these ROMs doesn't work with floppy drives at all. You can only save your programs with the cassette interface. Also IBM continued to ship this BASIC in ROM long after their systems no longer had cassette interfaces making it completely useless since there was no way to load or save programs.

To know if the 80c88 is actually doing something (pass the permanent infinite wait state you already fixed), I’d check ALE and M/IO pins. ALE (address latch enable) toggles every time the CPU is presenting an address in the A/D bus. That line latches the address to usually a 74ls373 (or some other transparent d-latch). If the CPU is stuck in a very short loop, the ALE will be toggling contrary to the most significant address lines, which were the ones you were probing. That would indicate the CPU is executing code. Also check if the DMA controller (8237) is sending hold requests to the CPU. If the DMA controller is sending hold requests to the CPU, that would indicate that all the logic and code for the RAM refresh was executed, thus validating the CPU is working or at least executing enough code to initialize the 82c37 (DMA controller, with accompanying DMA page registers), the 82c53 (timer), and 82c59 (interrupt controller). The DMA uses channel 0 for those, so I’d check DACK0 in the 82c37 to see those requests. After initial start, the BIOS writes to a bunch of the IO peripherals. I’d look to those getting their CS lines asserted, that would also indicate the CPU is writing/reading to/from those too. THE M/IO line indicates when the CPU is either accessing memory or an IO peripherals. If you see that toggling, then the CPU is stuck in a loop trying to get pass some IO device issue. That toggling indicates the CPU is going back and forth between memory (code) and the offending peripheral. The 80c88 has separate address spaces for IO devices and memory, sort the same as a Z80. M/IO line changes according to the type of access the CPU is doing, be that for memory or IO.

The first time I ever used any IBM PC was a 5150 in school in the early 80s and we saved and loaded our work on cassette tapes.

Great step-by-step wading through, backwards, the schematics and block diagrams. Where I usually bogged down is trying to understand signals that may not matter for the problem at hand, when really, what is required is to stay focused on only the path(s) that do matter. The trick is knowing the difference. You illustrate the technique well.

I did not own a 5150, but I was in a public tech school in 1983, and many of the 5150's in the computer lab were the 16K(?) no-drive variety. I’m sure these were cheaper and therefore easier to write an educational grant for. I took the very popular Basic Programming class, and we were required to use those machines at the beginning of the class. I think they assumed they would get the cassette tape completely working by the end of the semester (I tried to help since I had a new cheap-ified TI994a with a cassette interface at home), and we were close, but in the end they had to let us use the more expensive floppy machines. It’s a lot to expect a student to spent an entire lab entering 100s of lines of code for a class project to just turn off the computer and throw it all away.

I worked with lots of these in business environments, and never saw a cassette attached to them. Always a floppy drive, and later hard drives as well.

Thanks Adrian! Ive been waiting for a pc motherboard troubleshooting vid! Cant wait for part 2.🍺

Two thumbs up. Good old, straight to it, Adrian educational debugging video.

i love watching these repair video's its very interesting even if i dont understand a lot of it. I especially like your enthusiasm and joy when you actually find the problem. As you say a lot "it freaking works!" when we hear that we know you got the job done! lol

That was quite a journey to get to the 74LS00. But you found it. Not easy at all with all that complex logic. Will tune in to see where the next fault is. Looking good.

Fantastic, yes pcs are really complicated beasts, dma usually goes iffy on those, amazing analysis technique, looking forward for next part, tank you

it's a joy to watch how your analytic brain works, keep up the great work

In 2010 I purchased 10 IBM 5150 motherboards for $50. I only got one or two working in 2010 or so. I'll take a look at them again now.

I recently got a clone of this board at VCFSW from the free pile. I hope the fact that half of the RAM is MT means that that part of the RAM is the fault. If not, I will definitely rewatch this series for some guidance.

In the time that the IBM PC came out, my dad was working at a municipal automation center. I did a summer job there in 1985 and 1986, assembling IBM PC's. This was in the time that the AT was already released, so the original PC had been discontinued but the XT was still in the catalog. So I don't know for sure about the original 5150 PC (either the PC1 or PC2 which is what you have here) but I'm pretty sure that all XT's and AT's came from the VAR with floppy drives and hard disks installed according to the type number that you ordered. Of course there would also be the appropriate floppy disk and/or hard disk controller, but that was it. You had to add the display adapter (and set the dip switches accordingly of course), and the monitor and keyboard came in separate boxes too. I've never seen an IBM PC with no floppy drives. If you could afford an IBM PC, you could afford floppy drives and didn't need a cassette. The only official program that ever got released on cassette is the diagnostics program for the IBM PC; I've seen video's and pictures of it but I never had that in my hands, unlike a lot of other IBM PC related hardware.

Hi Adrian. I saw my first 5150 at my first job. We definitely had that model, but they were all equipped with either 1 or 2 floppy drives already installed.

Was enjoying this one dang that theres a cliffhanger can't wait for the next video!!

The IBM 5150 BIOS Post does several checks and they all must pass before it displays anything on the screen. If any of these test fails the BIOS will inmediatly halt the CPU. Some of these tests are: 8088 CPU Test, ROS Checksum Test, DMA controller check, Timer 1 check, RAM base 16k test, 8259 Int controller test.

I wish I had the time to dig out that old dead 5150 board I've got somewhere around here and do some more troubleshooting. All I did when I got it was check for shorted tantalums and make sure the voltage rails were all good, then put it aside. Unfortunately I'm right in the middle of another electronics project, with plenty of others in the queue, so I'm not likely to get back to it any time soon. Anyway, always good to see your repair videos, even if we don't get that "it freakin works" moment. I'll be very interested to see what else is wrong with this board.

~15 minutes in, the section you are reading is relevant to minimal mode. in this mode, you can only have one other master on the CPU local bus. The PC has min/mx pin strapped to maximal mode. On the pin schematic, the RQ/GT[01] column are the pins you should be looking at. They do this is so you can have DMA in addition to the math coprrocessor in the same system, as well as an external bus controller.

Brilliant! Great job!

And it's a 5150 with a hard drive. I had one as a kid (got it for $125 in the early 90s at a TRW surplus sale, back when their Carmel Mountain office was upgrading everything to HP-UX workstations) and it got me through fifth and sixth grade.

When debugging an early PC, hooking up the speaker will often tell you if you've got something seriously wrong. Back in the day, there was also something sold called a "POST" card. It had led lights that lit as the BIOS went through the power on self test. That would allow you to see exactly where the POST stopped. A nifty booklet gave further diagnosis by telling you what to look at with specific chips or areas of the board.

I used the cassette port to turn on/off a mains relay which was then used to control a desk lamp.

I'd check out S0-2 again at this point, but before that I would also connect a PC speaker. Allthough a lot of things on the 5150 fail silently with black screen, there's a bunch of stuff later in initialization that will give you beeps. And also, a POST diagnostic card will not work with 5150 PC and XT class machines, since a different IO port is used.

Hi Adrian. I enjoy watching your videos. They have been a terrific distraction for me since my mom died earlier this year. I have a strange problem and am not sure if you would be able to provide some insight into solving it. The problem is a Samsung LCD monitor. It started to behave strangely a few months ago. I'd be watching something, the screen would blank briefly, then it would come back, then it would blank again and come back, and then it would blank and not come back -- just solid black. I opened up the monitor and looked around and everything seems fine. I bought an ESR meter to check what appear to be the little tin can SMD capacitors and they checked out. I know the chips must work because there is text on the screen. I wonder if I could send you a short video on what the screen looks like with the problem happening. Also, I could show you a photo I took of the board. I did some voltage measurements based on what was listed on the board and they checked out. I wonder if you or one of your viewers might have some insight for me. Thanks in advance. Jeff

I know what those multiplexed address status lines mean, it's related to the x86 memory segmentation scheme. The "Code or None" one means that the CPU is using the code segment register (CS) to store data that's being read from memory, or it's doing nothing in relation to the segment registers. The other ones should be fairly obvious, Data meaning that it's using the DS registers, the Stack one meaning it's using the SS register, the one that confuses me is "Alternate Data" which I think means it's using some general purpose register like FS, GS or possibly the accumulator.

Yes, the 8088/8086 has the maximum mode and minimum mode selection. It actually does more in minimum mode as it is closer to a microcontroller and is a 1-chip computer. Maximum mode strips away the internal DMA and IRQ controllers so you can add your own and provide more functionality yourself.

My 5150 actually started out as a diskless one. It had an extra ROM with Datalight ROMDOS on it, and ran software for the MicroTICCIT interactive TV system. I believe it came out of an Air Force base on the east coast, as it had a DoD tag on it and I remember reading about a training system which was trialed by the Air Force which used MicroTICCIT. That schematic is a bit weird- there are NAND gates which look like XNOR gates.

Great work 👌👏

I've got a compaq portable with similar symptoms, though I don't have an oscilloscope to test with yet so I just keep replacing one chip at a time in the circuit until it works. My system speaker is squealing though and I'd be curious to find out if yours is too. I can't wait for the second half to see what you find! Hopefully your work will shed more light on my issue as well.

Brilliant 👏 👏

as you connect the ICs with the multi-pin clamp, would a logic-analyser (like SALEAE) be easier to work with?

Great vid! What is the PC-based oscilloscope that you’re using?

Just a nitpick here, but the CGA only did snow when you accessed memory during the raster scan, and (weirdly enough) it didn't happen in graphics mode. The IBM PC BIOS waited for the blanking interval before updating the screen, so you never saw the snow if you used BIOS or DOS screen output functions.

Wow what a ride!!

That. Was. Awesome.

Progress is progress!

Hi Adrian, what analyzer are you using with probes? Looks like VirtualBench from National Instruments, but you list the "DSLogic Basic Logic Analyzer" in the video description... ?

I used to work on many of these CPU boards for a clone manufacturer back in the day as a side job.

PC-compatible clones "Elektronika MS-1502", "Poisk-1" from Soviet Ukraine also used cassette port. Still remember loading Dam Busters and Zaxxon off cassette.

First thing I did when I got my 5150 was replace all the tantalum capacitors, they can go bad and short voltage rails to ground, Mouser sells identical looking ones. Also replace the ones on the cards. Next thing I did was re-cap the PSU, then replace the AMD 8088 with an Intel 8088, and add the Intel 8087.

Had them in school with no drive. My first semester was just the tape, but after that we had 5 1/4 floppy's and the tape.

well done sir

Were accelerator cards available for the 5150? In the style of c64 or amiga accelerators?

Hello Adrian, what is the PC in the back, with the Monitor ontop that looks like my first computer

Very good Video

Hi Adrian have you seen the multi functional monitor that checkmate are doing

At 29:30 Adrian says that RDY1 at ground is fine but that would not be correct. That would mean it is waiting for DMA. That line should be high. At ~31:30 Adrian says he doesn't know what Q0 and /Q0 means. It means the state of the Q and /Q lines don't change. Their state will be whatever state they were in before the inputs changed to the H, H, L state.

Strap on your oscilloscopes 😂. Reminds me of Joe’s Computer Museum lol.

I think to have one such board and also blew a tantacap - it has 3 legs - does that imply 2caps..3legs ? can it be replaced by non-tanta caps ?

I wonder if the bootloader (BIOS) on the motherboard is good or corrupted as I am suspecting that the processor is kinda expecting the bootloader to actually load, it may have to do with the motherboard design. The way you described the state machine status just indicated that the CPU was literally sitting and waiting for something. Wouldn't hurt to try a freshly flashed BIOS chip to see if it actually loads or not. Gotta love the early x86 computers, they were insanely complicated beasts (funnily enough, much of the complexity have since been completely moved to the processors themselves nowadays).