IBM PC 5150 repair: The motherboard doesn't have the typical faults

In a recent second channel video I tested four PC motherboards. This IBM PC 5150 motherboard was in that lot and I found it to not be working. Let's troubleshoot the problem and try to get this 1983 IBM PC motherboard working again.

Part 1: This part!

--- Video Links

My initial testing of this IBM PC 5150 motherboard:

Great resource for IBM PC information:

IBM PC 5150 Switch settings:

IBM PC 5150 Technical Manual: (64-256k)

RGB2HDMI: (What I'm using to capture CGA)

Adrian's Digital Basement Merch store:

Adrian's Digital Basement ][ (Second Channel)

Support the channel on Patreon:

-- Tools

Deoxit D5:

O-Ring Pick Set: (I use these to lift chips off boards)

Elenco Electronics LP-560 Logic Probe:

Hakko FR301 Desoldering Iron:

Rigol DS1054Z Four Channel Oscilloscope:

Head Worn Magnifying Goggles / Dual Lens Flip-In Head Magnifier:

TL866II Plus Chip Tester and EPROM programmer: (The MiniPro)

TS100 Soldering Iron:

EEVBlog 121GW Multimeter:

DSLogic Basic Logic Analyzer:

Magnetic Screw Holder:

Universal ZIP sockets: (clones, used on my ZIF-64 test machine)

RetroTink 2X Upconverter: (to hook up something like a C64 to HDMI)

Plato (Clone) Side Cutters: (order five)

Heat Sinks:

Little squeezy bottles: (available elsewhere too)

--- Links

My GitHub repository:

Commodore Computer Club / Vancouver, WA – Portland, OR – PDX Commodore Users Group

--- Instructional videos

My video on damage-free chip removal:

--- Music

Intro music and other tracks by:
Nathan Divino
@itsnathandivino

[capitanschetttino]

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.

[levieux]

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!

[kronos]

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.

[EinChris]

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.

[soulpach]

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!

[chrisdudarr]

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

[Colaholiker]

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.

[eayuwna]

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.

[Firthy]

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.

[fredknox]

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.

[FloatingFatMan]

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!

[tommythorn]

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.

[timcross]

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!

[silvestronsbitsandbytes]

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!

[cracyc]

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.

[brentpolk]

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!

[basecom]

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!

[KerboOnYT]

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!

[horusfalcon]

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!

[Dreamshadow]

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.