Let's repair a mystical issue with an old 386 mainboard 

Hi all, since the question came now about 4000 times ;) I'll just pin this answer here: my oscilloscope is a FNIRSI-5012H, it advertises 100MHz, which is a lie and remains just a dream. This thing works reliably for up to 35MHz, but it's more than enough to repair anything up to 486, since the FSB is usually up to 33MHz, or can be limited to for repairs. If the correct voltage is not important, but the waveform and the frequency, you can even use it for up to 75MHz. After all, even if it doesn't deliver 100MHz performance as promised, I'm still glad, that I have it and it's enough for what I'm doing.

Very unusual failure! I've also never seen a non working clock affect the system that way. I suppose we just don't know exactly how that chipset is using the 32khz signal -- it must be using it for other timers. And that tantalum being bad pulling down the rail. How interesting! Great fix!

When I have motherboard issues I'm annoyed, Necroware: very exciting 😀

After watching this video, I had the "inspiration" to fix my old IBM Aptiva P166 motherboard. The machine was posting VERY VERY slowly ( the memory test took more than 5 minutes, for example...). Using only my cheapo multimeter, poking through one of the clock circuits, I discovered a bad cap that was shorting (maybe grounding?) the 5v rail from the PS/2 connectors( a trace was also broken in this area). Trace fixed, cap removed, and now my board is working again! Thank you so much for sharing your knowledge with us! Greetings from Brazil!

Trivia: These clock crystals are usually 32 768 Hz, which is 2^15 That can be halved all the way down to 1 Hz if one so desire. They're quite universal and can be found in all kinds of stuff that only need a few kilohertz clock frequencies.

That was a really exciting repair with an even more surprising result. Even though I'm an electronics technician myself, I've learnt something. Liked and subscribed.

What a great didactic video. Thanks for sharing all this knowledge in such an educational way.

I saw a comment you made on an Adrian's Video and decided to check your channel. It's marvelous! Good to see someone still trying to work on old PC hardware like you. Sticking here now too. Good work!

Necroware repair? I'm all into it!

I really hadn't considered how it would need to switch from mains to battery. Every day's a school day!

The reason that the tantalum didn't explode in this case is likely because it's not tied to the PSU's 5V rail directly, but only through one of the clock power switching transistors (as you mentioned). As the dead tantalum heats up it becomes more conductive, which normally causes the even more power to be dissipated in the capacitor, leading to thermal runaway and eventually magic smoke release. But in this case the transistor has limited gain and is acting as a current source when overloaded, so the power dissipated in the capacitor actually decreases over time as the voltage drops, and the temperature reaches an equilibrium instead of increasing unbounded.

Some excellent diagnostic skills portrayed in this video! Those tants are of course always culprit #1, sabotaging old beasts like this. I've seen them produce some nice fireworks, short out, making the power good signal go low but I've never seen them slow down a machine to a crawl like this. Very interesting indeed! Keep up the good work!

Wow, who would guess? Very informative! Thanks a lot!

That was a good video. Thank you for spelling everything out for us pea brains.

Very interesting! I wish this video had been out last week, I have just finished diagnosing and repairing the RTC power supply circuit on a 286 motherboard I own. One if the transistors had shorted in my case, discharging the battery overnight.

I have learned so much from you. Thank you sir and God bless.

Awesome stuff. You’re the best at figuring out those old boards.

A good trick to measuring crystals with an oscilloscope is to add a 100k resistor in series with the 'scope tip. Just wrap one lead around the tip and probe with the resistor free end. You don't get the correct peak to peak voltage readings but you do get to see the waveform more correctly under a lighter load.

Kinda lucky there tantalums are not known to fail so gracefully. Very interesting repair, I had no idea the RTC was actually critical for the system beyond telling you the time

Great repair video. Very unexpected cause. Well done in trouble shooting this

You did a great job in finding the issue! Like some others said, I would be annoyed and at the same time propelled to find the problem. In the 90s, I had sooo many old computer parts for cheap. Then some life changes happened, along with losing many computer parts. Now, when I want to buy some of those components again... The prices are out of hand. What a shame! I especially like 386SX and also DX boards/CPUs. They can run 32 bit software while still being somewhat simple and repairable.

It was almost 25-30 years ago and I remember nothing significant since that time, but somehow I still remember this weird BIOS seconds refresh in many of the old BIOS of that time.

This was easily the most informative video I've seen on troubleshooting a slow voltage drop connected to the RTC. It always seems to be these filter caps by the power; the 5150/5160 has these fail a lot too these days. Excellent work!

First time viewer - your video popped up in my feed. I really enjoyed the diagnostic approach you shared. Thanks for doing what you do! I'm slowly picking away at a faulty Apple 128k motherboard. I'll try some of the techniques you're demonstrating.

Very interesting. Learning a lot from your videos.

Great work. Tantalum capacitors are notorious for failing after a number of years. Thanks for sharing.

I have the exact same issue with a Quantel 286 motherboard. In my case it was battery acid damage near the switching circuit that killed some of the traces. Board works now :) Thanks for posting this video!

I have a 386Dx40 board that will hang at startup when it's at full speed but works perfectly in "turbo" reduced speed. I think you may have lead me on a path to finally diagnosing it. Your videos are extremely educational. Thank you.

Wow, you did not promise too much, that was very weird and very interesting as well. Love the instructive bits with the graphics. Learning so much about how these things generally work, just on the side. One day I'll need to finally understand what Varta in which circuit I can replace how with which coin cell.

We need more of this.

Really interesting.....thank you!

Nice repair! This video made me remember that slow clock update in bios on old boards :)

Awesome fault to stumble on, certainly a first for me too. It makes perfect sense but only once you know where the problem is coming from.

Capacitors are a nightmare in IT for decades computers would have random issues I personally never noticed a clo0ck issue like that except battery's dying but I certainly learnt something new today. Remember the era of popping capacitors from particular Pentium 2 till after LGA 775 when they learn to use solid capacitors. Those jelly bean part capacitors I do know a site where you can buy them you can buy nearly anything EE related I will try and find the site amongst my millions of bookmarks and site lists. I would also support your channel I love your work and at the moment work is rare as rocking horse manure in Australia due to the Rona demolishing our economy but I am trying to do research on old computers and also start a charity building older computers for poor countries that don't have access to computers. Your knowledge helps us enthusiasts and IT pro's honestly I think you discovered a problem which has plagued IT since the late 80's and parts like motherboards were just ditched and a upgrade put in its place when a simple off the shelf jellybean EE part could of fixed it with the right trouble shooting techniques. One thing I do repair when I get my hands on them is old AT and ATX power supplies and power supplies are becoming short in supply they are a dangerous component to work on as you know and I do it on a EE hobby level despite my IT qualifications. I recommend preserving and or reconditioning any AT power supplies you have just make sure there is no residual power in them and know what you are doing as you would but for others reading this BE CAREFUL

Well... You've just answered my question as to how a real time clock can malfunction. And how to test for such a malfunction. Now to ask my boss for an Oscilloscope... Also +1 subscribed to your channel. Take care, stay safe, and have fun!

Great find, learned a lot from this video

I've had a similar issue on a different piece of hardware : a trident 8 bit vga video card The card was working but had a terrible video output. Some of that was due to the low quality ramdac, but when I swapped it and messed with the card I noticed that the image was getting darker and darker as I used it and there were some bad interferences in the brightness of the signal. I checked the tantalum caps and one reported a capacity way over its initial capacity (like 2-3times the initial value). When I replaced it, the card suddenly displayed a beautiful picture ! When they don't fail short tantalum caps definitely have some weird side effects !

Interesting repair, i'd like to see more

Super interesting and a pleasure to watch! Thank you for sharing !

That was really interesting, thanks for taking us along with you

Как всегда захватывающе, настоящий триллер ) Думаю, никто и никогда не расчитывал, что железо будет жить так долго, поэтому "необычных" глюков со временем становится всё больше. И это всегда увлекательно! )

great video!

the cylindrical oscillator is just a crystal that oscillates at a specific frequency, it can't oscillate by itself so it needs a driver for it, one that continuously "bangs" that crystal to keep it ringing, while the 4 pin crystal oscillators have that driver built in. when you see some voltage slowly fade away, it could also be temperature related, like a faulty transistor that stops conducting when it gets hot.

Wieder einmal ein super tolles Video. Wieder viel gelernt. Die ersten Tests hätte ich auch gemacht, also Spannung und Takt. Danach keine Ahnung. Vielen Dank!

Hey man, you deserve more subscribers, good video

I needed that info 30 years ago !!

Thank you for the video!, I LOVE IT!

Just found your channel and have already learned. That capacitor ringing tip I never thought of.

Thank you, I learned something new :-)

this brings back pc memories... the fast 386 was the way to go, then the 486, so... i really tuned in to see how the little Oscope works on troubleshooting PC pcb... thanks a lot...😉

Neat, I don't think I've seen the RTC stop like that before.

Great video, as usual !! love the repairing PC videos since pretty much no one does old PC repair stuff. !!!

2 digits should be read as pF and K is the tolerance code. It is +/- 10% I think that it should be a 22pF with +/- 10% tolerance.

Good bit of troubleshooting! 👍

Good repair!

Great video :)

Great video, Thanks

GREAT GUESS WORK

Just stumbled upon this channel. Great video and instant subscribe from me!

Damn, a 386. The good old days.

Very nice video! I liked it very much!

Great work 👌

It is interesting that the board even worked. I had a problem with mainboard where the crystal was gone so it didn't even boot. So it looks like that mainboard needs the RTC impulses only at POST.

Very very good job.

Absolutely awesome root cause analysis! I enjoyed and learned from that video a lot! Thank you! btw, from where did you get this interesting LCR meter? Is this a self made kit? cheers, peter

That was interesting. Even if I directly thought when the clock stops that it is something with the powerdelivery.

Once again a great video! Very useful information too. If I ever have this problem now... I will be repaired. BTW this was a leaky cap (Partially sorted). I hate these tantalum caps. I always go bad. Just replace them with electrolytic. These are extremely cheap too. Go online and get the full set of values. :)

Good job, very nice

1st timer here.. Enjoyed your video immensely. SUB + 🔔 was a EZ decision. Thank you. Cheers from So.Ca.USA, 3rd House On the Right.

yeah, the difficulties of finding replacement 32K real time clock quartz oscillators :-D Cool video, and a nice troubleshooting mystery, though

Personally, I would have never thought to check the RTC circuit. I probably wouldn't have even noticed the stopped clock in BIOS. But as for your fix, bodging a line direct to a known good 3V power source would have been my first choice for diagnosing the issue too.

Interesting..... This is a first for me seeing this as well.

Great repair and very interesting fault! 👍 This tantalum cap didn't explode, because of low current in that rail, but after power up, it gets shorted internally.. Now You get potentially explosive capacitor, just solder wires, put it in front of slow motion camera and turn on 5V supply! 😂

i was about to comment that blue capacitor was a ceramic 22 pikofarads in thermal insolation jacket probably capable of handling up to 1 kv actually , but you was smart on your own power :] anyway very interesting problem

Good Job 👍🏻

Cool, i have the same Board but not Powered after years. Thanks for your work! I think, i must play Doom again :-)

Tant beads were the bane of my life when repairing CNC controllers I had motherboard that wouldn't boot and wiped bios when the battery was flat, that battery cct can cause strange things

I remember I had a similar issue in a laptop, I didnt know what it was until I noticed the clock in the bios was running like a stopwatch... the laptop had liquid damage and the rtc circuit got affected by it, luckily a good soak with alcohol fixed it

Mysterious issue, but pretty well troubleshoot!

Thanks, it was a very interesting video!

Again Nice video F.

Yes refresh issue on the clock, my 386SX 16 had exactly the same issue on the exact same type of BIOS hehe

Cool video, thanks :)

If the quartz oscillator starts up and stops in several seconds, replace the 10 M Ohm resistor in parallel to the oscillator. A very common problem in 40 series based generators.

Good video. I learned some things to. :)

Maaan, I had one of these motherboards. This is actually the same and the only motherboard I have ever killed by accident. Basically I was doing some work on it and by accident left the VGA card partially inserted and (almost asleep on my feet) I decided to push it in while the system was still on instead of powering it down first. This proved to be a terrible idea, for the first (and only) time in my life I killed a motherboard. Never again that board booted or posted in any sense. In any case that resulted in me upgrading my system to a 486SLC 40Mhz from my SX16 386, which was not the best upgrade choice I ever made, should had gone with a 386DX 40Mhz, compatibility issues with the SLC proved to be a pain in the arse to deal with.

Important note: That Crystal is not a crystal oscillator, just a crystal. It behaves electrically like a capacitor and can be seen like a pendulum in a clock. A pendulum that doesn't receive a pulse of energy will just stay in place. You need a circuit that keeps pushing the pendulum. There is likely an IC on the board that keeps "pushing" that crystal creating a quartz locked loop.

Your oscilloscope seems to be so much better than my Hantek 2C42!

Beautiful

Nice detective work on a very strange fault. Personally, I always replace tantalum capacitors on very old boards because they become unreliable just due to their age it seems. (I'm not sure that actual use causes them to wear so much as just being old does.)

Good job, subscribed.

22:30 - ah, the good old Back-In-Dos OS

i was totally not expecting the system to rely on the RTC for timing interrupts for userspace applications, i guess i'm much more used to modern systems having internal interrupts and not needing to rely on such a thing.

This was awesome! As someone who is also interested in retro hardware and as someone who is currently an electrical engineering student, this is super informative and helpful. I absolutely was baffled by this issue at the outset, but found your explanation to be so compelling, simple, and interesting. Old tantalum caps sure can act weird when they go bad! Thank you for sharing this! I have a VLB video card that's suffering from a weird issue where its image fades and darkens to nothing, and I am now somewhat convinced it's a bad capacitor. I'll for sure be using these techniques and skills as I go! By the way, I absolutely loved that tester you used to check the capacitor. Do you perhaps mind sharing more info about it? It seems particularly useful! P.S. Thank you for showing some basics of probing motherboards! I haven't seen any other video that really shows the easiest ways to maneuver an oscilloscope around and, while I have one, I haven't really used it for board repairs.

That behavior on the seconds in the bios is normal. I have some boards with the same behavior.

Well solved!

2:35 "I try to use simple equipment as much as possible". Puts the board on life support. We love you Germans!

I imagine that the cap didn't blow up because the voltage is only 5v and the current draw is very low. It's the 12v ones that send themselves into orbit.

Punish that capacitor. Connect it to your lab bench supply and film it slowmo 😂

very interesting video... wonder how much hardware was "scrapped" through the years due to stuff that had "simple" solutions just because noone had the knowledge of this fine fella here.