286 Motherboard repair - ongoing w/ issues

[Nexxen]

Hello!!

I bought a 286 motherboard, advertised as "damaged". Pics from the seller were pretty straightforward.
He added that he couldn't make it POST, the cpu was scalding hot and that no BIOS reflash could help.

After a thorough inspection I found many undetected issues that might have prevented the board from POSTing at all.

I attached the seller's pics.

Most prominent is the jump wire.

[Nexxen]

Also, many solder joints were cracked - and that alone took quite some time to inspect and reflow.

It was a little dirty and had some mild corrosion on some other DIP-14 ICs. Nothing too serious but needed the vinegar+soap wash combo.
Everything came out spic and span.

The jump wire was soldered to the wrong leads:

the seller made a R5 to pin11 of a MC74HC14AN (https://pdf1.alldatasheet.com/datasheet-pdf/view/164836/MOTOROLA/MC74HC14AN.html),
that in turn connects to pin 13 of a SN7406N Datasheet (https://pdf1.alldatasheet.com/datasheet-pdf/view/27354/TI/SN7406N.html);
when in fact it was connected to R6.
-- my guess is that he followed the wrong trace and on the wrong assumption that is wasn't making contact he wired it.

I hope nothing happened there.

Traces:
some were actually good on continuity test, but when I scratched away the old solder mask a couple came off in bits;
another one was gone;
two others had indents of corrosion.

I tinned everything and restored the dead traces. Nothing fancy, it was easy.


Socketed chips:
- BIOS chips were so oxidised that my TL-866 struggled to read them indicating bad contact. Cleaned and perfect reads and writes.
- Keyboard BIOS, same, heavily oxidised. Cleaned and I hope good to go.

Sockets deoxidised thoroughly, let's hope it's good too.

I had to replace a cap that had a broken leg (it came straight out) (C63 pic), removed a broken 10K resistor (replaced with a smaller size - see R4).


Yesterday I was too tired to fire it up and watch it fail - LOL
Later today I'll test it.

[Nexxen]

Questions:

My last concern is the size of the R4 10K resistor, smaller size could mean that it will go smoking away.
I have no match for this one. I don't know the wattage of the bigger and of this one.

Also, in order to replace the two ICs involved in the jump wire, SN7406N is avilable, but MC74HC14AN looks way lessa vailable.
Is there a newer perfect replacement for this one?

[Nexxen]

I have issues with the crystals. I'll try to post pics from the oscilloscope.
BTW, board powers on but no POST.
Cpu goes up to 50+ °C (normal to me).
Edit: cpu is good

32KHz:
normal, 32 on both leads;
it displays a sine wave and on the other lead a square wave.

14MHz:
- on one lead sine wave, other oddly "dented" wave
Edit, the wave is squared by an ic and is correct (my mistake interpreting)

32MHz has only one out of the four leads issuing a signal, 32MHz, the other 3: 2 dead, one just Vcc in
edit: Datasheet is 2 nc 1xVcc and 1xOutput

8MHz (keyb):
one 94 KHz (but sometimes 9-8 MHz) and other lead dead.
Edit: without turbo, with turbo on it's correct

[Nexxen]

I closed the turbo switch and the 8MHz is now 8.

On the cpu, CLK is 16MHz; no change with turbo ON or OFF.

ISA Pin B2 is 94KHz, is this frequency ok for some communication?

Also, it won't RESET. Shorting the leads does nothing.
It does light up on pwron, and at poweroff (displaying FF 00, or FF --  for a split second).
Edit, even with bios chips removed; irrelevant then.

Next step: try some other BIOS. But I don't think that's the problem.

If BIOS still fails posting I'll desolder the cpu and socket it.


Edit: One Vcc on cpu is 5.17V, the other one, straight from the 5V rail as well, reads 3.17, 3.74V.
Maybe the cpu is dead after all.

[Nexxen]

I tested the BIOS on another 286 and it POSTS up to a certain point.
I get codes and that is enough to know it can work.

I checked the CPU Vcc and the one that is getting 3.74V on this board, gets the full 5.15V required.

The cpu is actually dead as per seller's prognosis or this is a motherboard issue.
I'm pretty sure it's the cpu at this point.

When I hit reset nothing happens (it has to be drawn HIGH), and it just loses "Frame" on POST card, no reset happening.
I'll desolder and socket the cpu. Maybe I'll try it on another board, but I also have plenty of PLC68 286...

[HwAoRrDk]

I wonder what the justification of the "fastest keyboard BIOS" tagline on that Jet-Key keyboard controller is. Not a brand I've ever seen before.

[Zero999]

Questions:

My last concern is the size of the R4 10K resistor, smaller size could mean that it will go smoking away.
I have no match for this one. I don't know the wattage of the bigger and of this one.

Don't worry. The voltage would have to be much higher to overheat, even a small 10k resistor, which would just dissipate 2.5mW at 5V.

Also, in order to replace the two ICs involved in the jump wire, SN7406N is avilable, but MC74HC14AN looks way lessa vailable.
Is there a newer perfect replacement for this one?

No, don't use the 74HC series. It has different logic levels than old TTL. The 74HC14 also has a push-pull output, where as the 7406 has an open collector. A modern equivalent, which is still available, is the 74LS06.

I tested the BIOS on another 286 and it POSTS up to a certain point.
I get codes and that is enough to know it can work.

I checked the CPU Vcc and the one that is getting 3.74V on this board, gets the full 5.15V required.

The cpu is actually dead as per seller's prognosis or this is a motherboard issue.
I'm pretty sure it's the cpu at this point.

When I hit reset nothing happens (it has to be drawn HIGH), and it just loses "Frame" on POST card, no reset happening.
I'll desolder and socket the cpu. Maybe I'll try it on another board, but I also have plenty of PLC68 286...

It does sound like the CPU. Have you tried removing it to see what voltages you get? The low voltage on VCC could be due to it drawing too much current and pulling the supply voltage down.

[wasedadoc]

No, don't use the 74HC series. It has different logic levels than old TTL. The 74HC14 also has a push-pull output, where as the 7406 has an open collector. A modern equivalent, which is still available, is the 74LS06.

The OP is not proposing to use a 74HC14 to replace a 7406. Both types of chip are already on the board and both are to be replaced.

[Nexxen]

1) Thanks! No, I'm looking for an equivalent replacement. I knew there was better and "more modern".

2) I desoldered the cpu, put it in another socketed board and it works 100%.

However I discvoered that the Vcc on pin 30 wasn't connected, or just a small piece of the trace was.

Later I'll press down the cpu on it to see if it was that.

[Nexxen]

No, don't use the 74HC series. It has different logic levels than old TTL. The 74HC14 also has a push-pull output, where as the 7406 has an open collector. A modern equivalent, which is still available, is the 74LS06.

The OP is not proposing to use a 74HC14 to replace a 7406. Both types of chip are already on the board and both are to be replaced.

Yes, exactly.
If nothing works I'll buy new chips.

[Nexxen]

I'm out of PLCC68 smd sockets. All used.
I'll have to order and wait 15 days. Well, that's it so far I believe.

Any other advice is welcome and don't feel cold posting any

[Haenk]

Obviously the battery leaked, and is missing completely now. A via has been drilled out. That really could be an issue.
Also several traces show damage from leakage, you certainly need to take care of that.
No battery at all might be an issue as well.

[Nexxen]

Obviously the battery leaked, and is missing completely now. A via has been drilled out. That really could be an issue.
Also several traces show damage from leakage, you certainly need to take care of that.
No battery at all might be an issue as well.

I'll desoler the sockets and chips and check for more broken traces, probably right to check more in the area.
As I don't have a desolering gun it's gonna take some time... Maybe next week, to give me some new view on the whole matter.

[Nexxen]

The cpu was proven to be functional. But the 2nd Vcc input was below minimum required (+5V).

I tested the smal 104 cermaic capacitor the +5V was coming from (a small trace from hole to pin), and it was a 130nF /0.1% /2.4ohms ESR, the V was 5.xxV (as intended).

Question:

if it wasn't being siphoned by this cap, why Vcc is lower on the cpu? What else could be affecting it?
Vss were = 0V.

https://pdf1.alldatasheet.com/datasheet-pdf/view/1284116/AMD/80286.html

[Haenk]

Maybe check/replace the electrolytic caps as well, 30 years is quite some time, and Samsung (SEMCO) was not exactly top-tier back then.

[Zero999]

No, don't use the 74HC series. It has different logic levels than old TTL. The 74HC14 also has a push-pull output, where as the 7406 has an open collector. A modern equivalent, which is still available, is the 74LS06.

The OP is not proposing to use a 74HC14 to replace a 7406. Both types of chip are already on the board and both are to be replaced.

Yes, exactly.
If nothing works I'll buy new chips.

Sorry. I misread. Replace the 7406 with 74LS06, but replace the 74HC14 with similar. It should be fairly easy to get hold of. There are better, faster, alternatives such as the 74AC14, but more speed isn't always better. The faster rise and fall times can cause instability on a board which is not designed for it. Don't worry about the MC bit, any 74HC14, in a DIP package will do.

The cpu was proven to be functional. But the 2nd Vcc input was below minimum required (+5V).

I tested the smal 104 cermaic capacitor the +5V was coming from (a small trace from hole to pin), and it was a 130nF /0.1% /2.4ohms ESR, the V was 5.xxV (as intended).

Question:

if it wasn't being siphoned by this cap, why Vcc is lower on the cpu? What else could be affecting it?
Vss were = 0V.

https://pdf1.alldatasheet.com/datasheet-pdf/view/1284116/AMD/80286.html

Have you measured the resistance of the VCC pin to the power supply rail? Perhaps it's a bad trace/solder joint somewhere.

[evac]

The cpu was proven to be functional. But the 2nd Vcc input was below minimum required (+5V).

I tested the smal 104 cermaic capacitor the +5V was coming from (a small trace from hole to pin), and it was a 130nF /0.1% /2.4ohms ESR, the V was 5.xxV (as intended).

Question:

if it wasn't being siphoned by this cap, why Vcc is lower on the cpu? What else could be affecting it?
Vss were = 0V.

https://pdf1.alldatasheet.com/datasheet-pdf/view/1284116/AMD/80286.html

Are you able to load test that supply rail to check if it can deliver the required current?

[Nexxen]

No, because I don't know how. I have to ask for your help on this (or link a guide).
I'd look it up by myself but no clue what I'd be looking at.

PSU is working 100%, AFAIK. I could check voltages once again to rule that out.

[Nexxen]

Thanks and will do as suggested.

It's a nice little neat board, I'm pretty sure it's a thing so obvious I'm not even thinking about...

[evac]

No, because I don't know how. I have to ask for your help on this (or link a guide).
I'd look it up by myself but no clue what I'd be looking at.

PSU is working 100%, AFAIK. I could check voltages once again to rule that out.

the point is just to get a realistic load on the rail and see if the voltage stays stable and doesn't drop
I don't know in this case if the rail is directly provided by the PSU or regulated on the board itself

ideally you would use an electronic load connected to the rail and increase the current until you either reach the expected load or the voltage starts to drop too much (indicating failure)
a barebones electronic load can be had for relatively low money from the usual places
alternatively, you can also use an adequately sized resistor (in both value and power dissipation) connected to the rail to draw some current

[Nexxen]

EDIT1: after checking voltages on another board:
1. -3.25 as recorded on CAP pin, a negative voltage is correct.
2. +5 on Vcc pin 30 is correct but not present on this board.
3. the 473 cap is there too, I guess that even 60nF should be good, as no short and no res is present (at least up to 2M)
4. without cpu CAP is = to 0.00V as no cpu is installed

Last issue is no Vcc = +5V on pin 30

What is down here is outdated.



I checked my notes taken a few days ago.

I have one cap (C43 - ceramic) with a reading of -2.4V (minus 2.4V), connected to CAP pin (see attached pic), that might be preventing the internal circuit to reach reset before POST operations.
Couldn't find its source, nothing beeps. I'll need a wider probe (i.e. a cable with smaller threads) to find where it is generated.

I'll post more in a while.

Edit: no cpu, cap43 readings are 0.00V on +.
Check Intel's CAP definition. Can't get what it means, looks like CAP is connected to Gnd via a ?? capacitor? Makes no sense to me.

Cap43 is a 473 value, 0.047µF or 47nf, reads 62nF. Maybe it was leaking to G?? Other 104 (100nF) read +20% or 30% higher. Vloss on all is 0.1%, ESR 2.1 to 2.4 ohms. Could be bad too??
I can buy new ones.

[Nexxen]

No, because I don't know how. I have to ask for your help on this (or link a guide).
I'd look it up by myself but no clue what I'd be looking at.

PSU is working 100%, AFAIK. I could check voltages once again to rule that out.

the point is just to get a realistic load on the rail and see if the voltage stays stable and doesn't drop
I don't know in this case if the rail is directly provided by the PSU or regulated on the board itself

ideally you would use an electronic load connected to the rail and increase the current until you either reach the expected load or the voltage starts to drop too much (indicating failure)
a barebones electronic load can be had for relatively low money from the usual places
alternatively, you can also use an adequately sized resistor (in both value and power dissipation) connected to the rail to draw some current

This is beyond my expertise.
I'll have to understand how in order to get there. Learning curve...

Thanks for the input!

[Shock]

So the only fault you are aware of is a missing voltage rail? So one VCC is good, ones missing? If you added a socket for the cpu then check your soldering work and that the problematic VCC is a low ohms connection and not shorted anywhere (to ground or a neighboring pin for instance). Then trace the VCC back to the power connector. You can also see if that rail is used elsewhere or other power rails are missing.

Perhaps a good time to take some better images of the board.

[Nexxen]

So the only fault you are aware of is a missing voltage rail? So one VCC is good, ones missing? If you added a socket for the cpu then check your soldering work and that the problematic VCC is a low ohms connection and not shorted anywhere (to ground or a neighboring pin for instance). Then trace the VCC back to the power connector. You can also see if that rail is used elsewhere or other power rails are missing.

Perhaps a good time to take some better images of the board.

+5V is present everywhere.
The Vcc from the capacitor (as on other 286 boards) is measured +5V on the leg; the pin of the cpu is 3.74V. 1.2 is missing.
PSU +5V to the capacitor is 0.06 ohm, powered off; powered on 0.00.

I probably measured Amps making a mistake, probes became hot, 0.25mA from G to +5V (I don't know how to use a multimeter to measure A, but I think I killed it some time ago when I got it sparking...). Have to buy a new one.

What, inside the cpu, could be drawing out 1.2V? Some short on a BUS line? Some 74 chips? LOL when they say learn by getting kicked...

Chipset isn't getting warm, probably because of a missing cpu.


I didn't solder a socket yet, I bought 5 and used them all elsewhere, I have THT sockets that can't be used. I ordered some and have to wait 15 days.
I also oredered new caps, in the meantime I'll use tantalums like on any other board.

[Shock]

Don't check resistance while the circuit is powered. It supplies it's own voltage in order to take a measurement and you will damage some multimeters by doing that. But 60 milliohms is about two test leads, so that is fine it seems like a good connection.

Swap the psu if you have another known good one to test if possible. If not you need to google how to load test a psu with a dummy load or electronic load.

I suspect if that checks out then there is either a partial voltage rail short on the board which could be caused by a component, a voltage rail short to somewhere it shouldn't. Or a missing ground or another shorted component connected via the cpu.

Lots of resistance testing or when you can... socket the cpu then touch test (being careful not to burn yourself) looking for other components getting warm.

[Nexxen]

Don't check resistance while the circuit is powered. It supplies it's own voltage in order to take a measurement and you will damage some multimeters by doing that. But 60 milliohms is about two test leads, so that is fine it seems like a good connection.

Swap the psu if you have another known good one to test if possible. If not you need to google how to load test a psu with a dummy load or electronic load.

I suspect if that checks out then there is either a partial voltage rail short on the board which could be caused by a component, a voltage rail short to somewhere it shouldn't. Or a missing ground or another shorted component connected via the cpu.

Lots of resistance testing or when you can... socket the cpu then touch test (being careful not to burn yourself) looking for other components getting warm.

How to load is something I'll look up. Learn once and forever done.
I ordered a load of sockets to test different components (maybe swapping from other boards). The seller connected a +5V to an input on the MC74, wrongfully thinking it was a dead trace but in fact it was the one next to it (even I firstly didn't notice). could have killed it.
While waiting I'll desolder other stuff and check for shorts and on a motherboard. Ruling out stuff is good + my TL-866 programmer can check those 74 logics and tell if dead.


I have a laser (or what tech it is) to test temperature. The days I was getting my finger burnt are over! 

[Shock]

Here is a simple test, but I recommend you learn the power or current ratings of each voltage on the psu. I'd not load more than 80% of the maximum power or current rating of each voltage. Reason why is you shouldn't be loading it more than that with the motherboard anyway. If there is a motherboard fault and it draws more than 80% then you got a motherboard fault or an underrated supply. Needlessly testing a vintage supply (if it is) at 100% may induce a failure so our goal isn't to create another repair unless you have some kind of intermittent psu problem.

Note that same color positive voltage wires may be the same rail connected inside the power supply. So if testing multiples at once careful to not needlessly overload the same rail.

So say you have a 60W 12V bulb. That is 5A or 60W load when connected to a 12V positive and common. You then check the voltage for DC and AC with a multimeter or an oscilloscope (don't blow your scope, remember never connect the ground lead of the probe to a grounded referenced, non floating voltage). The bulbs will get hot so put them on something appropriate and don't leave unattended. Voltage sag (loss of voltage) or excessive ripple can mean the psu is struggling to regulate.

You can buy or make fairly inexpensive electronic loads which can be adjusted but I still recommend reading how they are used. In your case you want to ensure it can handle the power/current/voltage ranges you require for psu testing.

[Shock]

Notice he is jumping the power good - PS_ON pin in the power supply so it starts up. You need to be aware that early power supplies used different wiring and standards. Don't just do random pin to pin without checking color coding. In the same way connecting some old psu to some old motherboard without checking can reveal that in fact they aren't pin compatible.

Automotive bulbs can be brought inexpensively, or acquired free if you know who to ask.

[Nexxen]

Ok, I think I got what you mean.

I also have other AT PSUs to try. I have a ATX-to-AT connector too.
Negative voltages aren't an issue for the sytem itself IIRC.

I'd like to have a bordview file of some 8086/286/386 now, I'm curious to see what it's all about with motherboard design.

[fzabkar]

Notice he is jumping the power good - PS_ON pin in the power supply so it starts up.

Power_Good (PWR_OK) and PS_ON are two different pins.

https://pinoutguide.com/Power/atx_v2_pinout.shtml

[fzabkar]

I also have other AT PSUs to try. I have a ATX-to-AT connector too.

Your 286 motherboard and AT PSU do not have a PS_ON pin.

https://pinoutguide.com/Power/MotherboardPower_pinout.shtml

[Nexxen]

I also have other AT PSUs to try. I have a ATX-to-AT connector too.

Your 286 motherboard and AT PSU do not have a PS_ON pin.

https://pinoutguide.com/Power/MotherboardPower_pinout.shtml

Ok, that I know too
It was to say that I have more and try more... just for good measure.


I desoldered all the electrolytics on the power rails. All 0.8-9% Vloss; ESR was between 0.8-1.2 ohm.

[amyk]

If the CPU is getting to 50C immediately, that doesn't seem normal for a 286.

I haven't touched one in a long time but I remember those early CPUs would only get warm to the touch, not burning hot, after prolonged use; late 486s and Pentiums is when heatsinks and fans started being necessary.

I'd like to have a bordview file of some 8086/286/386 now, I'm curious to see what it's all about with motherboard design.

Here's a 386: https://alexandrugroza.ro/microelectronics/system-design/isa-80386dx-sbmc/index.html

[Nexxen]

If the CPU is getting to 50C immediately, that doesn't seem normal for a 286.

I haven't touched one in a long time but I remember those early CPUs would only get warm to the touch, not burning hot, after prolonged use; late 486s and Pentiums is when heatsinks and fans started being necessary.

I'd like to have a bordview file of some 8086/286/386 now, I'm curious to see what it's all about with motherboard design.

Here's a 386: https://alexandrugroza.ro/microelectronics/system-design/isa-80386dx-sbmc/index.html

286 can get quite hot, not abnormal. On my working 286 board it's 50°C.

It gets to 50 on a curve, it's not 0 to 50. Plus I desoldered it and it works 100%.

As soon as the socket comes in I'll be able to do more tests.

[Nexxen]

I tried the keyboard microcontroller (Jet Key), in other boards it would not work (no POST) or give Gate 20 error.

Could this be the sign of a malfunctioning of the keyb controller?

[Nexxen]

I tried the keyboard microcontroller (Jet Key), in other boards it would not work (no POST) or give Gate 20 error.

Could this be the sign of a malfunctioning of the keyb controller?

In any board that has a keyboard micrcontroller, the Jet Key displays Gate A20, nothing or 0605 0807 post codes - and hangs there.

[Nexxen]

I tested the keyboard controller, Jet Key, and it won't work in any other motherboard.
Maybe it is this?

I can't wait to get the PLCC socket and test other keyb controllers in it.

[Nexxen]

DISCLAIMER: this is a pure "I have no idea but I'll try to push my solitary brain cell to do some work".

Let's reason on this:

my assumptions:

1) the 286 is divided into 2 zones, powered by 2 different Vcc inputs
2) each zone has a Vdd (gnd) - those are pointless here as they are GND

my thoughts

1) if zone2 is powered by Vcc2, and zone 2 is associated with Addresses (A0 to A23)
2) there is something shorted to ground in the ICs/other connected to Addresses
3) this could be avoiding zone2 from working properly and never reach the POST stage

My possible culprits:

i) Keyboard microcontroller (8042)

ii) 2 chips associated to it, located underneath

iii) some other transceiver/multiplexer to ISA bus

Nothing is shorted to ground, but internally something went wrong and an input is now associated to an output to ground, hence my transceiver/multiplex/8042 failed components.



Yes, this is baseless mainly. But it's a starting point to learn from mistakes 

[Nexxen]

PLCC68 socket came in today. From now on I can try cpus.

I put in the BIOS of an old Compaq 286 and I get a few codes from BIOS. 01/08/09, 09 0E 07....
With one Keyb controller it issues 00 01.
These Compaq BIOS images are 128 bit, not 256. Modifying the size via jumper changes nothing.
Reset LED lights up only at Power on and off. Is this good?

The bios images it came with work with another board but give checksum error. They should issue some code, probably they aren't this board's images.
I have to try new bios images.

There is a problem that I can't figure out, RESET does nothing when triggered. Only with one keyb controller it works, but just cycles IRDY and starts over.


Bottom line:

I can safely assume that the CPU is decoding something.

[Nexxen]

Chip HD14818P ha issues:

RESET should go LOW during power on, but it immediately goes and stays HIGH.

I'll desolder and socket it.

[Nexxen]

Tested the HD14... works in another board.

Issue is starting to smell like chipset.

[Nexxen]

I desoldered sn7406N and MC74HC14N (the two icd under the keyboard controller).
I tested them with XgPro, and they both passed.
IDK if this in conclusive or if I should buy replacements.

Whatever keybaord controller I stick in it won't have the reset line working anyway.

I'm out of ideas.

[Nexxen]

I desldered the keyboard socket and cleaned under it. Replaced with a new one.
Some traces on the back of the board had microcracks that became cracks and that was probably preventing some porcesses.

Now RESET works when triggered.
But IRDY on POST card isn't lighting up. Something isn't being read.

I socketed the cpu, but it broke. Bad quality stuff.

[Nexxen]

8MHz Xstal, connected to Keyboard wasn't working anymore.
When "alive" it had an erratic signal (not a nice wave but a very irregular shape, didn't take a pic so go with a sine that had many creases).

Ordered one, waiting to arrive and solder it back in place.

[Nexxen]

I replaced the xstal and still nothing out of it, no 8 MHz.

Next thing I'll check is a dead cap between xstal and keyboard controller.

I have RESET working, something is alive but without that 8 MHz nothing is happening.

[Shock]

If the clock is derived from the keyboard itself you will need it present. Some boards need the keyboard to boot. See if the keyboard leds flash or light up, the old quick test of toggling the caps and numlock is good to see if it's in a hung, off or normal state.

Don't assume some operation previous hasn't hung either. Keep updating.

[Nexxen]

To see if it works I simply use a POST card.

I needed some time to wash it off my brain. I'm planning on going back to it soon.
I'll test it witha keyboard connected. When I say it takes time to solve mysteries... it's my turn to hunt down one

Thanks for the input

[Nexxen]

Keyboard lights up but no signs of life after all LEDs lighting up at power on.
Numlock doesn't work.


My next step is to remove all the sockets and check for broken traces.

[Nexxen]

Latest news:

- RESET signal is asserteed both at power on and off.

- removing keyboard controller and RTC and BIOS chips (or any combination) makes no difference with the POST card:

reset lights on at power on and at power off;
FRAME is lit;
CLK is lit;
IRDY is always off

The keyboard controller that came with the board is dead, Fujitsu, Holtek and VIA make the 8MHz oscillator work, albeit @4.45V


My next step:

desolder all sockets and check for broken traces

Before I desoldered the keyboard controller and the RTC, Reset was stuck and POST card was stuck at 02 03 05 06 07 08 09 codes.
Cpu is good and working.


Any suggestions for a good desoldering gun? I have a hand pump but it doesn't work that well.

[Shock]

Hakko FR-301 is about the entry level point for something reliable.

[Nexxen]

Is it correct to be around 150€?

I discovered that two74F245N have +0V on OE, no voltage on output enable. Being on or off.
Are these broken chips?

https://pdf1.alldatasheet.com/datasheet-pdf/view/27848/TI/SN74F245N.html

Edit: I think it is correct as OE is controlled by the cpu, +5 is issued to direct this  74.

[Nexxen]

I removed all the sockets and dips.

I already have 5-6 microcracks. One is the pin 10 of 8042: high WR.
Connects to chipset #97 /IOW. But the other signal to keyboard wasn't.

IDK

I have a ton of stuff to check for continuity.
Also keyb pin 39 connects to this /IOW...

[Shock]

Is it correct to be around 150€?

I thought normally a bit more expensive than that, check the mains voltage it comes with (they are sold worldwide).