Look up 'tantrum capacitors' or correct name 'tantalum capacitors' and their propensity to blow up.

then just cut the offending part off, because it is likely to be causing a short on a power line, stopping the power supply from working properly, and test the mainboard again, and expect some other tantrum capacitor to blow.

and don't worry too much about it.
edit: that squeezed wire, is that white or yellow? either way it shouldn't matter for an old 386.

weedeewee wrote on 2024-02-23, 10:29:

then just cut the offending part off, because it is likely to be causing a short on a power line, stopping the power supply from working properly,

I don't really know what do you mean sorry... Cut the burnt yellow "component" and test motherboard again?
The component burnt is on the riser card, I have took apart the riser card and also no luck booting up.

and don't worry too much about it.
edit: that squeezed wire, is that white or yellow? either way it shouldn't matter for an old 386.

The wire is yellow.

Thankyou
Gus

zzgus wrote on 2024-02-23, 10:40:

I don't really know what do you mean sorry... Cut the burnt yellow "component" and test motherboard again? The component burnt […]
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weedeewee wrote on 2024-02-23, 10:29:

then just cut the offending part off, because it is likely to be causing a short on a power line, stopping the power supply from working properly,

I don't really know what do you mean sorry... Cut the burnt yellow "component" and test motherboard again?
The component burnt is on the riser card, I have took apart the riser card and also no luck booting up.

and don't worry too much about it.
edit: that squeezed wire, is that white or yellow? either way it shouldn't matter for an old 386.

The wire is yellow.

Thankyou
Gus

You can just cut the burned capacitor off from the board or de-solder it then boot the system to check its working, I believe the tantalum its only there for filtering and as you can see there's 2 other in parallel with the blown one.

Cut the burnt component off of the riser board. The one you pointed an arrow to on the photos you posted.

You say the mainboard without the riser is still not giving you anything.
Does the power supply come on and can you measure the voltages? you should find +12v, +5v, -5v & -12v on the power supply mainboard connector.

weedeewee wrote on 2024-02-23, 10:53:

Does the power supply come on and can you measure the voltages? you should find +12v, +5v, -5v & -12v on the power supply mainboard connector.

Yes I can see the fan spin... I'll measure the voltages.

Thankyou
Gus

After testing the voltages all of they are correct.

Have re-attached the power supply and it does boot !!!

Will replace the "component" burnt of the riser and try again.

Thankyou
Gus

BTW what's this ?

You dont even have to replace it, in the eighties there was no power analysis software used when designing motherboards, engineers just sprinkled capacitors on the whim/hunch.

zzgus wrote on 2024-02-23, 17:11:

BTW what's this ?

It's a 'tantrum' capacitor (tantalum) and as rasz_pl said, you don't have to replace it.

zzgus wrote on 2024-02-23, 17:11:

BTW what's this ?

It appears to be a 4.7uF 16v tantalum capacitor. Readily available:

https://www.mouser.co.uk/c/passive-components … g%20dc=16%20VDC

But, as said, they are not always essential components and were often fitted to "cross the t's and dot the i's" - i.e. belts and braces approach. They are often doubled up and scattered around the board seemingly at random.

As you've found out, unlike normal electrolytic capacitors which usually don't go bang (but instead leak, and puke their conductive/corrosive guts up), tantalums tend to go BANG, and sometimes fail in a shorted state.

Thankyou all !!!

Gus

megatron-uk wrote on 2024-02-23, 17:26:

unlike normal electrolytic capacitors which usually don't go bang

You haven't tried hard enough 😀 They do go bang too, and tend to leave quite a mess when they do. Though it is very rare for low voltage electrolytics to go bang with nominal voltage applied.

It looks like that yellow wire was pinched.
Is the wire exposed? if not, don't worry about it.

I serviced a few TCX17 power supplies, can you measure the 5 and 12 V rails during a cold boot? Sometimes they are far too low. When warmed up, they become within tolerance range.

Deunan wrote on 2024-02-23, 17:49:

megatron-uk wrote on 2024-02-23, 17:26:

unlike normal electrolytic capacitors which usually don't go bang

You haven't tried hard enough 😀 They do go bang too, and tend to leave quite a mess when they do. Though it is very rare for low voltage electrolytics to go bang with nominal voltage applied.

Tell me about it. A wrongly wired PSU on a freshly recapped Sharp X68000 (courtesy of a mistranslated wiring diagram and swapped +12v / -12v colours on the schematic) and I had 3 or 4 cans blow up and launch their innards across the office.

Motherboard ended up toast. That was an expensive failure.

Smelled great, as well!

It is a Tantalum capacitor (value = 4.7uF/16V). It is common for these to go bang with time. Nothing to worry, most of the time they just fail this way but don't take anything with them. So just replace the capacitor (or just remove it completely) and the board should start right up. Also it would be best to replace them all with electrolytic ones. Cheers!

rasz_pl wrote on 2024-02-23, 17:18:

You dont even have to replace it, in the eighties there was no power analysis software used when designing motherboards, engineers just sprinkled capacitors on the whim/hunch.

but there already was a thing called oscilloscope... and this thing had the ability to show noise on power rail, you know 😀
so it's still a good idea to replace it. but i admit it could probably live without it with no big issues.

Thankyou all !!! great community.

Will replace the 4 capacitors.

Thankyou all !!!
Gus

Deunan wrote on 2024-02-23, 17:49:

megatron-uk wrote on 2024-02-23, 17:26:

unlike normal electrolytic capacitors which usually don't go bang

You haven't tried hard enough 😀 They do go bang too, and tend to leave quite a mess when they do. Though it is very rare for low voltage electrolytics to go bang with nominal voltage applied.

A capacitor burst on one of my Acer USB cards, spraying molten gunk over the neighboring card, a Highpoint RAID card, also killing it.