Microstar 486 motherboard repair

[roadrash]

Hi everyone, I wonder if you could help me with repairing a old 486 pc that this year started giving 3 beeps at startup. 3 beeps i understand is a ram error. I checked everything and was about to scrap it when I remember at one time its cmos battery leaked over the pcb. I was wondering if maybe there was still some acid under the ram slots so I took them off and I found a tiny bit and so far found one bad contact between a pcb track and a smd component. I also found smd components had fallen off. Trouble is they are missing but wonder could you tell me what they are. There are others on the board that are identical if you can identify them in the 1st pic if you look between where the ran slots were you will see BC4. What is it? In the other picture you will see some bits with labels starting TC1 TC2 etc what are these too?

[SluggerB]

Re: Microstar 486 motherboard repair

Looking at https://en.wikipedia.org/wiki/Reference_designator I don't see "BC" listed. I would assume being in between RAM slots is a capacitor ("C"). Measure it and try replacing the missing spots with those.

TC is listed as "thermocouple", I would assume it's part of the temperature sensors on the board to prevent overheating

[RJARRRPCGP]

Re: Microstar 486 motherboard repair

I would assume it's part of the temperature sensors on the board to prevent overheating

Usually, no such thing as temperature sensors for anything pre-Pentium II or Pentium Pro!

[stj]

Re: Microstar 486 motherboard repair

nicad strike!
the caps are probably .1uf but you really need to pull the floppy socket etc and check that area!!

[eccerr0r]

Re: Microstar 486 motherboard repair

TC8 is a capacitor?

[Topcat]

Re: Microstar 486 motherboard repair

How the hell did you get the memory slots out of that board in one piece?!

[stj]

Re: Microstar 486 motherboard repair

electric desoldering pump can do that - i'v salvaged pci slots that way

[roadrash]

Re: Microstar 486 motherboard repair

How the hell did you get the memory slots out of that board in one piece?!

Thanks for all that info guys, getting the memory slots off is not easy but you definitely need a tiny desolder pump like my tiny Abec one and watch out as those sockets are made up of lots of tiny square plates with a single pin on and they can fall out and almost impossible to see on the bench of floor and a magnet dont pick em up off the floor. Ive lost a few.

[roadrash]

Re: Microstar 486 motherboard repair

electric desoldering pump can do that - i'v salvaged pci slots that way

Hi STJ its computer motherboard repairs this time.

Ive not been able to get a capacitor readings from my meter and had to use my multimeter x1k diode scale to get a reading from any of the smd components. For example BC4 gives 352k one way and with probes reversed i get 432k. One of the other parts i thought was a resistor also gave different readings both was ie: TC1 also gave 432k & 353k. This makes me think these are either diodes or maybe electro caps. C21 also gives dual readings of 755k & 866k. These are just examples and all the smd components are either grey or brown coloured with no markings or green coloured with a value marked on them which I gather are resistors.
So how do i find the value of say BC4 which is the same as the one between the momory slots that had fsllen off and gone missing. Ive ordered myself a dedicated capacitor and ESR meter to see if that can get a better reading. Here is a pic of BC4.

[stj]

Re: Microstar 486 motherboard repair

BC are caps - probably .1uf given the age.

TC may be inductors

[momaka]

Re: Microstar 486 motherboard repair

I think both BC and TC are capacitors.

The "B" and "T" pre-fix in front of the "C" is probably just for specifying its use in the circuit diagram for whoever designed this. I've seen a similar scheme on LCD monitor boards and video cards, where sometimes caps would be referenced with "CE" or just "C"... and sometimes, there would be a pre-fix to the "C" like this case. I don't know if I have any pictures that show it, but I'll dig around in my collection, if I can even remember which monitor board had these kind of weird designator naming.

In any case, just replace these with ceramic caps. 0.1 uF might work, but 1 uF should work too for the power filtering ones.

How the hell did you get the memory slots out of that board in one piece?!

I know, right!

I've pulled PCI slots out (that AMD Kadoka motherboard in a Gateway Select PC I found a while back, if you remember), but memory slots... never. Perhaps because this is an old motherboard, it doesn't have such thick ground and power planes as newer motherboards. Now I'd like to see someone remove a memory slot on a current gen mobo without damage. Probably still possible, but likely will be much much harder.

[roadrash]

Re: Microstar 486 motherboard repair

I think both BC and TC are capacitors.

The "B" and "T" pre-fix in front of the "C" is probably just for specifying its use in the circuit diagram for whoever designed this. I've seen a similar scheme on LCD monitor boards and video cards, where sometimes caps would be referenced with "CE" or just "C"... and sometimes, there would be a pre-fix to the "C" like this case. I don't know if I have any pictures that show it, but I'll dig around in my collection, if I can even remember which monitor board had these kind of weird designator naming.

In any case, just replace these with ceramic caps. 0.1 uF might work, but 1 uF should work too for the power filtering ones.


I know, right!

I've pulled PCI slots out (that AMD Kadoka motherboard in a Gateway Select PC I found a while back, if you remember), but memory slots... never. Perhaps because this is an old motherboard, it doesn't have such thick ground and power planes as newer motherboards. Now I'd like to see someone remove a memory slot on a current gen mobo without damage. Probably still possible, but likely will be much much harder.

BC are caps - probably .1uf given the age.

TC may be inductors

Thanks everyone for this useful info which is greatly appreciated , cheers.
I was wondering, I thoght I saw 2 little bits fall of and its obvious BC1 was one but if there were 2 bits what was the other? All the other pads dont look like they have been used except TC5 which is strange as it has no pads for anything to solder to. Its possible it might have completely fallen off in one lump I suppose. So what do you think should i replace it? Checking with meter it seems the stud (if thats what its called) next to where the pad was has contact to the memery slot hole closest to it and just below to the right. Is this a part I should also replace or is it not nessesary or can be left out without any effect? See photo.

[stj]

Re: Microstar 486 motherboard repair

unless your overclocking, you probably dont need all the .1uf caps.
they only add stability anyway.

what does the board actually do?

[eccerr0r]

Re: Microstar 486 motherboard repair

I hate the dreaded beep beep beep ... reminds me of this old 286 board I had. Was really intermittent, sometimes it'd work, frequently not. Was a RAM issue but since it was intermittent, it probably was a broken trace somewhere... finding where is the question.

The board ended up finding the round file.

[stj]

Re: Microstar 486 motherboard repair

on those old boards, you need to make sure the plating on the ram matches the sockets.
gold on gold, or tin on tin.
if you mix them you get a galvanic reaction and you have to clean the black coating off your simms every month or 3.

[RJARRRPCGP]

Re: Microstar 486 motherboard repair

I hate the dreaded beep beep beep ... reminds me of this old 286 board I had. Was really intermittent, sometimes it'd work, frequently not. Was a RAM issue but since it was intermittent, it probably was a broken trace somewhere... finding where is the question.

The board ended up finding the round file.

LOL the same here with a socket 775 motherboard, I upgraded the RAM and then was met with "bleeeep-bleep-bleep". Meant it was reseat-the-RAM-time.

[roadrash]

Re: Microstar 486 motherboard repair

on those old boards, you need to make sure the plating on the ram matches the sockets.
gold on gold, or tin on tin.
if you mix them you get a galvanic reaction and you have to clean the black coating off your simms every month or 3.

Thats a new one on me stj. I did give all the contacts a clean with alchohol and it did remove a lot of tarnish but still had the beep beep beep. I took the ram out and tried it in another desktop and it was working fine so not faulty ram which is why I removed ram slots especially as I spotted the green battery marks on things.
I converted it to a 3x AAA battery pack instead of the pcb battery so wont happen again if I can fix the 3 beeps.
Its a good motherboard a d has ISA and PCI slots and takes the x586 cyrix processor.
If any of these capacitors are missing does affect the ram modules by letting in noise etc. Would these caps falling of cause the 3 beeps?

[stj]

Re: Microstar 486 motherboard repair

the problem with AAA is if you use anything other than eneloops they will probably leak when you arent looking!

[momaka]

Re: Microstar 486 motherboard repair

unless your overclocking, you probably dont need all the .1uf caps.
they only add stability anyway.

But that's the problem here... these old boards don't have so many numerous small ceramics like modern boards do, and they don't have a whole lot of electrolytic caps for capacitance either. So even with just a few missing caps, you might already be hitting stability problems. Better to have more capacitance and filtering than needed than have less.

Again, I'd just go with something more standard, like 1-4.7 uF 16-25V ceramic caps and call it a day. If you have any spare broken GPUs or laptop motherboards you can use for parts, just grab some of the ceramic caps from there.

Also, even if you happen to replace an SMD inductor with an SMD ceramic cap, the worst that could happen is th board will not POST or work. But on the other hand, if you replace an SMD ceramic cap with SMD inductor by mistake, you'll likely create a short-circuit from a power rail to ground, and that can burn traces. So when it doubt, just use a ceramic cap for everything... that is, if you don't feel like doing it the more proper way and checking where each trace goes to determine what the part should be.

on those old boards, you need to make sure the plating on the ram matches the sockets.
gold on gold, or tin on tin.
if you mix them you get a galvanic reaction and you have to clean the black coating off your simms every month or 3.

Which makes me wonder... what if we plate both contacts with leaded solder? Can that be used to solve plating mismatch? (Granted in this case, it would be a ridiculous idea, given how hard it can be to get solder inside on the contacts of the memory slots, lol.)

the problem with AAA is if you use anything other than eneloops they will probably leak when you arent looking!

+1

Unless he keeps them with long wires away from the board. Then they can leak (and they will) all they want.

[roadrash]

Re: Microstar 486 motherboard repair

Unless he keeps them with long wires away from the board. Then they can leak (and they will) all they want.

Thats what I do. I either put AAA's in a battery holder/pack usualy stuck to the back of the computer or sometimes put a coin battery holder. I have to do this for some really old computers that use a Dallas RTC/CMOS chip that.has cmos battery combined with the RTC chip. There is a tutotial online that shows just the right places to grind into to get to the chips internal battery connections and then just stick a coin battery in a holder on top of the chip. Google it.
Done a few of these.