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Let's restore my 486 motherboard - AOpen Vi11

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Reply 20 of 87, by feipoa

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There should be some general guidelines or calculations for what wire guage to use for what current and for run lengths at DC. Generally, though, you can get some estimate for the thickness of the traces, their lengths, and try to at least use equal wire cross sectional area.

For AC, I think there are also guidelines for current capacity and run length based on operating frequency. I remember something called the skin effect which states that the majority of current runs along the surface of the conductor, and the amount is a factor of the frequency. Maybe this is where the benefit if stranded wire comes in? I haven't really researched this much recently.

Concerning the inductors - I cannot say for certain because I just formulated this theory and have begun implementing it. I replaced the 4 uH keyboard inductors on my board with 2.2 uH and didn't notice any issue. The fact that the board manufactures go the extra step (sometimes) to put 1 uH for the mouse and 4 uH for the keyboard does have me curious. I have noticed, though, that many motherboard manufacturers merely follow the circuit implementation example provided by the manufacturer of some device, e.g. the keyboard controller, voltage regulator datasheet, etc.

Plan your life wisely, you'll be dead before you know it.

Reply 21 of 87, by CkRtech

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Desolderfest 2017 is (most likely) complete.

This image gives a pretty good idea of the total surface area that the battery leakage covered. The board has been scrubbed several times and therefore doesn't show as much, but the trace damage remains.

My fiberglass brush ran out. Refills are on the way. I still have some small green areas to scrub off. I have also done some partial work on adding fresh solder to bad/oxidized areas and then removing it in order to create a cleaner joint for when the components are replaced. There are a few holes that still have some of the old solder in them.

If anyone attempts to do a repair like this one, I highly recommend removing (and if possible, replacing) as many of the components in the affected areas as possible. This will help you see the PCB, scrub it, and get a good idea of how to route repairs. I am very glad to have this component side photo to go along with the solder side photo when it comes to trace repair. A cleared board photo will definitely come in handy when adding components back along with jumper wires.

EDIT: I lied. I might pull another resistor and cap just North of the RAM banks.

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Reply 22 of 87, by CkRtech

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feipoa - Also from Ian.M regarding corrosion, the components, and notably the inductors we have talked about -

If you can ID and obtain an equivalent component, even if you have to pull it from your pile of scrap boards - *REPLACE*. This is especially important for small inductors due to the fine wire inside them.

Somewhat confirms your concerns regarding those inductors.

I wish I had more data - like a datasheet with typical circuits - for the MEGATRENDS MEGA-KB-H-WP keyboard controller. I could probably just steal the values that I need off of that. The parts 386 board I have uses an NEC D8041AHC chip, one less inductor, and potentially a different crystal. There is a datasheet available for that (and it is rather thin), but I don't feel like I can make assumptions from it.

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Reply 23 of 87, by feipoa

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OK. I guess because of their thin wire gauge they get get open circuited pretty quickly. I'm wondering, though, if those black cased inductors are sealed in anyway. They almost look like there is an opening right into the canister due to how it funnels inwards. So I would imagine the battery juice gets in quite readily and stays in.

I assume the battery juice only runs along the surface of the conductor, so for sealed IC's, like an inverter pack, can the corrosion makes its way inside to the various transistor configurations? If so, then no matter how much cleaning we do, the damage is already done and the components must be replaced.

Plan your life wisely, you'll be dead before you know it.

Reply 24 of 87, by CkRtech

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Did you ever get any schematics for any other 486? I saw that you were asking in a thread a few years back, I think.

I would like to confirm the crystal value (which could be 32khz and be different depending on chips used, clock dividers, etc). I guess I will throw in inductors from another couple of boards and assume they will work. That leaves the various diodes - I am not familiar with diode values, etc. Other components like resistors, pins, jumpers, hex inverters I should just have available or be able to pluck easily off something else. That leaves only the keyboard ic as a possible ic to replace.

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Reply 25 of 87, by feipoa

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Did you look at the work I did when troubleshooting this 386 board? This is the board that I'd really like to redo and cleanup my trace repair.
Date/Time in BIOS counting slowly (seconds)

Anyway, in that thread it was determined that the crystal was 32.768 KHz.

Plan your life wisely, you'll be dead before you know it.

Reply 26 of 87, by CkRtech

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Holy cow. What a read regarding your trouble with that crystal - and the timing thereof. It is going to be a bit of time (thank you, I am here all night) before I reach that point, but your thread did cause me to do some Googling.

I found this to be eye-opening regarding crystals, RTC, and drift: https://www.maximintegrated.com/en/app-notes/index.mvp/id/58

It talks about running fast, running slow, the importance of the ground plane, oscillation potentially stopping due to the extra load of an oscilloscope probe, and *noise* coupling into the signal due to other high frequency signals on the board. I suppose it is possible that your board/components have been altered in such a way that it would cause that fast clock condition, but I also wonder if your jumper wires could pick up some of that noise. If so - man, what a pain.

I suppose further specific discussion on this could be done inside your thread. I kept it here as I will be having to deal with the same thing in the near future. Also note regarding replacement batteries that my manual for this board I am working on says 3V ~ 4.5V for the replacement external battery.

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Reply 27 of 87, by feipoa

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Yeah, that is an interesting application note from Maxim. I try not to obsess myself too much with all the helpful hints out there from IC manufactuers because one can easily run into information overload. Though I did spent 5 minutes reading that link, none-the-less.

In my case, replacing the crystal fixed all the issues, so I don't think there is a problem with my sloppy extra jumper wires.

Right now, I have a UMC 481/482 board which I performed the CR2032 on some time back. I'm noticing that it doesn't keep the time all that well, perhaps off by 3 minutes per month. It is the only 386 board which I have kept a single CR2032 in it (the others I used a double-stacked CR2032). I need to spend some time to trace out the battery circuit to see how many series diodes are use to drop the voltage down and to determine if I can get away with 6 V (double-stacked CR2032) clock backup battery.

Basically, I'm wondering if 6 V is ever too much on these 386/486 boards and will it break something? The lazy path might be to stack a 3 V with a work out 1.5 V battery and forget about the voltage investigation.

Plan your life wisely, you'll be dead before you know it.

Reply 28 of 87, by CkRtech

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Fiberglass refills arrived. LOVE these things. Went to town with vinegar and burned through two full refills of fiberglass. This really helped me get a grip on the conditions of the traces - which ones had surface damage vs total destruction. I had to do a small bit of work on the back as well. These traces will eventually be protected again with some PCB lacquer.

It is stripped bare, but gorgeous compared to what it once was. Upcoming steps include trace removal and repair for -5V (with copper foil, most likely), tinning repair/replacement joints, and resoldering ISA slots + IC socket for keyboard controller.

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Reply 30 of 87, by CkRtech

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I had heard of it but never seen or used one until it was recommended to me for doing some of this work. It is also called a scratch brush. It is a very rigid set of fibers that can be deployed from a pen-like utensil and used for abrasive work like the removal of corrosion. So I suppose it is quite similar to steel wool, but meant for use on a very small area. The description listed on the one I have is literally "Our scratch brushes with pen-shaped handles are used for removing rust or corrosion in hard to reach places."

After using it (plus refilling it twice), I highly recommend having one in your arsenal. I hope I don't have to clean corrosion like this (or worse) again, but I have a feeling it is inevitable.

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Reply 31 of 87, by feipoa

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Never heard of it. Is this it? https://www.amazon.ca/Scratch-Brush-Fiberglas … y/dp/B0019V18D2

And does the wearing out of the fibres make a mess?

Plan your life wisely, you'll be dead before you know it.

Reply 32 of 87, by CkRtech

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Yep. Pretty much. Here is the one I ordered - https://www.amazon.com/gp/product/B0019V18D2/

I had a pool of vinegar over the areas I worked, and I kept the fibers quite short. As they were brushed across the surface, they would disintegrate and cloud-up the vinegar pool. The refills I ordered that were also meant for that particular brush didn't seem to be as high of quality as the one that came with the pen as they would actually fall out of the pen in clumps at times. That being said, the refills got the job done. They only cost $4 more than the pen and came in a pack of 24. I recommend buying refills up front. If you look at earlier photos of the board and compare them to the most recent scrubbed photos, it basically took 3 full pens worth of fibers to get it to this point.

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Reply 33 of 87, by CkRtech

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Progress update as I await an order of parts -

1: 16 bit ISA resoldered. 8 bit ISA replaced from 386 donor
2: Resistors R4, R5, R6, R13 replaced with new components
3: J1 (External battery) and JP16 (Clear CMOS) pin headers replaced with 386 donor
4: Capacitors C3, C4, C13, BC1 replaced with donors from 386
5: Crystal Y1 replaced with donor from 386
6: Socket added for keyboard controller IC
7: Inductors L1, L2, L3, L4 replaced with donors from 386
8: AT keyboard connector replaced with donor from 386
9: 486DX BIOS placed back in board since all scrubbing is complete.

Acrylic conformal coating is here. Will complete wire-based trace repair prior to applying. A large volume of continuity testing is in my future!

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Reply 34 of 87, by feipoa

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"Acrylic conformal coating" - website/part number? Is this intended for PCBs specifically, or is it generic?

Plan your life wisely, you'll be dead before you know it.

Reply 35 of 87, by CkRtech

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EDIT: If anyone decides to use conformal coating on a project, please read the safety warnings.

https://www.amazon.com/MG-Chemicals-Acrylic-L … /dp/B008OA77NG/

Last edited by CkRtech on 2017-04-21, 01:32. Edited 1 time in total.

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Reply 36 of 87, by SSTV2

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CkRtech wrote:
Progress update as I await an order of parts - […]
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Progress update as I await an order of parts -

1: 16 bit ISA resoldered. 8 bit ISA replaced from 386 donor
2: Resistors R4, R5, R6, R13 replaced with new components
3: J1 (External battery) and JP16 (Clear CMOS) pin headers replaced with 386 donor
4: Capacitors C3, C4, C13, BC1 replaced with donors from 386
5: Crystal Y1 replaced with donor from 386
6: Socket added for keyboard controller IC
7: Inductors L1, L2, L3, L4 replaced with donors from 386
8: AT keyboard connector replaced with donor from 386

How bad that 386 MB had to be, compared with this motherboard, that you had to use it as a donor? I can't even get my hands on a dead one!

Reply 37 of 87, by CkRtech

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Not too bad. It did have a leaky battery, but I wanted this 486 board to work more than I needed the 386 board to function. I can order replacement parts for the 386 in the future if I want it to become a project, but I do have other 386 boards that are more likely candidates for a build.

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Reply 38 of 87, by feipoa

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CkRtech wrote:

Looks like something I could use.

Why is everything free shipping in the USA and how do they get away with this practice from so many companies, e.g. eBay, Amazon, etc? Why doesn't Canada have free shipping so frequently like this?

Plan your life wisely, you'll be dead before you know it.

Reply 39 of 87, by CkRtech

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Success.

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