VOGONS


First post, by majestyk

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From my heep of unrepaired mainboards I recently pulled a FIC 486 SC-P that´s based on the "Symphony Haydn" chipset.
I ran some tests and to my surprise everything seemed to be fine. But after a while the thing started acting absolutely crazy. It either wouldn´t POST at all or give some random AMI BIOS beepcode like defective RAM, missing video adaptor, defective video RAM, CMOS read error and many more- some of those might even not exist. The next day all was fine again.
During some further troubleshooting with coolant spray I found the culprit: As soon as the chip that handles all the peripheral stuff like RAM and ISA-bus, a "SL82C362", was cooled before startup, the system worked in about 95% of the cases.
First I checked each and every pin of this chip but all solderings were still in perfect shape. So replacing the chip it is!
This is the original situation:

SL82C362in.JPG
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My usual "strategy" is as follows:
1. cut all the pins where they come out of the chip with a Dremel.
2. carefully remove the remaining ends of the pins with a regular soldering station
3. clean all the flux and dirt carefully

Here´s the mainboard after I did this:

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and here´s the old chip:

SL82C362cut.JPG
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4. carefully position the new chip and solder all pins the oldschool way.

Problem in this case is I cannot find a new replacement for this chip today so I will have to use a chip from a donor-board. I´m quite sure I have some damaged Haydn board in the basement.
But how do I safely and carefully remove the chip without damaging it by overheating or mechanically?? I have never done that before because I was always able to find new chips for replacement or I had to deal with far smaller QFPs thad could be desoldered easily.
Any tips welcome here!

Last edited by majestyk on 2022-05-01, 08:30. Edited 1 time in total.

Reply 1 of 16, by TrashPanda

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Flux the chip up then heat it up with a hot air station using a circular motion making sure to only use enough heat to let the chip come loose. (I wouldn't be removing a chip like this with a soldering iron)

If you are careful with the hot air station then it shouldn't get excessively hot.

Oh noes, the cap let the shmooo out 😁

Reply 3 of 16, by keropi

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oh that takes me back... I used to do the same in the 2000s
but like others said, hot air is the best way
even a cheap station will do wonders in cases like this

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Reply 4 of 16, by TrashPanda

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rasz_pl wrote on 2022-05-01, 08:48:

+heat from below

Depending on how many ground planes the board has this may not do anything or take an excessive amount of heat.

Oh noes, the cap let the shmooo out 😁

Reply 5 of 16, by vstrakh

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TrashPanda wrote on 2022-05-01, 09:22:

may not do anything or take an excessive amount of heat.

Why? The board definitely will dissipate some heat, making hot air gun less efficient, and tempting user to raise the air temperature.
And if for bottom heat you use something that's not too powerful (I was using a spare heatbed for 3d printers), it just slowly preheats everything to a safe ~100 deg. The air gun would then add just enough heat to melt the solder, and overall thermal stresses would be minimal.

Reply 6 of 16, by TheMobRules

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In my experience these chips are usually pretty resilient, you really need to go overboard with the hot air gun in order to damage them. Keep in mind these are designed to survive the factory soldering process where they are basically baked in an oven for a few minutes. As long as you keep the hot air gun moving around the chip to heat the legs evenly it has a very good chance of surviving.

Recently I desoldered a couple of S3 Trio64V+ chips from donor cards for the custom VLB card project by Madao and they both survived the ordeal. I just pre-heated the donor card around the chip, then set the rework station to 350C and after a while the chip came out without damaging any traces. Just make sure you don't use any force when removing it, although you may not care about the pads in the donor board surviving, you may also bend or even break the legs on the chip. It's better to wait a while until the solder on the joints is fully melted.

Reply 7 of 16, by BitWrangler

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How much do you like the donor board? Cutting out the chip from the board leaves you a lot less ground plane pain. ... and if there's nothing else of value left on the board, taking a blowtorch to the backside and banging on it until the chip falls off can work.

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Reply 8 of 16, by Shponglefan

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As others have said, flux + hot air station is the way to remove SMD chips like the above. Plus can also be used to re-solder them more efficiently than an iron.

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Reply 9 of 16, by majestyk

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Thanks to everybody for your help!
I will dig out the donor-board as soon as I have some spare time and check it´s condition. It would be useful to do some basic tests to make sure the chip is in good working condition - this means to get the donor-board running first.
I will report here as soon as I have any news.

Reply 10 of 16, by whaka

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not more than others said 😀
BUT... a safely thing you can do, is put the donor card in oven for 1 or 2 hours at 50/60°C before desoldering.
why ? because of moisture. strongly heating old chip that have been in storage for long can have absorb moisture.
and when you strongly heat them, mositure turn very quickly into steam and... can go out of the chip like a gun bullet... and potentially damage it.

Reply 11 of 16, by majestyk

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O.k. - yesterday I got hold of the donor-board. The BIOS chip was missing, so I gave it a try with the Symphony BIOS of my FIC 486SC-P. It started, but cache and RAM were not recognized correctly. I was unable to find an original BIOS online and now I knew the donor-chip was working so I decided to start the slaughter.

First I drilled 4 holes from the upside to mark the area to be cut out without damaging any legs of the chip. Then, 4 quick cuts later the chip dropped out:

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Before, I made two diagonal cuts also to split the area in 4 quarters and each row of pins could be easily removed after heating the pins carefully with the hot-air station.
This is what I got:

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There was very little heat needed for a short time only and no leg was bent!
Then all the pins had to be cleaned carefully and I applied a little fresh solder to all pins.
Finally the transplantation could take place and the result is as good as if I had used a new chip:

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So far the system is working flawlessly. But I will have run tests for a while to make sure all issues have been solved.
For the first time the L2 cache waitstate can be jumpered for 0 ws now. This had resulted in loads of errors before.

Reply 14 of 16, by 386SX

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I never even tried desoldering a single pin like those on a Am386DX-40 to disable it while people can replace them entirely.. I wish I had more soldering skills and tools. 🙁 😁
Not to mention to change modern DDR3 modules on a OrangePi SBC to increase ram size.. I'd like to try that but I suppose it's too much difficult.

Reply 15 of 16, by majestyk

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maxtherabbit wrote on 2022-05-18, 14:38:

so you took a fully working board and cut it to pieces to fix another one? seems dumb

Not necessarily, if you´re 100% sure which one you want to keep and if you have been searching for a new (NOS) replacement chip for 5 months with zero success.
Unfortunately this chipset is quite rare and there are just a few mainboards using it.
And I think this beauty is worth it:

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Still wondering what to do with the donor board - maybe I´ll put it up on Ebay as "not tested" for $ 300.00 😉

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Or I could harvest the KB-connector, 8 SIMM sockets, the power-connector, the other two Symphony chips and 8 ISA sockets.

Reply 16 of 16, by maxtherabbit

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majestyk wrote on 2022-05-18, 15:34:
Not necessarily, if you´re 100% sure which one you want to keep and if you have been searching for a new (NOS) replacement chip […]
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maxtherabbit wrote on 2022-05-18, 14:38:

so you took a fully working board and cut it to pieces to fix another one? seems dumb

Not necessarily, if you´re 100% sure which one you want to keep and if you have been searching for a new (NOS) replacement chip for 5 months with zero success.
Unfortunately this chipset is quite rare and there are just a few mainboards using it.
And I think this beauty is worth it:

486SC-p_tot1.JPG
Still wondering what to do with the donor board - maybe I´ll put it up on Ebay as "not tested" for $ 300.00 😉

dead_dead_good.JPG

Or I could harvest the KB-connector, 8 SIMM sockets, the power-connector, the other two Symphony chips and 8 ISA sockets.

it's your property and you're free to do what you want of course, but still seems ridiculous to me

why not wait until a board ravaged with battery damage comes along to lift a replacement from? or simply remove the chip using chipquik so as not to destroy a perfectly functional PCB? just so many better options

(I have a young micro MB with the SYM461 chipset, they are cool I agree)