Reviving some 80's electronics - Page 1
Forum section: electronics / beginners

author: PhilA
author_rank: Newbie
timestamp: April 20, 2013, 06:57:27 pm
content: Howdy, all. First post here. Recently came upon a stash of computer goodies from the 80's at work, via a colleague.I've inherited various electronics in the past, but most have been stored in either a temperate climate or climate controlled areas. These have been stored in the humid subtropics of coastal LouisianaIMAG1113byrenault9gta, on FlickrFirst off, a TRS-80 model 3. First thing that colleague wanted to do was power it up. first thing I did was take it apart and point at the capacitors connected across the mains input that were likely to go bang. That's currently in a state of non-assembly awaiting caps for the power supplies.Had a couple of not-so-interesting laptops that either did nothing or booted up. The did-nothing ones were inadequately interesting to even bother with, plus, when the paint is peeling off the case and it smells of cat wee... I'm not going to bother.Next two were halfway written off by me as soon as I saw them. A Compaq Portable 386 (the small beige case, bigger than a shoebox with the keyboard that clips into the front and orange gas-plasma display) and an original Compaq Portable "Luggable".Those two came home in the back of my pickup truck (filthy!) and were subjected to "let's plug them in outside and see which one produces the bigger bang" because they had both been stored outside in a garage for about a decade.They both went bang and let out a not-inconsiderable amount of smoke. the larger of the two, after some thought, was brought inside and stripped down. The little 386 sat outside, in the rain, overnight.Threw the case off the Portable and deposited it into the dishwasher. Stripped the boards down and discovered the source of the pyrotechnics.Pop!byrenault9gta, on FlickrSeveral 10uF 16V tantalum capacitors, connected across various power rails went pop. Had a dig about, decided that seeing as the board actually powered up off a spare "known good" PSU without any further signs of distress they were ok to replace with aluminium can wet electrolytics. Plugged it in and got a series of POST beeps. Grabbed the video card, which has CGA/Mono output on a 9-pin D-sub connector externally and a row of pins inside to connect to the internal monitor.Noticed that it also had a composite output on the card. After replacing a few bad caps there, the board came up, plugged into a TV set.Compaq PC first signs of lifebyrenault9gta, on FlickrIt's not ground bonded, it's missing two of the +5v stabilised rails but that's definitely signs of life right there.Replaced yet more exploded tantalums on the power supply, carefully reassembled everything the powered up just the motherboard and VDU board.Compaq portable is alivebyrenault9gta, on FlickrOk, so that's what the message on the TV set was meant to be.Found yet more bad caps on the floppy drive controller board, got that in along with a the floppy drives. Hard disk required several thumps to free up the read arm... it booted up into Compaq PC DOS 3.10 right there.At this point I looked outside, the following day and the 386 is still sat beside my mailbox in the verge. I headed out with a screwdriver, dismantled it out on the tailgate of my truck, liberated as many of the earwigs and ants as I could and brought it in out of the elements.After another capacitor hunt-and-replace from the pile of new spares here:10uF 50V electrolyticbyrenault9gta, on FlickrIMAG1120.jpgbyrenault9gta, on FlickrCame up fairly clean. Interesting form factor. Came with a couple of bonuses- 387 co-processor and RAM upgrade (2Mb!) with a two-ISA-slot expansion "carbuncle" that clips to the back of the case.Compaq 386, alivebyrenault9gta, on FlickrI guess I was too quick to write it off. Even the hard disk tests 100% free from errors, the case and keyboard have cleaned up to looking like new, only need to get a few driver issues sorted for the plasma display (640x400 MDA) but it boots into 3.11 for Workgroups...Fought with the Luggable a bit more last night as the keyboard refused to work. Took it to bits and it's a capacitative keyboard, apparently as used in some Sun computers, Acorn/BBC, Tandy etc. A little silvered Mylar disc attached to a foam pad attached to the underside of the key. Pressing the key brings the metalized disc into proximity with a pair of pads on the keyboard PCB, altering the capacitance, causing the thing to register the keystroke.Fine, but a) the foam has disintegrated and b) all the silver has fallen off the Mylar. Pushing a mylar disc up towards a PCB doesn't do anything! Discovered that touching the pads causes the keyboard to register keystrokes.*deep intake of breath*Well, that was a lot of bamf to read through.Now, I have a question. The RAM expansion/Serial port/real-time clock board has a battery soldered to it, which has caused the surrounding board to be covered in greenish crystals. Plugging it in causes "PARITY CHECK 2" on the screen, so I'm guessing there's some real bad connections on the RAM sockets, which are green.I'm guessing the battery is a NiCd, being 3 cells rated 3.6V - any clue how best to remove this muck?battery_dreckbyrenault9gta, on FlickrI'm not a chemist, but I'm guessing that's a component of copper there- don't know if it's acid or alkaline, what will remove it without dissolving the rest of the board and damaging things..?Thanks for reading, by the way--Phil

author: SeanB
author_rank: Super Contributor
timestamp: April 20, 2013, 07:08:13 pm
content: Dilute white spirit vinegar will neutralise it, but you will have to first clip the battery leads and then wash the board afterwards with clean water, then rinse a few times and dry in a well ventilated area for around 48 hours at 50C or so to remove moisture. Remove any socketed IC's before washing, and note where they go. Clean them the same way and keep in an antistatic bag unti lyou put it back together.Look for broken tracks on the memory side, check continuity on the sockets ( most pins aside from data bus wll be commoned together) and repair them.

author: PhilA
author_rank: Newbie
timestamp: April 20, 2013, 07:44:38 pm
content: SeanB, thanks.It would appear that the lacquer layer on the board has protected the traces that I can see. The RAM chips are all logically identical, so once I find my IC pliers I'll lift them all out into the bag all the boards and bits came home in.I'm going to de-solder the battery from the board and probably run flying leads to a separate battery, zip tied to the chassis somewhere.What dilution vinegar? I have regular grocery-store 5% acidity vinegar. I was thinking probably 1:4 vinegar to water. Too strong?I don't have a high temperature room like that. Best I have is my kitchen oven which has a LOW setting of 160F/70C so that's out of the question, I guess. I could stick it up in one of my central air vents. I've had the heat on lately, and that's a gentle zephyr at about 95F/35C.--Phil

author: PhilA
author_rank: Newbie
timestamp: April 20, 2013, 09:37:40 pm
content: Battery and RAM removedbyrenault9gta, on FlickrGot all the RAM off and in a safe place, de-soldered the battery and got that set aside. Going to have to take care with the liquids because there's a DIP switch on the other side of the board that I think will probably stop working if it gets wet.Battery reads 69mV disconnected, after all this time.--Phil

author: mikes
author_rank: Regular Contributor
timestamp: April 20, 2013, 10:14:31 pm
content: I'd start with isopropyl and a toothbrush.

author: PhilA
author_rank: Newbie
timestamp: April 20, 2013, 10:39:53 pm
content: Quote from: mikes on April 20, 2013, 10:14:31 pmIn the end I used dilute vinegar and Q-tips (cotton buds) to clean up. I then rinsed it with tap water gently, shook it out and stuck it in the oven on low until the oven almost came up to temp, switched it off and let it sit. It's currently drying out.I need a new toothbrush, so when I'm next at the store I'll see about hijacking my old one to give it a bit of a better cleaning, but the large caked up dirt is off. the black dye in the chips has been affected, the plastic casings are now brown!Interstate carry a direct replacement battery. I found a spare one in an old portable phone handset that we don't use any more. 3.6V 800mAh. The original 3.6V 80mAh. That should keep the clock ticking away for a rather long time.--Phil

author: SeanB
author_rank: Super Contributor
timestamp: April 21, 2013, 07:02:29 am
content: Use flying leads to connect it this time, and tuck it in a small plastic bag in a box elsewhere in the chassis. I still have one of those plasma displays here, probably still works.

author: PhilA
author_rank: Newbie
timestamp: April 22, 2013, 02:02:59 am
content: Quote from: SeanB on April 21, 2013, 07:02:29 amI think I've found the place to locate the battery- there's a nice gap up on the outside by the fan which is isolated from any electronics and also accessible without having to crack the case open. Before the days of fully tool-less cases for home electronics, taking it apart is a chore.Pulled down the datasheet for the RAM chips used (http://generalthomas.com/Old_Test_Equipment/Micron4264.pdf) and started tracing the RAM bus for shorts and breaks. Tedious work! 864 bus connections, tracking them back to the chips that drive them.... oiyee!--Phil

author: ptricks
author_rank: Frequent Contributor
timestamp: April 22, 2013, 12:59:37 pm
content: Check the corroded parts for shorts. Older dip packages had an issue where the material for the casing allowed corrosion of the internals.