Ancient Atari 800 repair - Page 1
Forum section: electronics / repair

author: spinnaker
author_rank: Regular Contributor
timestamp: April 10, 2019, 12:50:32 am
content: I am looking at getting my 30+ year old Atari 800 up and running. Still waiting on my video connectors but I am pretty certain I am getting composite video out an I get keyboard clack sounds out of the internal keyboard so I am reasonable certain the CPU is alive.Filter caps don't appear to be leaking but one problem I noticed was the 5V is a little low. I am reading 4.9V. Could bad caps be a cause of the low voltage?Also how does the composite signal look? Does it seem OK? Been awhile since I worked in video.

author: TK
author_rank: Super Contributor
timestamp: April 10, 2019, 01:00:06 am
content: Video signal looks OK and 4.9V is more than OK for a 5V supply

author: spinnaker
author_rank: Regular Contributor
timestamp: April 10, 2019, 02:03:23 am
content: Thanks. What about those 30 year old electrolytics? Replace now? Assuming the rest of the computer is working.

author: TK
author_rank: Super Contributor
timestamp: April 10, 2019, 12:36:54 pm
content: If you don't see any leaking or bulging capacitor, there is no need to change them. I have several vintage 8-bit computers (Apple, TI 99/4a, commodore) and never replaced any capacitor.Tantalum capacitors (yellow ones) can blow when they age, but you will hear a loud bang and see the burned one immediately.

author: spinnaker
author_rank: Regular Contributor
timestamp: April 10, 2019, 10:35:24 pm
content: Got my RF connector in today. It's alive!A little buzzing but as I recall that is normal with text, Waiting on the 5 pin DIN, hopefully with even better results.

author: CJay
author_rank: Super Contributor
timestamp: April 11, 2019, 05:45:02 am
content: Quote from: TK on April 10, 2019, 12:36:54 pmThat is not good advice.I've had many repairs with short circuit, open circuit or leaky electrolytics where the capacitor looked absolutely perfect to the eye, they don't always bulge, in fact in over 30 years of repairing 'stuff' I think I've probably seen more that didn't bulge than did.Similarly tantalums can and will go short with no sign of burning let alone an explosion, the only advantage of that is the power rail the tantalum has shorted will show significant voltage drop if the PSU isn't capable of driving many amps into it.If the PSU is capable of that then it's a toss up if the tracks, psu, tantalum or all burn

author: spinnaker
author_rank: Regular Contributor
timestamp: April 12, 2019, 02:32:20 am
content: Well I was a little nervous about replacing the caps on a working system. The radials are easy to replace. I don't think I am even going to have to desolder those. I will just clip the leads and solder the new ones in place. Yeah not professional but the least invasive.The two axials and the tantalums will be slightly more challenging. So not to risk damaging a working board, I just purchased a working replacement. It has the old caps too but I got it cheap. Figure I could always sell the other refurbished or keep it as a backup.

author: 6PTsocket
author_rank: Regular Contributor
timestamp: April 12, 2019, 03:08:58 am
content: Quote from: spinnaker on April 12, 2019, 02:32:20 amThe standing rule where we maintained tons of electronic equipment was: If it works, don't **** with it.Sent from my SM-G900V using Tapatalk

author: CJay
author_rank: Super Contributor
timestamp: April 12, 2019, 10:41:49 am
content: Quote from: spinnaker on April 12, 2019, 02:32:20 amAbsolutely, I wouldn't mess about if I weren't sure I could do it.My point was that the advice given about electrolytics only being faulty if bulging or tantalums always going bang was bad advice, many faults are caused by completely 'innocent' looking parts.

author: spinnaker
author_rank: Regular Contributor
timestamp: April 12, 2019, 10:53:27 am
content: So then should I just pack the backup away and don't mess with it either? I mean I am not exactly sure I even want to mess with disconnecting the current one. Or should maybe I refurb the new one in the event it is needed?Could a bad cap cause the rest of the system to fry or would it just take out eh cpu?