Leaderboard

A quick write up of a few Ace boards I've been working on being as the workshop is like an Arctic outpost at present.🥶 These ones have caused a lot of headaches due to the fact that they keep failing even after long soak tests and always different faults, no doubt due to the battery vapours over time. One particularly troublesome one was working fine when first tested but then started to develop fault after fault which started to drive me demented but thankfully not enough to jump in the Thames! First thing that became apparent was it would intermittently reset and especially so when the board was slightly flexed. These type of faults can be a real headscratcher due to their intermittent nature and patience is a handy thing to have. Trying to get the fault to stay on was impossible and sometimes just a slightest touch to the board was enough to reset it. To cut a long story short by monitoring the sensor lines from all the voltage inputs I eventually found that it wasn't any of them causing it. The actual fault lie between the connection between collector of TR9 and IC3 (9). The actual voltage measured at either end varied on flexing the board, the temperature, time of day, how many glasses of wine I had (only kidding) but you get the gist. So following the track this one appears on the top side of the board just underneath where the battery was and although it looked clean there was obviously a microscopic break between the track and the via! A patch wire sorted that. Thinking a quick functional test and wrap up and on to the next one. Switch on and no boot! The reset is now stable but now what's happened? Checking around and after a few more power cycles and it starts making load white noises and other intermittent grungy sounds. No lamps were coming on and no 7 seg display so it just looked dead bar the wierd noises. All the ADD and DATA appeared to be running but nothing else was happening. A good trick is to break the meter link as this will set off an alarm if things look dead. I could tell by the sound changing to a different horrible noise that this was actually doing something and I also noted that the board would reset itself it the door switch was operated. This is when the old lightbulb went off in my head as I've had this before. IC38 (74LS32) was the culprit as pin 6 (IOWR) had no output. Switch on again and no boot, no noises, no nothing! Recheck everything again and now I see that Address line A9 is low?? This line measured 55 Ohms to ground whereas it should be in the meg Ohms. To find this every device connected to A9 has to be removed which is the CPU, all the PROMs, the RAM (which wasn't socketed but is now) and IC26 (74LS32). As soon as IC26 was removed the low resistance disappeared? I know these boards eat 74LS32's for breakfast but this is getting silly. So, switch on again and it boots but now no mux lamps or 7 seg display and none of the user switches do anything either. This is usually down to IC5 (CD4538) which monitors the updates from the CPU and switches off the mux if there's a failure. In with a new one and bingo it all works. None of these devices looked bad with no crumbly solder either so it just goes to show you can't tell anything by physical appearance. More to follow.....

Yep it's the silver component here. If you google zn404 you will see what it looks like. It's like a top hat with a locator tab sticking out. Always take a pic and refit it the way it was (if you have seen it working) and ignore the silkscreen writing on the board as it's wrong on some boards. Think I've repaired +30 plus and always fit a newer one regardless. I replace the electrolytic caps, new voltage regulator, new ztx transistors. I do that before I even test them. The Cpus do occasionally die aswell. 1.5 in 10 dead clean cards will be the cpu or socket contacts... by that I mean the sockets loose tension and end up slightly oval shaped rather than square.