Leaderboard

Another one that caused an inordinate amount of head scratching. Usual symptoms of no boot due to rotten components. All the really bad stuff was swapped out until the board booted and ran OK on the test rig. On return all was not well and came back with No Boot again? Back on the test rig all was good so all the rest of the chips were swapped out around the battery area just in case. Back it goes and guess what, it comes back again as No Boot? Sometimes you think there must be a easier way to enjoy your hobby. I had to get to the bottom of this but couldn't see any reason why it would work in the workshop but not in the machine. Sometimes these boards can go conductive and need a serious clean on the top surface. The slightest bit of humidity in the atmosphere can make all the difference between it working or not. I sometimes recommend just a quick blast with a hairdrier around the battery area just in case. Back at the ranch... So after a good few hours on and off with this one and the previous one I was getting pretty fed up. Then something magical happened, it wouldn't boot. For most folks that would be a disaster but for me it was what I'd been waiting for. Being careful not to disturb anything I evenually discovered that D24 wasn't connected to IC3(6) anymore! There was a microscopic break on the top pad of D24 underneath the etch resist. The copper had been eaten away under the etch resist but looked good from the top. The bad bit is just below where the marking for R77 is, you can hopefully see where I've scraped the etch resist. Anyway a patch wire was fitted and it all burst back into life. Let's hope it doesn't come back again.

Hi all, Currently I am digging into the schematics of the JPM SYS80 CPU board. I do have quite some experience in 70's , 80's microprocessors, concepts , memory mapping, bank switching but the TMS9980 is new to me. Looking at the SYS80 memory decoding I cant get my head around how it works (can be the age as well 😉). This is a memory map I found of the SYS80 CPU boards (the 14K is not in it but I can guess that one). So the 16K addressing space makes sense here : Looking at the schematics of the 6K , 8K, 12K memory decoding I see the use of a few 74LS138's. Here the 6K, 8K and 12K memory decoding logic. So far so good but what I don't understand, for instance with the 6K board (but same question for other boards) is how you can decode the 2nd EPROM , on address $0400 , which is from A13-A0 -> XX00 0000 0100 0000 by just using BA0 to BA3 which are the lowest 16 addresses ?? Anyone who knows this and is willing to explain ? Kind regards, Bram

Well there I was this morning, swimming trunks on ready to jump in the Thames and all because of a System One board that was driving me nuts....... This one started with being stuck in reset, pretty much standard fare for these Ace boards. All rotten components swapped out and board running OK, at least as far as my test rig was telling me. I could play games on it and all switches and outputs appeared to function correctly. So back it went and to cut to the nitty gritty there were still reported problems. The main one was when it booted the reels were slow and the lamps dim? Not seen that one before and it was difficult to see it on the test rig not having any reels. So on the third return things started to get worse, it would still boot OK credits could be clocked up but the START button did nothing? Even during the game's test function it wouldn't recognise the START button? After a few days of head scratching and basically getting nowhere the sound went off! You couldn't make it up. By this time all the small logic chips around the battery area had been replaced, mainly out of desperation but of course the fault persisted. I'm now thinking I'm getting too old for all this stuff and I've lost my touch but before I jump in the Thames I thought a fresh head on a new day might be a good idea. OK, thinking cap on, hang on tight as it gets a bit techy from here on. I thought I'd take some readings on a good board to compare so starting with the lamps and then the reel pulses I saw that they were approx half speed?? OK, that explains why the reels run slow and the lamps are dim but why?????? Next step was to discover what was holding up either the bus or the interrupts. This is where my understanding gets a little hazy but I know from the docs that if IC12 sends a pulse out on pin7 (74LS138) then it can prevent NMI from occuring which controls the reels and multiplexer (lamps). Checking pin7 and there was a constant pulse. My notes state that in normal running there shouldn't be one so this is looking promising. What on earth is happening though to cause it? Another day gone and still no resolution so off to bed. Now most people count sheep jumping over gates but I see circuit boards and logic gates. Anyway in my dreams a voice said surely it has to be an input, perhaps a stuck gate on one of the input chips? Previously I'd already swapped out two of them trying to track down the reel sensors and the START switch and also IC37 (8255) thinking that might be it. Nope! So this is where we came in, trunks on and ready to walk down to the river. Before I go had I missed anything? That's when the old lightbulb lit up and I got all excited again. There is another input albeit it's not mentioned in the docs and also it appears on the diagram in the outputs section!! This is the DATA IN which goes to pin 13 on IC24, under normal running it should be HIGH but it was stuck LOW. It's fed via a 220K resistor and clamped HIGH by a 1K resistor to the 5v rail. On the DATA IN side it was indeed HIGH but LOW on the chip. I've no idea what caused that as I doubt whether anyone has a DATA pack connected. So it was out with the 8255 and in with a new one and checking the pulse back on IC12 pin 7 it came on for a second after starting to boot then went off. Whoopeee!! Reels now definitely running a lot faster and the lamps also brighter, what do you know the START button now works too. Definitely one for the Midibob notebook. Hmm, still no sound so it's out with the AY-3-8910 and in with a Yamaha YM2149 and bingo sound restored. Trouble is it's my only spare and prices on Ebay are ridiculous. If anyone wants to sell me some I might bite your arm off as long as they aren't Chinese of course, the chips not the seller. I think I deserve a few glasses of vino tonight.

i remember 2 of these machines on 2p play! gonna give this one a good bash

Hope whoever gets them posts about them, be good to know whats what

Great to see some true classics on offer to play .

Brilliant dx thank you.

Thanks for the great dx.

Well there you go,do apologise for me emptying them that night and pissing the money up on Pernod and black.

Sometimes I think it must just be me and I'm being punished for a misdemeanour in an earlier life.

Because when power rails on the board fail or get noise CMOS components are very fragile. They need very stable voltage levels on vcc and gnd or they will break. Common thing to happen is latch up, this is where pins are driven on an IC that has no power supply, it causes the internal transistors of the IC to fail. On top of that, battery damage to a maygay is usually widesrpead because the traces are thin to begin with , very easily corroded through.