Leaderboard

Well, one of the functions that IC24 does is to monitor the lamps for short circuit bulbs and turn off that bulb as a precaution. It does this by monitoring the voltage dropped across the mux voltage feed resistors. This is fed into a little two transistor sensor circuit which then puts a signal on pin 10 of IC24. In this case both the transistors were iffy and the signal on pin 10 was constantly low. This is then fed back to the CPU which told IC12 to send permanent wait states up the CPU's own backside! Both transistors (T68, T69) had black legs and the BC337 (T69) tested leaky. Once swapped out along with the 5.1v zener as a precaution IC12 didn't send any more pulses on pin 7 and you could see the lamps at full brightness along with all the switches now working. To say I was relieved was an understatement.

A little update i have hopefully got the 10p roms coming later this week i will let you know if they work ok

Maybe dump the roms and some hi res pics as don’t think we’ve this version emulated yet

I can't believe it's been over a year since the last thread but this one was a corker! This board had been to & fro quite a few times with various faults but this time it was just 'stuck in RESET'. That proved to be easy as the little voltage supervisor chip had failed. Thinking it was ready to go back, testing it the next day proved otherwise. It was now running slow, dim lamps, and the Start switch wouldn't work? 3 days later and swimming trunks at the ready for the jump in the Thames I was geting nowhere! For those who follow this thread may remember this is very similar to a fault a few posts back and this was due to IC24 (8255) which had failed. The plot somewhat thickens here as an old note stuck to the bottom of the board had all these symptoms on it and a previous repairer had alread been around that area changing chips obvioulsy to no avail. All these had to be removed as they needed tidying up and I couldn't be sure there wasn't something untowards underneath them. So once that done the same issues were still there. So what is causing the problem. Well, as was described in the previous fault, a pulse coming from IC12 (7) puts delays in the NMI line to the CPU which is used for various functions. I'm thinking it must be something to do with IC24 as it controls the lamp multiplex and the switch multiplex, but what? The chip is OK as it's already been replaced. Whilst in bed last night it came to me in light bulb moment and this morning I found it straight away. I was correct in my diagnosis that it was something to do with IC24 but up and until then didn't understand why. Now I do. What caused the issue? (just two components)

Afternoon Ron, I will take a look, hopefully it will be a simple as hoped! Thanks for the reply 👌