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Please

Thanks again Bob. I’ve rebuilt the Reset Circuit again this afternoon, and also put T59 and R79 back in. Upon testing I’m getting garbled or no text on the display, and the voltage at the R76/T57 junction climbs quickly to 14.86v at switch on. I am getting 5.26v almost instantly on the RESET line. I’ve spent time this evening mapping out voltage readings to 0v at various points within the Reset Circuit as per the attached pic. I’m just hoping this may highlight something to your experienced eye? After this I logic probed the RAM chips again and they gave the same outputs as when the board was running. RAM voltages are a solid 4.98v. Cheers, Steve.

I already have a couple of working mpu3 machines but no experience of fixing anything inside the metal box. I decided to have a go at building some experience so I’d have the confidence to try and fix any issues that might arise with my working machines. I purchased a non-working Royal Exchange and didn’t pay a lot for it but reasoned with myself that if my project failed then I could always sell it on or break it for spares. But I only want to preserve it and get it working!! I have no contacts to call on for help, but I’ve found this forum is a brilliant resource for people like myself who are willing to have a go. I’ve been logging everything I’ve done so the following posts detail my journey so far. The story so far part 1… Power on and Royal Exchange not working, only 2/3 flo tubes lit. I replaced the ST111 starters and all the flo tubes, then checked/replaced all the PSU fuses. The mpu3 board is a Mod 5 with a daughter board and 6117 RAM. No battery on the board but it was evident the old battery had leaked and done some damage as there is some masking missing. Someone has previously cleaned the board up, but upon first inspection I found quite a few green legged components, also T60 had a broken leg and C41 was missing. I replaced T60, T61, C41 and R92 – R95. I also cleaned between tracks with a fibre pencil and mopped up with IPA. Checking the white power plug I found that the 12v return pins 1 & 9 had burnt. I then fitted a bypass lead to 0v on the board as suggested elsewhere on this forum. I checked pin voltages on the white power plug and all OK. I fly wired a NiMh battery, confirmed 5v at the link and pin 24 on 6117 which dropped to 2.4v on power off. After cleaning all the plugs I connected the board and switched on… it booted but was stuck in reset. Reels stepped up & down and remained stiff, optos lit and display was now showing ‘Resetting RE 4.0’.

No, it should start at 0v and then switch to 5v after approx 1 second ish. No point keep changing the transistors as it's unlikely they are bad again. Looking back through the thread I see you had removed the spark circuit so that's eliminated. The timing of the RESET pulse is set by R76 and C36 which you can monitor by reading the voltage from R76/T57 to ground on switch on. It should start at a low voltage and rise to somewhere 12-15v if I remember. If that's happening then T57 should be switching when it's collector voltage reaches the threshold of the 9v1 zener on it's emitter which in turn switches T58 on and T59 off....bingo 5v on the RESET line.

Does the RESET circuit function correctly, sounds like it doesn't? It should go to approx 0v on switch on and then after about a second switch to 5v. You shouldn't need to 'jump' the RESET obviously but I would make sure that part is all working first as it will save you time. Depending what mod state you have will depend on what parts may be faulty. Moving on to the no boot is where it can get tricky. Assuming your ROM chip is OK, test by substitution, then you are getting in to logic probe or even scope territory. I wouldn't recommend a scatter gun approach either of swapping all the support chips (unless things get desperate, been there a few times) as that can lead to all sorts of added problems of pad lifting, broken vias etc unless you have all the right gear of course. It'll be a good idea to do IC9 & 24 anyway as you may hit lucky. Watch you don't pull any pads off though as some of the pins are soldered both sides