So, here's what was wrong with this machine:
- Bad keyboard - I managed to get a replacement PCB from somebody off stardot and rebuilt it from the ground up. All keys good, though 4 switches needed 2 days soak in contact cleaner!! The kbd is fantastic I have to say.
- Dead CPU - Replaced with a new one.
- Bad 16Mhz master clock - One of the caps was dead, so I replaced both caps and the 16Mhz crystal.
- A very gnarly short between the 6845 /RESET line (+5v when switched on) and the hsync line from the 6845 to a number of other ICs in the video circuit - track cut and bodge wire used to skirt the dodgy area.
A few other ICs got changed during the troubleshooting procedure, but I think they were probably ok, so I won't count them in. I also replaced sockets and jumpers with new as I went as they were all pretty nasty.
As a little extra I did the colour composite upgrade to both No2 & No3 and it looks great :)
I also fitted the TurboSPI disk emulator to No2 so I could test it out. It's a 2 part kit which has a ROM and an MMC card reader which plugs into the user port. This was a double whammy because it tested the ROM slot and the user port. I might be shopping for a 2nd TurboSPI.....
I even managed to get the joystick port tested as I have a dual analogue stick, though I think that'll be next for a refurb as very dusty and grimy, and no doubt the pots would benefit from a spot of contact cleaner.
No2 is not great cosmetically, the case is very yellow, and the black plastic keyboard surround is pretty grotty. However, it lives, and somebody on stardot is working on getting the surrounds re-made, so it will get one of those when they are available.
I've also got a bus reader/adapter from
@cmorley that I'll get built next so I can have a poke around, plus one of his 64K EEPROM kits to play with too.
I want to have a crack at assembly language on the Beeb too, as I have a lot of 6502 machines and a good deal of what I learn will be transferable.
I'd like to say I only used basic tools, and mostly I did, but whilst the Fisher Price oscilloscope did have its uses, as it showed TTL logic nicely, it also showed that you do need to have a minimum spec for looking at frequencies, and it was hopelessly inadequate for what is present in the BBC.
I had been intending to invest in a frequency generator/counter for some time now, especially for when I'm in my new workshop, so I splashed out on one for £66.
Its frequency count function was just what I needed, as it showed all the clocks were correct (16Mhz master clock, 2, 4, 6 & 8Mhz from the video IC) and enabled me to look properly at the vsync & hsync signals coming from the 6845, and that's what got me to the point where I could see that hsync was dead, and from there helped me figure out that it was a short of the hsync feeds.
If you can't afford a scope, then a frequency generator/counter is a reasonable halfway house (Cost half of what my 'scope did).
Having a decent schematic and chip data-sheets was one of the other major reasons I could fix this, along with the service manual and a very specific troubleshooting guide I also found. The basics will get you there, I'm no electronics expert, but I have learned how to read a schematic and understand logic gates. If you can do that it'll take you a long way.
Trouble is, I'm running out of machines to fix! IIRC my last 2 broken machines are a Spectrum and my Mac Classic. Both are in storage, so they will be on the list once I get into my new workshop.
One final thing I learned? Rifa capacitors scare the crap out of you and smell real bad when they enter smoke generation mode!!!
Today has been a good day :)
:girldance: :girldance: :girldance: