BBC Model B No2. Will it live again!

[rubber_jonnie]

Turns out I have spares of all the ICs bar IC41, a 74LS02, so that's ordered just in case.

It's not like you won't find it used elsewhere, so it can be a parts bin item if I don't use it.

I've decided too that I'll enable colour composite on both machines once the No2 machine is fixed and showing video again, since it's only adding a capacitor and a jumper, nd both already have the capacitor :)

Usually it's just the jumper.

Yeah, both have C58 fitted already :)

I have jumpers to hand too. I'm going to leave it until No2 video out is fixed and verified though.

Managed to try my TurboSPI in the No3 over the weekend, nice cheap little device, next I need to try @cmorley's Sideways ROM too.

[rubber_jonnie]

Looking at the No2 machine, it seems I have already replaced IC44, so Just need to check for shorts along the way.

[stephen_usher]

Did you socket it? Maybe remove the 6845 and IC44 and do a resistance test. You may be able to determine how close to each end the short is.

[rubber_jonnie]

Did you socket it? Maybe remove the 6845 and IC44 and do a resistance test. You may be able to determine how close to each end the short is.

I did socket it, and it's on my list to test.

Oddly, 37, 41 & 44 are all pretty close together, near the rest of the video circuitry, but 33 is over the other side of the board near the econet end.

Won't be able to do much tonight as I am training, but fingers crossed I can have a look at some point in the week.

[rubber_jonnie]

I managed to get a very quick look in after lunch and noticed something off about IC44.

Pin 11 should have hsync coming in from pin 39 of the 6845, and those pins are connected ok.

IC44 pins 10 & 13 have 5v, but based on a test with the good machine, are in no way connected to pin 11. On the bad machine, pin 11 is connected to pin 10 & 13, so I'll fix that next, see where we are.

Can't help that the hsync line of IC44 is being pulled up to 5v, so that would be a fairly good reason that hsync isn't working.

One step at a time, I'll do that and see if the problem goes away, and if not, move onto the next step.

[stephen_usher]

:dualthumbup:

[rubber_jonnie]

And great, every last hysnc connection is somehow connected to 5v!! No wonder there's no video. There are no obvious shorts, and aside from IC44, they are all soldered in.

Of course I can't buzz out each chip, because they VCC is shorted to Hsync, but maybe a resistance test might give a clue.

[stephen_usher]

I'd pull IC37 as this isn't needed by anything other than the modulator as far as I can see in terms of video out. (In fact, if I was desoldering chips I'd do both IC37. IC33 and IC41 and install sockets as then that would completely factor out any chips.) I'm sure that there should be a pull-up somewhere but I can't see one. Maybe the 6845 is doing the pull-up internally.

[rubber_jonnie]

I'd pull IC37 as this isn't needed by anything other than the modulator as far as I can see in terms of video out. (In fact, if I was desoldering chips I'd do both IC37. IC33 and IC41 and install sockets as then that would completely factor out any chips.) I'm sure that there should be a pull-up somewhere but I can't see one. Maybe the 6845 is doing the pull-up internally.

The plan is going to be to pull the chips one by one I think, then re-check to see if the short has gone as each one comes out. They'll all get socketed, but I really do want to find out the cause. The board seems really too simple to have an internal short, and of the tracks I've looked at so far, none seem remotely damaged or have obvious shorts.

[rubber_jonnie]

Ok, that is disappointing.

All of the ICs are out (33, 37, 41 & 44, plus IC2) and I still have a short to VCC. That means that there must be a physical short somewhere as every single hsync connection to every IC is connected to VCC when I test with either continuity or ohmmeter.

The ohmmeter shows zero ohms if I measure between the hsync connections at each chip location and VCC. This is not the case on the good machine.

What I have also noticed is that the reset line on the 6845, pin 2, is somehow connected to pin 39, the hsync out pin.

More detective work required.