BBC Model B No2. Will it live again!

[stephen_usher]

Oh, that's interesting. I guess that the only recourse is tracing the motherboard traces. That's tedious.

[Tomswork]

I would look for bad decoupling caps just lift one pin out and test

Oh is there a transistor used to mix the h sync near the out put check that to

Could be a circuit like this

Screenshot_20210602-083329_Gallery.jpg (61.24 KiB) Viewed 3255 times

Tom

[stephen_usher]

IC41 is used to mix the two signals.

h-v-sync-mix-bbc.JPG (15.08 KiB) Viewed 3251 times

However, they both go to the teletext chip, IC5...

h-v-ic5.JPG (25.21 KiB) Viewed 3250 times

Pin 2 of the 6845 (RST) is connected directly to the +5V line, so if you have a short then that would obviously be connected.

I'd look around the teletext chip now. Pull that too.

[rubber_jonnie]

Ok, so I did some motherboard tracing and think I've found it.

The trace from IC2 pin 39 goes under the socket, across to a via, under the board, then back up again and across to pin 9 of IC33.

I cut the trace just before the via, and it fixed the problem with IC 33. Of course it would you say, but that means the problem is on the other side of the board, so I cut the trace just inside the socket, and it fixed the problem for the remaining chips.

Now, where that trace goes under the socket, it passes by /RESET for the 6845, which is at 5v when the machine is powered on, and buzzed out to where it shouldn't before I started cutting tracks.

TBH it was a bit of a hail Mary, but it had to be a physical short on the tracks, and I didn't want to desolder the 6845 socket since I don't have my de-solder gun and was worried about damaging the board.

I've fixed the cut track between pin 39 of IC2 and pin 9 of IC33 with a bodge wire and I am now getting hsync at all 4 chips. I'm just off to test it and I'll let you know the outcome.

[rubber_jonnie]

And

We have video out



Got to go and run an errand, but I'll be back later. Very happy indeed!!!

[derkom]

Got to go and run an errand

Buying champagne?

Good work! This has been fun to follow, and I'm sure a lot more fun to do.

[Tomswork]

@rubber_jonnie

Congratulations

Tom

[stephen_usher]

Yay!

[PhilC]

[rubber_jonnie]

Got to go and run an errand

Buying champagne?

Good work! This has been fun to follow, and I'm sure a lot more fun to do.

No, fish and chips! Had a ball with this one, nice to be able to apply techniques to non ST machines