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Re: BBC Model B No2. Will it live again!
Posted: 02 Jun 2021 15:54
by stephen_usher
Oh, that's interesting. I guess that the only recourse is tracing the motherboard traces. That's tedious.
Re: BBC Model B No2. Will it live again!
Posted: 02 Jun 2021 16:18
by 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
Tom
Re: BBC Model B No2. Will it live again!
Posted: 02 Jun 2021 16:44
by stephen_usher
IC41 is used to mix the two signals.
h-v-sync-mix-bbc.JPG
However, they both go to the teletext chip, IC5...
h-v-ic5.JPG
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.
Re: BBC Model B No2. Will it live again!
Posted: 02 Jun 2021 17:11
by 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.
Re: BBC Model B No2. Will it live again!
Posted: 02 Jun 2021 17:19
by rubber_jonnie
And
We have video out :)
:girldance: :girldance: :girldance:
Got to go and run an errand, but I'll be back later. Very happy indeed!!!
Re: BBC Model B No2. Will it live again!
Posted: 02 Jun 2021 17:22
by derkom
rubber_jonnie wrote: 02 Jun 2021 17:19
Got to go and run an errand
Buying champagne? :lol: :cheers:
Good work! This has been fun to follow, and I'm sure a lot more fun to do. :bravo:
Re: BBC Model B No2. Will it live again!
Posted: 02 Jun 2021 17:22
by Tomswork
@rubber_jonnie
Congratulations
Tom
Re: BBC Model B No2. Will it live again!
Posted: 02 Jun 2021 17:32
by stephen_usher
Yay! :yay: :dualthumbup: :yay2: :bravo: :girldance:
Re: BBC Model B No2. Will it live again!
Posted: 02 Jun 2021 17:45
by PhilC
:cheer: :cheer: :cheer:
Re: BBC Model B No2. Will it live again!
Posted: 02 Jun 2021 18:23
by rubber_jonnie
derkom wrote: 02 Jun 2021 17:22
rubber_jonnie wrote: 02 Jun 2021 17:19
Got to go and run an errand
Buying champagne? :lol: :cheers:
Good work! This has been fun to follow, and I'm sure a lot more fun to do. :bravo:
No, fish and chips! Had a ball with this one, nice to be able to apply techniques to non ST machines 😁