Building a BBC

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rharper
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Building a BBC

Postby rharper » Sun Nov 12, 2017 5:19 pm

I am building a BBC from some spare parts.
However, the motherboard is not playing ball. The seller claimed it was from a Torch pack.
I have read the Torch install instructions - it has the DFS installed but just a standard 1.2 OS not a riser board.
There is a cut off cable on IC76, possibly from a ROM/RAM board.
I am using a PT-65 replacement psu which is giving 4.75v at the motherboard. There is no speaker.
On power on the Caps light comes on but the Caps/Shift lock keys are unresponsive, there is no RBG image.
There is a wire on the rear of the board connecting IC6, pin27 to IC10 pin1. I don't know what this might be doing.
I have tried replacing the 6502 & OS with known working versions and S25 south.
Any suggestions welcome.
Thanks,
Ray :)
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BeebMaster
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Re: Building a BBC

Postby BeebMaster » Sun Nov 12, 2017 5:40 pm

There's no BASIC in there. That won't stop a BBC B working, but it will limit what you can do in terms of testing blind if the video isn't working (like typing in a SOUND command to make a noise, or even just pressing COPY).

IC6 is the early VC2023 ULA so the bodge on the solder side of the board is the "fix" for that chip. It might be worth checking that the mod has been done correctly. It sounds right from the description, but also make sure S26 is open.
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tricky
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Re: Building a BBC

Postby tricky » Sun Nov 12, 2017 5:53 pm

Wiggle any chips to clean their legs, swap the 6522s, suspect faulty ram is where I am with beebs now. There are loads of troubleshooting threads around here, but if you can burn an EPROM, you could try my os replacement ROM and see what you get.

cmorley
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Re: Building a BBC

Postby cmorley » Sun Nov 12, 2017 5:56 pm

S21 south jumper is missing. Should be E-W. This is for the gate/enable on the IC20 ROM chip select chip. Leaving the input floating may be problematic.

rharper
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Re: Building a BBC

Postby rharper » Sun Nov 12, 2017 9:13 pm

Will check S26 and I hadn't noticed S21 was missing.
I will swap the 6522's and resit other IC's with sockets.
Thanks for the replies,
Ray
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CMcDougall
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Re: Building a BBC

Postby CMcDougall » Mon Nov 13, 2017 4:39 pm

as said previously , S26 is NOT needed due to the jumper hack on back of MB & VC2023 video proc.

when got a speaker on it , press CTRL G makes a beep, & type *T. , *. ,should make tape relay click, if not then ram /6522 faulty....
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