Date   : Sun, 13 Apr 2008 11:05:48 +0100
From   : mike@... (Mike Tomlinson)
Subject: Reviving a dead BBC+

In article <F3871F6F5CF045DA9A4D8A70ACF69108@...>, Ken Lowe
<ken.lowe@...> writes

>Step 1: Power Supply is OK
>Step 2: Can't test - don't have a PET tester!
>Step 3: IC78 Pin 39 (HS) held low, Pin 40 (VS) held high

Meaningless if the OS has not booted and initialised the 6845.

>Step 7: Address lines are showing signs of activity

Use a scope to take a look at the address and data bus and clock signal
levels.  Are they wiggling nicely at decent TTL levels?  Can you find
and identify 16M, 8M, 4M and 2M clocks?

>Step 8: Clock divider is working fine

>Step 9: CRTC notCS (IC78 pin 25) does NOT pulse low after BREAK. This signal
>stays high.
>
>I thought about the system VIA, but I'm not sure if that
>would prevent the cursor appearing on screen?

The BBC B will boot to a blank screen with a cursor with the System VIA
missing.  Not sure about the B+.

> What about the Ferranti
>Vidproc ULA? I assume this wouldn't prevent the Beeb from booting, and would
>only affect the display?

Not sure, but the ULA does the clock division, and as you say the clocks
are present and correct, that part of the ULA is working.  It's also a
very reliable chip.

M.

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