Date   : Fri, 28 Jun 1996 05:12:50 +0100
From   : Mike Tomlinson <mike@...>
Subject: Re: Hardware problems

In message <9606271427.AA08096@...>, jsr4 <jsr4@...>
writes
>>I'm still trying to get this old Beeb I bought cheaply a while ago working
>>properly.  Problem is that after starting a game (problem only seems to
>>occur during games!) for example Cylon Attack, after a few seconds into
>>the game, the images of the attacking aliens start spreading all over the
>>screen, as if they are being redrawn without the old image being erased. 
>>After a minute or so, machine locks up solid.
>
>check the RAM in the machine... quick test is to feel the tops of the 
>ram chips after the machine's been on for a while, dram tends to get
>rather warm once it blows... 
>

On most machines, ram chips get very hot when they fail. Unfortunately
this is not the case on the Beeb. However, the ram chips themselves are
usually very reliable.

Try changing ic14, 74LS245, to the right of the 6502. This is an 8 bit
octal latch which buffers the data bus to the memory. It's heavily
loaded and is a common failure point.

Also make sure the heatsink on the video processor is securely mounted,
and that the power leads which connect to the PCB in the area of the
DRAM are clean and tightly connected.

A good test is to put the machine into mode 0 and leave it for a bit.
Bad memory will show up as single dots appearing on the screen, bad
address select ICs will show up as lines or blocks of corruption
appearing. You can also try this program:

10 *TV 0,1
20 MODE 2
30 REPEAT
40 GCOL 0,RND(15)
50 PLOT 85,RND(1279),RND(1024)
60 PRINT "ON TEST"
70 UNTIL 0

Because the Beeb's high res modes take up 20k of the 32k memory, any
test which exercises the screen is usually sufficient to show up faults.
Faults will show up as random corruption, lines appearing, failure to
scroll properly, etc. and are very obvious with the above. You can leave
this running as a soak test.

Troubleshooting the BBC's memory is not straightforward, as there are
two CAS lines for each bank of 16k (the Beeb was originally produced as
a 16k Model A machine), there are three 81LS95 octal buffers which
select high resolution (bitmapped) graphics and teletext modes, and
there are a lot of ram chips to deal with.

The Master's memory was much simpler to deal with, a lot of the Beeb's
discrete components being combined into one memory controller IC and
four DRAM chips.


-- 
Mike Tomlinson
constructive email to mike@...               ; flames to /dev/null.