New subject: Getting bent (ON topic if not thread)

13 Mar 1998

On Mar 13, 16:54, Keith Whitehead wrote:
...
  Ok...here's the problem
 The machine come up with garbage on the screen.It would also seem that the
 video is inverted, I can see the retrace lines etc, so the horizontal
 blanking is not working either by the looks of it.

 What I suspect is wrong is that either U42 (the 6845 labeled in my machine
 as a motorola SC80757P) or U102 (the 4.3 video support chip) is faulty (or
 both?). 
The 6845 isn't much more than a programmable video timing generator.  Since you
have *some* video, it's unlikely to be at fault.

It could be a memory-addressing problem, or a bad connection, or...  My first
suggestion would be to *gently* prise each socketed chip from it's socket, and
reseat them all.  This helps clean any oxidation off the pins at the point of
contact.  Used to be a favourite problem with Apple ]['s, but it's a
widely-applicable technique.

I don't wish to teach my granny to suck eggs, but I'd also suggest you take
care that *all* the pins go back in the socket when you reseat anything; it's
not hard to bend a pin underneath sometimes.  For many years I was a
component-level repair technician for an education authority, and I once had
someone bring me a malfunctioning machine which had that problem.  When I
pointed it out, the response was "but NEARLY all the pins are in, and it
doesn't work AT ALL".

-- 

Pete						Peter Turnbull
						Dept. of Computer Science
						University of York

TRS80 Model 4P gate array help