CHARGE Pixel Store Repair

Having successfuly repaired faults on two Misubishi CRT monitors from LCP spares the  opportunity presented itself to have a go at fixing a CHARGE Pixel Store card that went U/S a month or so ago.

CHARGE stands for Compact High-resolution Advanced Raster Graphics Equipment. It is the system in the LCP that produces the graphics for the four displays.  It was devised and developed by Ferranti design engineer Peter Don-Duncan specifically for the Argus 700. Each of the four displays has a Controller card  (first below) and two Pixel Store cards (second below).

CHARGE Display Controller wm.jpg

 

CHARGE Pixel Store wm

We do have the circuit diagrams but no test specifications or special-to-type test equipment so with the density of the boards it is easy to understand why repair is approached with some trepidation.

Looking at the faulty display for this Pixel Store,which would have swapped out by substitution in the LCP, the symptom was recognised as being similar to a fault we had seen a year or so ago – doubling up on the video.

Pixel Store Fault wm.jpg

After creating a document with a suit of reference logic patterns using a serviceable Pixel Store comparison was made with the U/S card. The previous fault had been a 7483 4-bit adder so testing focussed in this area; lo and behold replacing  IC 107 it as another of the same device type.

Here you can see part of 3rd Line Servicing – part of Peter H’s conservatory; he already has a spare bedroom but while ‘the boss’ is away ….   The display now shows a solid test display (although the monitor linearity now needs setting up) with no doubling.
Pixel Store 3rd Line Test Rig wm.jpg

Back to mechanical stuff next week!

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