compucolor

[Jim Battle]

Tony Duell wrote:

>>Vintage Computer Festival wrote:
>>
>>
>>>On Sat, 7 May 2005, Jim Battle wrote:
>>>
>>>
>>
>>...
>>
>>>Mine produced smoke too :(  Where did yours smoke from?  I want to get
>>>mine working again.  It seemed to have come from the CRT section.  I'll
>>>bet I have a smoked cap somewhere.
>>
>>I didn't see any smoke.  The previous owner reported it, so it was just 
>>speculation on my part that it was dust cooking off the CRT neck.
> 
> 
> I still wonder about the diodes/capacitors (assuming a conventional 
> circuit) in the raster correction area....
> 
> 
>>A very common problem reported is that the switching power supply 
>>sometimes wouldn't start oscillating and it would result in a cooked 
>>machine.
> 
> 
> If an SMPSU fails to oscillate, it gives no outputs (well, an isolating 
> one does anyway, and anyone who designs a computer that's not isolated 
> from the mains input is criminally insane!). It will not cook anything 
> else. But it'll probalby blow its own chopper transistor and assorted 
> other parts...

When I said it results in a cooked computer, I didn't mean to say it 
fried the whole thing, it just broke it.  I believe the failure 
mechanism is exactly as you say, the commutating transistor in the 
switcher overheats and dies.

The compucolor also another failure mode; it is a rare example of how a 
software bug can cause physical damage.  The compucolor logic is stuffed 
inside a normal TV case (there are even empty holes for 
tint/brightness/contrast along the bottom and a convenient carrying 
handle on the top) with the board driving baseband video to the normal 
TV logic.  The CRT timing is generated by a chip that has timing 
registers loaded from an on-chip ROM.  However, the CPU is free to write 
new values to the timing registers, and if the wrong value is entered, 
it causes something dire to happen in the TV section's logic (eg, if the 
horizontal frequency is too low then the flyback transformer's impedence 
drops and draws a lot of current and smokes).