Malfunctioning VT240 - help please

[Nigel Johnson]

One trick I found with the -5V if it is driven by  a charge pump: check 
the voltage. If it is being pulled down since the charge pump cannot 
supply the current, just disconnect the charge pump and put a lab supply 
in its place.  The increased current will clean out whatever is shorting 
it to ground without harming any good chips.  If you are lucky a puff of 
smoke will identify the chip. Otherwise there may be enough memory 
running to give you a diagnostic message to say which bit.

I did it once on an $1800 board, all chips soldered in, results in minutes!

cheers,

Nigel


On 11/06/2020 16:33, Ethan Dicks via cctalk wrote:
> On Thu, Jun 11, 2020 at 2:37 PM Adrian Graham via cctalk
> <cctalk at classiccmp.org> wrote:
>>> The 4116's are soldered to the board, too....
>> You’ve just mentioned the magic 4116 word, I’d bet some of your dollars that it’s either one of those that’s gone south or the -5V required to run them
> Definitely check the -5V for the 4116s.
>
> -ethan

  

-- 
Nigel Johnson
MSc., MIEEE, MCSE
VE3ID/G4AJQ/VA3MCU

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