[Coco] MPI voltage problem

[Gene Heskett]

On Sunday 12 May 2013 08:22:29 Luis Antoniosi (CoCoDemus) did opine:

> Hi, it turns out that i found my issues with the MPI. I found out that
> after running for a while the +5V pin drops to around +1V or less.
> 
> The +12V/-12V seems to be ok all the time.
> 
> What could it be ? Should I replace the PSU ? Can I use a PSU found on
> digikey or any other local store ? What PSU should be ?
> 
> Or could it be just a transistor or bad capacitor ? I can't find the
> problem alone :/
> 
> Thxs,
> 
> Luis

Luis, those that I have seen used a "TO-3" cased power transistor as the 
regulator stage.  This has a bolt through the ear on each side to mount it, 
and I have seen them with loose bolts that could cause a poor connection.

Also, with the amount of heat they can make, check the two solder pads on 
the bottom of that device with as strong a magnifying glass as you can 
find, maybe even one of those pocket microscopes, looking for hairline 
cracks in the solder. The wavepot solder used was pretty bad stuff, so 
while at the shack (I am assuming there is something like a radio shack 
near you), get some solder-wick so you can remove that stuff and replace it 
with decent, modern alloy, rosin fluxed electronic solder, with the best 
grade having a couple percent of silver in the alloy.  When replacing the 
solder, heat the device pin first, apply solder there and slide the tip of 
the iron down to the board applying more solder so you have a nice puddle 
and everything is "wet" before removing the iron & letting it cool.  That 
ought to be good for another 30 years.

Cheers, Gene
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