[Coco] Erratic MPI Behavior

[Robert Gault]

Boisy G. Pitre wrote:
> Robert,
> 
> I saw three power transistors on the MPI, took my soldering iron and let 
> the solder reflow on the underside of the board precisely where these 
> transistors were.
> 
> I also noticed that the card edge connectors were loosing their "silver" 
> to a reddish color.  I retinned all 40 of the card edge connector pins, 
> then slapped everything together.
> 
> The behavior is now slightly different.  Before, 90% of the time it 
> would act as though it wouldn't come up.  Now, it's about 50% of the 
> time.  40% of the time now, I get the ECB screen and the OK prompt 
> cursor flashes once or twice, then stops solid, as though the CoCo locks 
> up.  It will then either go into the ROM pak or crash the CoCo.  The 
> other 10% of the time it actually works.
> 
> Multi-Paks can be weird beasts.  I sure would like to find the culprit 
> here.  BTW, I checked the voltage on the transistors, and am getting 
> readings on my multimeter that makes me seem to think that they are 
> working.
> 
> There was minimal oxidation on the underside of the board.  One place in 
> particular was one of those four large electrolytic capacitors by the 
> transformer.
> 
> Could this possibly be an issue of these capacitors drying out and 
> loosing their ability to hold charge as quickly?  It sure seems as 
> though the MPI is kicking in a little on the late side (after the CoCo 
> has powered up for a second or so).
> 
> Boisy
> 

Looks like you changed something with the retinning and as things have 
improved, you seem to be on the right track.

If all the bus voltages seem good, try turning the MPI on before the 
Coco. If the problem is a delay, this will bypass it. If the order of 
turn-on makes no difference, then there must be some marginal part but 
not the caps. Still, if you have a scope, check for AC grunge on the DC 
lines.

Are you saying your MPI does not bolt the main power cap to the circuit 
board? The nuts I spoke of clamp a TO-3 transitor to the board and act 
as the electrical connection; no solder on this collector. Model # 26-3024

What happens, if anything, if you move the disk controller to a 
different slot and change the selector switch? Different performance or 
the same? What happens if you put some other self starting ROM pak in 
the MPI in place of the disk controller? Will it start better than 50%?