[Coco] Erratic MPI Behavior

[Boisy G. Pitre]

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

On Mar 16, 2004, at 3:56 PM, Robert Gault wrote:

> Boisy G. Pitre wrote:
>> Folks,
>> I have a 26-3124 MPI, upgraded for the CoCo 3, that behaves in the 
>> following manner:
>> 90% of the time, with a working disk controller and DOS in slot 4 and 
>> the switch at that position, the CoCo 3 fires up in ECB, not Disk 
>> BASIC. If I turn the CoCo and MPI off and on enough times, it will 
>> eventually fire up into Disk BASIC, but a power cycle usually results 
>> in the ECB screen again.
>> This is a major annoyance.  Another MPI of the same model, also 
>> upgraded, works consistently in place of this one.  Something is 
>> wrong with this Multi-Pak.
>> I've scrubbed both sides of the card edge connector with an eraser 
>> thinking that dirty leads might be the cause.  Still, the same 
>> behavior persists.
>> Does anyone have any possible ideas on what this might be?
>> Boisy
>
> The main power transistor in the MPI is tied by two nuts with lock 
> washers to the circuit board. These connections can oxidize after 
> which there won't be power on the MPI. My MPI was "repaired" after I 
> loosened and re-tightened these screws with a socket wrench. Take care 
> not to over tighten.
>
>
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