[Coco] Erratic MPI Behavior

[jadonaldson at charter.net]

Boisy,
  Power supply caps are VERY BAD about loosing their 
ability to hold a charge. When I used to be a repair tech
(a long long time ago), the first thing we did when a unit
came into the shop is replace the Caps.

John Donaldson

> 
> From: "Boisy G. Pitre" <boisy at boisypitre.com>
> Date: 2004/03/17 Wed AM 02:19:25 GMT
> To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
> Subject: Re: [Coco] Erratic MPI Behavior
> 
> 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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> > Coco at maltedmedia.com
> > http://five.pairlist.net/mailman/listinfo/coco
> >
> 
> 
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