Re: IIe power supply problems

[peterm@tlp.apana.org.au (Peter Maloney)]

kjhass@sandia.gov (Joe Hass) writes:

> 
> Wizards of the Net:
> 
> I'm trying to help a local elementary school keep their IIe's running and
> have found what appears to be a bad power supply.
> 
> The machine will boot properly when it's warm but the power supply won't 
> come up if it's cool.  When it's cool the +5 output is at about 1.1V, the
> -5 is at about -1.1V and the +/- 12V outputs are proportionately low.
> Sometimes, if it's not too cool, the voltages will slowly rise and then
> the supply will suddenly come to life and be fine.  I know it's the supply
> and not the motherboard because I've swapped them with another machine,
> and I've tried heating the supply alone with a hair dryer. Also, when the
> supply is cool but disconnected from the motherboard the positive voltages
> are about 1% above nominal and the negative voltages are about 10% more
> positive than nominal.
> 
> I can't see any cold solder joints or obviously bad components.  The
> machine is of 1985 vintage and the supply was made by Astech.
> 
> Any diagnostic suggestions would be appreciated, as well as pointers to
> a low-cost vendor for a new supply. I'm an EE so I have a passing
> familiarity with switching power supplies, but I don't have the schematics
> for this guy.
> 
> TIA,
> Joe
> 
> -- 
> Joe Hass               | library clever_sigs;          | My opinions are my o
> kjhass@sandia.gov      | use rock_and_roll.verses.ALL; |   not Sandia's and
> Sandia National Labs   | use lineprinter.art.ALL;      |   not the DOE's

Theis problem is usually caused by a failed capacitor (220uf/16V?) 
that sits near the big power transistor. Test it by warming it up 
before you power on.
Regds,
 Peter M.

peterm@tlp.apana.org.au                TLP - The only *REAL* A2 BBS in 
Peter Maloney                          Melbourne 
A2 Hardware Freak