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Conference 7.286::atarist

Title:Atari ST, TT, & Falcon
Notice:Please read note 1.0 and its replies before posting!
Moderator:FUNYET::ANDERSON
Created:Mon Apr 04 1988
Last Modified:Tue May 06 1997
Last Successful Update:Fri Jun 06 1997
Number of topics:1433
Total number of notes:10312

371.0. "Power-up problem on Mega-2" by MUNCSS::BURKE () Tue Jan 10 1989 14:26

    I recently replaced my 1-meg ST with a Mega-2, and have the following
    problem:
    	After power-up, the fan starts OK, but the drive does not seem
    to come 'ready' (ie. no green lamp or drive access). Sometimes it
    does come ready after approximately 45 seconds after power-up. However,
    mostly it does not access the drive at all (SM124 screen blank since
    the power-up sequence has not completed).
    	I removed the casing and checked for loose connections, etc,
    without any luck. Now hear this: when I gently blow (yes - BLOW)
    in the area of the power supply, all is well !!!  This would seem
    to point to a hair (?) shorting something, or something similar.
    	Has anybody any ideas, or experienced anything similar ?
    I don't want to send the machine back for obvious reasons (delay
    etc.), but mainly because it must be something minor and easily
    fixable.
    		Help !
    
    Thanks in advance to a helper
    Jim Burke
    
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371.1Maybe your cooling down a hot spot?TENERE::DEIGHTONWed Jan 11 1989 13:3531
Your unusual and temporary fix implies that there may be a "whisker" of
something causing a short circuit or more likely that some component in the
PSU area is heating up and functioning only when you blow on it
(and hence cool it down!).

Maybe this could be isolated using cooling spray (such products exist
exactly for the purpose of tracking down heat sensitive problems) on
components until you find the one causing the problem. Blowing on the
whole area is not selective enough.

Boring bit is to try and find out if one supply rail only is going down
ie +5, +12 or -12) and then see if there are faults in PSU related to
this......the key is that voltage convertors can shut down when they
get hot and this may be caused by a broken track or a bad cap or resistor
(amongst many others!....your breath may cool it enough to turn back on).

I assume that the 5v, 12v and -12v are supplied by 7805, 7812, and 7912
convertors which normally have "safe operating area" protection....this includes
thermal shut down....maybe one is getting two hot (could be bad thermal
connection to heat sink, faulty components or a short circuit causing too
much current to be drawn, also consider that they may be being supplied
by too high a voltage! The power dissipation of such devices is governed
by the difference between their supply voltage and the voltage they
supply and the current demanded....simply put if you supply a 7805 with
7 volts and your circuit draws 100ma at 5 volts then power dissipation in the
7805 is approximately 7volt-5volt x 100ma = 200 mwatts.......if you supply the
7805 from 25volts then you get 25volts-5volts x 100ma = 2 watts (ie YOU NEED A
HEAT SINK!).

Hope you get the problem fixed,
	Nigel
371.2Problem fixed (?)MUNCSS::BURKEFri Feb 17 1989 08:3821
    	I'm a 'softie' and so I don't have the confidence to stick probes 
    into the PSU. Therefore, I admitted defeat and took the Mega-2 in for
    repair after trying some of Nigel's suggestions.

    	The engineers report was brief and not detailed. He mentioned
    something like "modified Interrupt Control IC" - he didn't replace
    anything. What could he have 'modified' on a single chip ?
    Anyway, it works OK now, but only on first (cold) power-up.
    A second power-up displays some of the original problem - it takes
    as much as 30 seconds for the drive to 'come ready' after switching
    on the unit. There is a 3-month warranty on this repair, so I'm
    not sure whether to take it back. 

	The bit I did not like, especially in view of the fact that
    no parts were replaced and that I gave him a detailed description
    of the problem, plus showing him Nigel's note (.1), was the CASH.

    		****  50 quid (about $80) !!@?*!!  ****

    Jim Burke    
    
371.3Life after deathTIMMII::RDAVIESAn amateur expertMon Sep 02 1991 10:3025
    This was the nearest text under dir/title=prob, so I'll re-open this
    note! :-)
    
    T'other week I had problems with my STe. It had never previuosly shown
    any problems, and the day before had been working fine. I came to use
    it and it failed to do anything except light the power light.  No
    cycling of the floppy, no lighting of the floppy indicator, no activity
    on screen NOTHING!    (MAJOR PANIC!)
    
    I opened it up, checked physical connections, checked the power supply
    outputs, OK. That's all I could do as it's mostly all VLSI IC's
    (including at least 1 surface mount). Removed the two SIMM modules I'd
    fitted at the beginning of the year, still no difference.
    
    I did lift the two EPROMS slightly and reseat them, no difference, so I
    removed the clips off of two other chips sitting in square sockets (I
    think one was the actual CPU), and virtually removed and reseated the
    EPROMS again. 
    
    Thought, nope, nothing wrong, started putting it back together, powered
    on to re-confirm problem......It Sprang into life!!!
    
    Must have been re-seating, could it have been the EPROMS?. 
    
    Richard
371.4EPROMS sounds goodUFHIS::BFALKENSTEINTue Sep 03 1991 05:429
    
    It could have been the EPROMS. The symptom is alike when a EPROM has
    no contact (what to boot where from?).
    Also I made the experiance after pressing the RESET-button that some-
    times the same happens. It does not come up again. In that case usually
    another coldboot brings it back to life.
    
    Bernd