braner@batcomputer.TN.CORNELL.EDU (braner) (08/20/86)
[]
Atari has issued a technical note to its dealers about the high-
pitched whine in the 1040. I hope I am not stepping on anybody's
toes in posting this here.
The whine is at about 16 KHz, which most people who grew up with TV
cannot hear. I love the SM124 B/W monitor because it's silent (35 KHz
scan rate), but the whine from the computer still bothered me.
I thought it comes from the main power-supply, but the note says it's
from the circuit around the "converter" chip on the mother board (U62).
This chip makes negative voltage from the positive (using an oscillator and
a "multiplier" rectifier arrangement?). The fix for the whine is to raise
the frequency, by RAISING the values of a resistor, a capacitor and an
inductor. The latter claim is against all my electronic instincts, but I
tried it and it works. Here is how you do it:
Obtain a 5 ohm 1/4 W resistor, a 330 pF capacitor, and a small, axial,
220 microH inductor. Take the top cover off the 1040ST (unscrew the short,
self-tapping screws from the bottom). Take the metal cover off the
power-supply module (one little screw holds it down) and remove the module
itself (two screws at the two corners on the keyboard side).
Just "north" of the power-supply-module connector you will find U62.
Locate R17 (north of U62, 1 ohm), C28 (third component on the west, 100 pF,
looks like a resistor), and L4 (east, 100 microH, looks like a big resistor).
Replace them with the new components. That's it.
There is a pair of wires, blue and red, going from the U62 area towards the
RS232 interface area. The red wire carries +8V or so, relative to ground.
The blue carries the negative, and I measured -7V on it after the fix.
(I expected -12V, but 7 is close to 8 and the computer works fine...).
If you don't get good negative voltage on the blue wire, relative to
ground (not the red wire), you may have stopped the oscillations in U62.
Try a different L4, perhaps?
- Moshe Braner
Corson Hall, Cornell University, Ithaca NY 14853
(607) 272-3487
For electronic mail, my address is:
braner@amvax.tn.cornell.edu (ARPANET)
braner%amvax.tn.cornell.edu@WISCVM.BITNET (Bitnet)
{decvax,ihnp4,cmcl2,vax135}!cornell!amvax!braner (USENET) collinge@uvicctr.UUCP (Doug Collinge) (08/25/86)
In article <906@batcomputer.TN.CORNELL.EDU> braner@batcomputer.UUCP (braner) writes: >Atari has issued a technical note to its dealers about the high- >pitched whine in the 1040. >... >Replace them with the new components. That's it. My friend just did this and it worked. No problems encountered. Thanks Moshe! -- Doug Collinge School of Music, University of Victoria, PO Box 1700, Victoria, B.C., Canada, V8W 2Y2 decvax!nrl-css!uvicctr!collinge decvax!uw-beaver!uvicctr!collinge ubc-vision!uvicctr!collinge