TYSON@SRI-STRIPE.ARPA (Mabry Tyson) (09/25/86)
I had no problems with the high-pitched whine described by some people until I plugged a generic null-modem onto the cable attached to my RS232 port. When I removed the null-modem, the whine would stop. It wasn't always there when the null-modem was attached. (I don't think the problem was related to the particular device attached to the other side of the null modem.) The null modem passed pins 1 and 7 through, swapped 6 with 20, swapped 2 with 3, and tied 4 and 5 (same side) together and fed them to 8 (on the other side). The null modem was symmetric (ie, it looked the same from both ends). I'd guess the problem was because 4 and 5 were tied together. I have since constructed my own null modem that did only the pin switching that I needed. It is connected to the same device as before. The whine is no longer around. Moral: if you have the high pitched whine, unplug your RS232 cable (if any) and see if it stops. By the way, I've had my 1040ST since March and have only powered it off about 4 times (mostly because of power failures). I've had no problems with overheating (room temp is usually 65-80 degrees F). I chose to try rarely power cycling my 1040 as a way of minimizing component failures. So far I have had no problems (but this is only one sample!). I do turn off my monitor when it is not in use.
braner@batcomputer.TN.CORNELL.EDU (braner) (09/27/86)
[] According to Atari (in a memo they sent to dealers), the high-pitched whine comes from the "inverter" chip, a little DIP on the mother-board that converts +12V to -12V. It oscillates to do it, ==> whine. The only (?) load on the -12V line is the RS232 port and related driver chip (1488). So it is not surprising that whatever is plugged in there affects the whine. Atari has also sent the dealers instructions on how to eliminate the whine, which I posted here a while ago. - Moshe Braner
ZSR@PSUVMB.BITNET (09/30/86)
Hmmm...does anyone have any ideas why my Color Monitor whines once and a while? It only does it about 15-30 minutes after I turn it on and only lasts for about two minutes, but it is a DEFINITE whine. The picture's great, but the sound needs work :-)
braner@batcomputer.TN.CORNELL.EDU (braner) (10/09/86)
[]
Due to repeated requests, a reposting:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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
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