mason@robots.ox.ac.uk (Ian Mason) (03/14/91)
I am posting this for someone. However he reads this (comp.sys.ibm.pc.hardware) newsgroup. -------------------forwarded message-------------------- I have bought a 2nd hand XT with a 135W power supply, and then upgraded the motherboard to a 386SX-20MHz. At the moment it is running in a configuration of a Hercules-compatible graphics hard, a floppy disk controller (5.25in 360k drive), and a MFM 20M Seagate hard drive. The problem is that the power supply is giving out high-frequency noise when idle. i.e. when neither the hard disk or the floppy disk are in use. e.g. The noise dies when I do a dir on a:. [There was no such noise with the XT board.] Can anybody tell me what is the problem and cure? I understand that the noise could well be from the switching elements of the switch-mode power supply. It doesn't seem like a not-enough-power problem, since it only happens when idle. Any hints welcome. Thanks in advance. Delman. ------------------------------------------------------------------------------ Delman Lee Dept. of Engineering Science, Oxford University, Parks Road, Internet & BITNET: delman@sun.eng.ox.ac.uk Oxford, OX1 3PJ, U.K.. JANET: delman@uk.ac.ox.eng.sun ------------------------------------------------------------------------------
dd2x+@andrew.cmu.edu (David Eugene Dwiggins) (03/17/91)
Well, I wouldn't play with it--I'd replace the power supply immediately. Power supplies are lots cheaper than motherboards/hard drives/some cards. Don't frustrate yourself by being too cheap and frying parts. David
kenw@col.hp.com (Ken Wyatt) (03/21/91)
In have designed switching power supplies in one of my past lives. I am not sure what the switching frequency of XT power supplies is, but let's assume that it is around 20 kHz (a common freq). What you are likely hearing is a subharmonic (10 kHz) or the fundamental (20 kHz - if your hearing is good!) causing the power transformer to vibrate. Transformer manufacturers reduce this effect by dipping them (under pressure) into a varnish-like goo which acts to hold all the transformer "hardware" together tightly. Some trans- former manufacturers skip this step to "save cost" and the consequence is sometimes a high-pitched squeal. By the way, this squeal could also be coming from any of the magnetic components, such as output filter chokes; but most likely, the power xfmr. If you are daring (!) you may try to locate the offending "squealer" by probing around with a PLASTIC rod (such as a BIC pen, or equivalent). A slight pressure may stop the squeal and perhaps jamming a NON-CONDUCTIVE object under/over/between the transformer may cause the squeal to cease. NOTE: To perform the above experiment requires the removal of the top lid of the power supply (about a dozen screws) - do not remove the mounting screws for the fan. There may be as much as 500 VOLTS connected to the power transformer and main switching transistor/FET (largest device near the power transformer). BE CAREFUL - KEEP ONE HAND IN POCKET - REMOVE ANY RINGS - HAVE A SECOND PERSON THERE READY TO UNPLUG THE LINE CORD - ETC. Interesting Note: Switching power supplies may be modeled as a negative resistance. Thus, they often draw more power when lightly loaded than when fully loaded. This might explain the squealing symptom during "idle". By the way --- my XT power supply started to squeal at start-up and then after a couple minutes fades away until it is not objectionable. It has been doing this for a year now with no bad effects. From my experience, squealing components do not usually mean "doomsday"; they are just a bother. One design I did for the MX missile required a switching frequency of 5 kHz. Now THAT was a bother! There was nothing we could do to reduce the "audio" from this supply. Bottom Line: In my opinion, the squealing does not mean impending failure. If it bothers you and you don't wish to fool around inside have it replaced. ----------------------------------------------------------------------- | Ken Wyatt | HPDesk: Ken Wyatt / HP0800 | | Product Regulations Mngr | UNIX: kenw@hpcol.hp.com | | HP/Colorado Springs Div. | FAX: (719) 590-7679 | | PO Box 2197 | Compuserve: 75665,1727 | | Colorado Springs, CO 80901 | Amateur Radio: WA6TTY | | Phone: 719-590-2852 | | -----------------------------------------------------------------------