At 09:20 -0800 11/30/09, Ivan X wrote: >However, when I have the board in, there's an irritating high-pitched >whine from the speaker. It isn't on for a couple of seconds during >power up, but then it cuts in -- it appears to be related to the >display. In fact, if I change the resolution of the display (you can >set 560x384 if you have an Apple IIe card), the pitch of the whine >changes. Likewise, if I let the screen turn off (from the Screen >control panel), the whine goes away. > >When I use the original board, no whine. >
What you describe is almost surely leakage from the horizontal deflection coils of the CRT monitor into the audio circuitry on the motherboard. It's possible that the magnetic field is exciting a vibration in some metallic element that is not the speaker but when you mention that it goes away with the old mother board that becomes unlikely. The analog board supplies power to the motherboard at +12, +5, and, I think -5 volts. It's possible that some of the horizontal drive signal is showing up on one of those wires and being coupled into the audio driver. Marginal filter capacitors on those power lines are a real possibility. There is a lot of information around on cleaning boards and replacing capacitors for older Mac 128 through SE/30's I don't know about the more modern classics but if the board has those aluminum electrolytics on it - the usually blue round cans that stand up fairly high - it's worth consideration. It's also possible that the analog board has weak components. The flyback transformer has been implicated on older "classics". If so a somewhat higher power usage on the new board may be stretching the current (two meanings there, both appropriate) capability of the analog board. Increasing the load on the power supply, like adding a IIe card, would be expected to make the symptoms worse. An oscilloscope looking at the power wires between analog and motherboard would answer a lot of questions but not everyone has that kind of equipment. Perhaps a friend who's a ham radio operator? A voltage check with a good meter might help. The +5 volts needs to be accurate. The others have a lot of tolerance. And. Your description of the problem is excellent. I wish others would pay that much attention to details. One other check you could make is to see if the whine shows up in headphones. -- --> There are 10 kinds of people: those who understand binary, and those who don't <-- -- ----- You received this message because you are a member of the Vintage Macs group. The list FAQ is at http://lowendmac.com/lists/vintagemacs.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To leave this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/vintage-macs Support for older Macs: http://lowendmac.com/services/
