lupienj@hpwadac.hp.com (John Lupien) (04/05/91)
I don't normally read this group, but I have come across a hardware problem that seems unusual, and I was wondering if anyone can offer some insight or guidance. A friend of mine has a Mac which works fine until it warms up. Once warm, the monitor intermittently goes blank. A rap on the side of the box (sometime a couple of raps) brings it back, and if you slant the system to the right, the symptom does not occur. I took the back off and checked the cables and connectors: all seem to be seated firmly, with no loose wires. Having no expertese regarding the circuitry, I did not want to poke around any deeper without guidance (or help). If anyone recognizes the symptom, please let me know. If not, I would be glad to hear about folks in the eastern Mass or RI area that could be trusted to properly trouble-shoot the problem at not-too-exorbitant rates. If you have any help to offer, please send e-mail. I will monitor the newsgroup, but since newsgroup dissemination can be rather slow, a note would be appre- ciated. Thanks, --- John R. Lupien lupienj@hpwarq.hp.com
steveh@tasman.cc.utas.edu.au (Steven Howell) (04/13/91)
Assuming its a mac plus, then the chances are you have simply a dry joint. ie a bad solder connection between component and the copper circuit board This is a common failure point to the macintosh (128,512,plus) and simply repaired by removing the old solder and applying new solder to the joint. For the screen to go out, but the mac still operates as normal, even though you cant see it, points to several locations of the power/sweep board. Very Commonly, and usually is J1. This connection has a tendancy of failing on the horizontal outputs (The two connections on J1 closest to the top of the board) due to high current oscillating voltages. You may have to peel up the insulation support adhesive pad to get to it. Along the same lines is C1. It can develop bad joints around its two leads, same goes for the in series L2, and unusually L3. all located at the top of the board. Also prone to bad joints are T1 J2 and commomly also J4. The information comming into the sweep board from the logic is oscillating at approx 22KHz. This causes the the Horizontal, Vertical and Video joints on J4 to fail. Try these first. the majority of faults related to video are usually within this area. This of course is assuming that a component has NOT failed, but basically has a dry joint, which i suspect. See How You Go ay'. Steve h