Twice in the past three years I have experienced failure in the electronics of the printer used with my TS2068 system. The second time, the Oliger printer port card failed as well, so I wrote to John for advice. The part of his letter that follows may be of interest to others as it was to me:
“I would suggest replacing the 74LS374 data output buffer chip on the printer port. I would guess your problem is being caused by a grounding problem. I have heard from at least one other who was having this kind of problem, and it was traced eventually to a power strip that used varistors as a filter within the strip for surge protection. The varistors many times are wired both across the power lines and to the grounding “U” ground of each outlet. Because the 2068 uses a DC power pack it is itself not grounded, but can become grounded by connections to printers and disk drives that do have U-grounded cords. If one of the power strips is not grounded as well as another one of the strip outlets is, or if, say, a monitor is plugged into a different power source than a disk drive power supply is, then this ‘noise’ removed from the incoming power lines can be shunted from, say, the printer to the drive enclosure through the 2068! Bottom line: If you have more than one device with U-ground plugs connected to your computer, be sure they are plugged into the same outlet or use a power strip on all such devices that does not attempt to filter the power line.”
Following John’s suggestion I found that the probable cause for the port and printer being zapped was via a mechanical Centronics switch between the C. Itoh printer on the same electrical outlet used by my TS2068 and the Epson printer on a different surge protected outlet. I did find that replacing the 74LS374 on the TS2068 printer port card restored it to operation, but the printer which I was using at the time it failed still gave out garbage, printing only random characters. This indicated that the printer buffer chip had gone out also, as it did about three years ago when I had to enlist the services of Dan Elliott to get the printer going again.
I experimented with a digital voltmeter and an extension cord between the outlet I use for my TS2068 system and a surge protected outlet that is supplied by the other side of the main service. I got a reading of not 110 volts, nor the 220 I expected, but 440! This excess voltage appears to be a sort of surface potential, but certainly is enough to zap an IC — and possibly you! The circuit capacitance, combined with a varistor used as a surge protector, apparently acts as a voltage-doubler. While this is not the same as the surge or ‘line noise’ John refers to, it is well to keep in mind that this voltage potential exists between devices which happen to be connected to opposite sides of electrical main circuits.
I now have isolated my entire TS2068 system from any surge protected circuits not on the same outlet with it. It appears very possible that the failure of the data buffer chips on some of my disk drives might have been linked to this source also.
Why these problems occurred only at such widely-separated intervals remains a mystery to me. I’ll leave it to you to decide whether or not your system may be prone to similar ‘accidents just waiting to happen.’