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7.0 Appendix: More Random Info

7.1 Using PC and Surplus Parts with Classic Macs

Finding parts for Classic Macs can be difﬁcult at times, so it’s occasionally useful to know

how to adapt components intended for the PC world to ﬁx a Mac.

7.1.1 Monitors

The 9” CRT in the classic Macs is deﬁnitely cute, but it is just as deﬁnitely small. Several

manufacturers once made adapters and monitors that not only expanded the screen’s size,

but also permitted more text to be displayed at any given time. Unlike those products

(which are no longer available), the modiﬁcation described here only gives you a larger

screen, period. It’s the electronic equivalent of putting a magnifying lens in front of the

existing CRT.

Most older PC monitors are at least 12” in size, and the added area is signiﬁcant. Unfortu-

nately, they're also often green or amber, and most people ﬁnd them a little hard on the

eyes when used with a Mac. If you can locate a black and white unit (somewhat rare),

you’ll be a lot happier with the results. You can also interface to a modern color monitor.

Beyond that consideration, there are two technical issues to contend with. One is that the

Mac produces horizontal sync and video signals that are logical inverses of what TTL PC

monitors expect (the vertical sync signal is just ﬁne, however). The second consideration

is that the ~22kHz horizontal sync frequency generated by the Mac is substantially higher

than the ~16kHz expected by old PC monitors, so you'll almost certainly have to ﬁddle

with the timing network in the monitor to insure synchronization (this issue usually disap-

pears if you are interfacing to a multisync monitor, but the instructions here don’t give the

pinouts for that). Inside some monitors, there is a small trimpot to adjust the frequency. In

others, you will have to change a resistor. Clearly, such a modiﬁcation is only for those

with advanced circuit skills. Schematics of the monitor are a big help, too. In many old

monitors, a 555 timer IC is used as the horizontal oscillator. Look for one (there may be

two) and, in particular, look for precision 1% resistors (typically there are two of them)

and/or a precision capacitor near the 555. These are almost certainly the timing elements.

Increasing the frequency by a factor of about 1.38 requires a reduction in RC product of

that same factor. The easiest method is probably to place a potentiometer in parallel with

the larger of the two precision resistors. Keep the leads absolutely as short as possible to

avoid interference from other signals generated nearby; you can fry the deﬂection circuitry

if you do not observe this precaution. Choose a potentiometer whose maximum resistance

is about 5x larger than the ﬁxed resistor it is to parallel. That should give you enough

adjustment range to get the monitor to lock to the Mac's higher sweep frequency. Set the

pot initially to the maximum resistance (and never allow the pot to go too low in value, or

you will once again risk frying the works; always start the pot in the position of maximum

resistance, and work down, taking care not to go too far), and then power up the monitor.

Adjust the pot until the display stabilizes. As an additional check, turn the monitor off then

on again, and verify that lock is maintained. If not, readjust the pot and repeat the test.

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Macintosh SE User Manual Page 11