The week of repairing calculators

[David Ramsey]

Well, so far:

-- 9830 tape drive problem: Paused until I can drive to San Jose and have a debugging session with EE friend that actually has a logic analyzer.

-- 9810 printer repair: Awaiting Cold Shrink™ tubing. Printer removed and will clean today.

-- Secondary 9810 display repair: see item #1. This parts machine might not have been my wisest purchase.

-- SR-60 card reader gummy wheel repair: finally got around to this yesterday. Turned out to be much simpler than I'd thought it would be, about on par with 9810 card reader work. Typical O ring repair that works perfectly.

-- SR-60 power supply(?) repair: sadly, the SR-60-- which, for a few shining minutes, was completely functional-- promptly died in the middle of running a test program. Display blanked out and calculator appears dead. When turned off and on the display will flash for a brief second, and sometimes the printer stutters. Will check PS voltages today.

So, overall...not doing that well (though certainly not for the lack of helpful suggestions from the folks here!) I think I really need to learn some basic electronics and graduate beyond a $10 Radio Shack multimeter.

[Dave Shaffer (Arizona)]

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graduate beyond a $10 Radio Shack multimeter.

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Depending on your investment so far (in the machines themselves!), look into used test equipment on TAS. I got a 200 MHz dual-channel Tektronix scope a few years ago for not much more than $100. The front panel is a little beat up, but it works fine.

[David Ramsey]

The SR-60 thing seems to have been a bad connection, which is fixed (crossed fingers) now. Unlike HP machines of the same era, the SR-60 is not full of gold.

I assume a "dual channel" scope simply means I can view two simultaneous inputs, right?

[Dave Shaffer (Arizona)]

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I assume a "dual channel" scope simply means I can view two simultaneous inputs, right?

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Yep. Typically, you can view them in alternate mode (one channel shows, then the other, as triggered either on the signal or with some kind of external sync), add them (not so great, usually, although you can invert an input, so they are in fact subtracting), or "chop," where the display alternately selects one or the other input at a rate faster than the sweep, so you are really viewing them both (nearly) simultaneously. Works best for relatively slow signals.

[Tony Duell]

Test equipment is undoubtedly very important (in that you can't 'see' the logic signals without it), but remember that there is no magic box you can plug into a calculator which will say 'Replace U3 on the Memory Addres PCB' (or whatever).

IMHO the most important piece of 'test gear' is a brain. Seriously. You can get a long way with simple instruments if you (a) know exactly what they tell you and (b) know how to interpret that information.

That said, I do find a good logic analyser to be very useful when repairing CPU/memory problems in the 9800s. I don't find a 'scope all that useful actually, but that's a personal thing.

[Eric Smith]

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IMHO the most important piece of 'test gear' is a brain.

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Agreed!

I feel like I need a new one, though. The one I've got now is definitely past its "best by" date. The data retention has substantially deteriorated, and I don't think the processing is working as well as it once did.

Eric

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"Yes, an electronic brain," said Frankie, "a simple one would suffice."

"A simple one!" wailed Arthur.

"Yeah," said Zaphod with a sudden evil grin, "you'd just have to program it to say What? and I don't understand and Where's the tea? --- who'd know the difference?"

"What?" cried Arthur, backing away still further.

"See what I mean?" said Zaphod and howled with pain because of something that Trillian did at that moment.

"I'd notice the difference," said Arthur.

"No you wouldn't," said Frankie mouse, "you'd be programmed not to."

-- Douglas Adams, The Hitch Hiker's Guide to the Galaxy

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[David Ramsey]

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That said, I do find a good logic analyser to be very useful when repairing CPU/memory problems in the 9800s. I don't find a 'scope all that useful actually, but that's a personal thing.

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The ability to use a logic analyzer effectively in cases like this requires a degree of hardware knowledge and experience most of us don't have. I've been a programmer since the 70s, but that doesn't help much!

Still, I wrote numeric display drivers for the Motorola 6800, so I know the basic principles behind them (turn the segments on and off real fast and let persistence of vision convince the user they're looking at an "8" instead of 7 segments flickering on and off). So I might be able to repair the 9810 display board since I actually understand how it works.

I might also add that in addition to a brain, a community of helpful folks like this is important, too.

Edited: 25 Aug 2011, 12:16 p.m.