Electronic Design

Bob's Mailbox

Dear RAP:
Recently I was shown a memo explaining that all manufacture of polystyrene capacitors will cease in five years, because the only manufacturer of polystyrene film stock has quit the business. If your circuits really need polystyrene, you should contact your favorite capacitor supplier to make your one-time buy now. Then start redesigning your circuits.
LTC, Milpitas Cailf.

For many applications, teflon, polypropylene, or NPO ceramics are just about as good as polystyrene. But in special cases where the tempco or the linearity of tempco of polystyrene is ideal, you may have to go Back to the Drawing Board. Or the Bench. Or both. — RAP

Dear Mr. Pease:
In your September 16 "Bob's Mailbox,", there was a letter from Robert L. Nuckolls, III, Consulting Engineer, Wichita, Kansas, who told about having a Tandy Model 100 that he bought from Compuserve that held 32k of RAM. He took it along on trips because it fit in his briefcase. He could do lots of writing with this Tandy 100. then on his return from the trip, he could dump the contents into his computer.

Well, I called both Tandy and Compuserve, and neither one of them could tell me anything about such a device. I wondered if you can identify it further so I might look into buying one, or something similar. I do a little writing myself, although retired, and would find it very handy.

Incidently, I enjoy your columns, and have read all of your "What's All That Stuff About..." and found them very entertaining. I sent that magazine containing those articles to a writer friend, Sam Wilson; I'm sure he will enjoy them.
Dunmore, Penn.

Read the next letter. — RAP

Dear Robert:
Regarding your September 16 column: I have info that might be useful in finding yourself a Tandy 100. It seems this early laptop is still used by newspaper reporters, and a small company exists to buy, sell, and support it: Richard Hanson, Club 100, P.O. Box 23438, Pleasant Hill, CA 94523-0438; (510) 932-8856 voice, (510) 937-5039 fax, (510) 939-1246 bbs.

I use a Tandy 200 for the same reasons as reader Robert Nuckolls. It's a model 100 with a bigger 15-line display. It's harder to find than a 100, but I think the extra lines are worth the trouble. The model 102 is a "slimmer, lighter" 100. I'm told the 600 is an orphan.

In case you'd like further details, enclosed are a spare Club 100 catalog, a copy of their most asked questions page, and copies of the pertinent pages in a 1987 Tandy Computer Catalog.
Santa Ana, Calif.

P.S. Did you hear Kaiser Steel and Intel are merging and are going to produce Liberty Chips?

Roy, thanks for the info on "Appropriate Technology." I may buy one myself, since it is apparently easy to interface a Model 100 to a DOS computer. — RAP

I was very glad to see the anthology of "What's All This..." Regarding your "Mother May I?" article, here is my favorite MMI story.

A long time ago, there was this old computer system connected to a well-used bit of analytical instrumentation. The user had run out of pre-formatted 8-in. floppies to store the data for his dissertation, and the company that made the system was no longer in business. He tried to format a standard 8-in. floppy using the system and had no luck. Thus came the request for help to me.

I used the "help" command on the system and found the brief instructions on how to use the format command. I tried formatting the disk and t asked me, "Are you sure?" I answered "Yes". It asked me "Are you positive?" I again answered "Yes."

Nothing happened. I tried again. "Yes" twice again. Nothing. "Y." "yes." "YES." "yEs." Nothing got me past the second MMI. I sent the user on a futile search for the manual. It was long lost.

I finally got so peeved at the system that I started typing all sorts of affirmative responses to the "Are you positive?" question. Some of them couldn't be printed in mixed company.

You may have guessed it by now. I finally tried "positive" as the answer to "Are you positive?", and I was rewarded with the "formatting..." response and a happy chunking in the disk drive. Needless to say, I was less than impressed with that computer system. Keep up the good work.
Corvallis, Oreg.

With Computers like these for friends, who needs enemies? When old computers get balky like this, they either make that happy chunking sound, or I "chunk" them off a roof. — RAP

Dear Bob:
I've been following your discussions of audiophile power amplifiers and speaker wires. Perhaps you can clarify a question that's been nagging me for several years on this subject:

If we look at the whole system, the controlled output we really care about is the position of the speaker cone, not the amplifier output voltage. In the limit, why not put a position sensor on the speaker cone and close the loop around that? For normal voice-coil-operated speakers, the displacement of the cone is proportional to the current through the speaker coil (though this is probably a first order approximation, it is certainly closer than saying the displacement is proportional to the voltage across the speaker). Therefore, why aren't audio amplifiers deigned with closed loops around their output currents?

About 15 years ago, I worked for a company that made linear magnetic-deflection amplifiers (i.e. closed-loop current out, voltage in). Some of the techs took these home and had good results using them as power amplifiers for their stereos. I never delved into this further. An added benefit of closing the loop around the output current is that the amplifier is intrinsically short circuit protected. Shorted outputs used to be, and probably still are the leading cause of power amplifier failures. This seems so obvious, I'm sure I'm missing something. Am I?
Phillips Scientific
Ramsey, N.J.

I am sure some engineers have done this, or even closed the loop with a sensor for cone position. Maybe this did not produce "the right sound." I've heard that the natural low-frequency distortion of most speakers "sounds right," and speakers that don't distort at 20 Hz do not sound very "impressive." Maybe less distortion is not as right as more. — RAP

Dear Bob:
Re the Sept. 16 Bob's Mailbox: The "thunk" Mike Middleton hears when he turns on his TV is the high inrush of current to the degaussing coil. Hit the degauss button on your large-screen color monitor and you will hear the same sound as when it's initially turned on. It sounds impressive, but probably doesn't stress any components except for the MOV in the degauss circuit.
Durham, Conn.

Still, if you turn on the TV or monitor 1000 times a day, that's probably not a good idea. If the degaussing coil or the MOV don't wear out, the switch probably will. — RAP

All for now. / Comments invited! RAP / Robert A. Pease / Engineer

Mail Stop D2597A
National Semiconductor
P.O. Box 58090
Santa Clara, CA 95052-8090

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