[sdiy] Tandy modem cap tale (longish)

[Paul Schreiber]

 Speaking about modems, did you ever tell your "funny modem capacitor story" as
> mentioned in the old concertmate post?

If not.....

My first real design job at Tandy was to design a "direct connect" 300 baud 
modem. 95% of modems used acoustic couplers, where you crammed the handpiece 
into 2 rubber "cups". The only real consumer (not AT&T) direct connect modem at 
the time was a S-100 buss version from Hayes (old farts nod head in 
remembrance), but it was $399 (gulp!). Mine was to be $99 retail at RadioShack.

Once I designed the DAA (transformer-based circuitry that couples to the 
telephone line) I wrote a BASIC program in a TRS-80 Model I that was a filter 
optimization for the receive & transmit filters (modem tones are in the 1000 to 
2000H range). I wanted to use all the same cap values (there were 6 in the 
receive and 4 in the transmit) so the program stepped through standard cap 
values and calculated the resistors. I then manually looked for the "best fit" 
that didn't give too wacky resistors (like 5 ohms or 800K). The best fit was 
4700pf.

At my previous job at Data General I was working on CRT circuits. The vertical 
deflection amplifier had a clever, patented circuit that used a ringing bandpass 
filter to "warp" the sweep to compensate for the curvature of the CRT glass. It 
used 2 polypropylene film caps from a company called ElectroCube. They were 
about 1" long and big around as a pencil. We used 2% @250V and paid around $1ea 
at 25,000.

So they were the first company I called. I asked for a quote for 250,000 4700pf 
1% 50V "in your smallest case size". They were VERY skeptical because of the 
large quantity but after a week or so quoted 89 cents. Well, that meant $8.90 of 
caps and my entire budget was $25. Not good.

I then got out the EDN Gold Book directory (more old fart nodding) which was 
like a "Yellow Pages" for the electronics component industry. There were about 
15 vendors listed for Capacitors - Film - Precision so I called every one. Over 
*half* of them did not even BELIEVE ME because their *annual production* was < 
50K. Not one person was able to beat $1, much less 89 cents.

I started to panic a bit. I then pulled out the Japanese equivalent book, a 
massive *28 pound* hard-bound book (over 4,000 pages) called the EIAJ Sourcing 
Guide. In there I saw an ad from Panasonic about their "new, lower cost film 
caps". So I sent them a Telex (Google that you young turks).

A few days later I got a Telex back. The quote was for 18 *cents*.

Well, I figured THAT was wrong, they probably quoted +-10% not +-1%. So I sent 
the quote again, spelling out these were *ONE PER CENT* tolerance. The next day, 
here comes another Telex:

"Please accept utmost humble apology for incorrect previous quote. Corrected 
price is $0.16".

Well, I laughed and more as a "get lost you idiots" gesture asked for 100 
samples, sent Fed-Ex with FULL test data on EVERY CAP in the sample lot.

1 week later, here came the package: a 120 page *hand typed* report (1 cap per 
page!), 100 samples, each with a *tiny* serial number. And the caps were about 
1/4th the size of the ElectroCube. So I bought 1.7 million of them over 4 years.

Epilogue: When the ElectroCube sales guy called looking for the order, I told 
him I found another source "under 25 cents each". HE didn't believe me, he had 
the president and chief engineer of ElectroCube call me. THEY didn't believe me, 
either. After the design was in the stores, they went and bought one. They 
called me up asking if Tandy would consider *reselling* them the caps!!! I 
LOL'd.

Paul S.