AW: The merlin project. Update and tip.

[The Dark force of dance]

Y-ellow Juergen 'n' diodes.

At 04:38 PM 2/9/98 +0100, Haible Juergen wrote:

>Hey, that's a good idea! 
>But what's good on a Wurlitzer Piano cannot be bad on electronic
>components. Heard about cutting parts off ceramic capicitors,
>but not filing resistors, so far.

Now that's a new one on me. I've not heard of any way of trimming caps.
Please illucidate. :)

>Question is do we risk any other damage from the vibration that is
>involved. Or, taking it one step further, would it also work with the
>resistors already soldered in? (vocoder "file-tuning", opamp and ota
>offsets ...) I guess the solder connections would be stressed too much,
>and a file needs space to work ... so maybe fixing the resistor with
>epoxy on board, and using a little rotary grinder ?

As I was just explaining for one reason or another, to Paul P. This
technique of filing resistors really does date back in my family. My Late
farther use to work for what is known as WRE. Weapons Research
Establishment. This once clasified organization is quite famous and was
responsible for launching all those rockets at Woomera. In particular the US
Red-stone rockets of the 1950s.

Back in those days, the best resistors available were 5% tollerance. Most
were 10 and 20 % tollerance. Let alone the 0.1% tollerences you can get
today. (6 bux each for a quater watt resistor down here) Because of this
they didn't have the E96 range of values. I don't even think they had the
E24 range. Only the standard toy shop variety E12. 

So often it was the case that they would file up a resistor to the exact
value they needed for this reason but there was another. Trim pots were big
and bulky in the 1950s. Rockets take off with considerable G-force. Which
teneded to rip components from the boards. The stuff you call "silly putty".
(That stuff that's like plastercene and yet bounces.) Was actually invented
by some chemical engineers out at WRE. Only the original stuff, some of
which I have here, is also a liquad. Over night it will run to fill up a
cavity. So they would pour this stuff into circuit board cavities to cussion
everything. But as you can imagine, big bulky trim pots were not a real good
idea in those days. So instead they'd file the resistors. Coating them with
a little nail pollish and then put them into rockets.

My point here is that if they can withstand being fired up at 50 G and then
survive crashing to earth a thousand miles down range, I think they'll
survive almost anything you could do to them in the process of filing.

It is true that metal might get into your circuit board if you filed them
in-circuit. But I never actually thought of doing it like that. It's worth a
thought though. The main idea here is to trim up a resistor before you
insert it in circuit. Creating a custom value resistor for the exact job.
Filing requires little effort. It's not like you're filing it with a power
file. Some gentle rubbs with a needle file is all it takes. Once you're
through the outer coating and into the resistive material you can watch the
resistance climb on your meter. As you approach the desired value you can
get even more gentle unitl it just tweeks up. Once you've got it there you
can put some nail polish on it and make sure it's stable. In reality I've
not had one that didn't remain stable. Even in variations in temperature.

The point here though is that I was always under the impression that it
couldnt' be done with metal film resistors for some reason. Perhaps my old
man had thought metal films equated to "wire wound"? I'll never know but I
just thought I'd try it last night and it worked fairly spectacularly.
Because the metal film resistors are tougher to file, you can get more
acuracy in filing. It should also be noted that most metal films, apart from
the crap you get at toy shops, are actually .6 watt these days. Not .25
watt. So you needent worry about loosing the rating. Which, as an asside,
brings me to wonder. "Where do these toy shops get their crap components
from?" I mean is there a "Crap-componet" wholesaler somewhere in Asia that
specializes in selling junk components on the basis that the average
hobbyist will never realize? But that's another story.

So that's basically the deal. Now Juergen, Tell us about the ceramic
capacitor thing.

Hope this helps.

Be absolutely ICebox.





>JH.
>
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