Punch cards

[Tim]

On Mon, 2008-04-07 at 12:53 -0400, Gene Heskett wrote:
> I had an alcoholic uncle who, during WW-II, worked as a postal delivery type in 
> downtown Des Moines, and he usually had a radio in the bottom of his mailbag, 
> either going back to a customer after repairs, or given to him to be repaired 
> while he walked his route.  I think I was about 8, maybe 9 when I asked him 
> as he was changing the filter capacitor in one of those 'all american 5 
> tubers', what was actually wrong with the part he was taking out, and he 
> couldn't tell me!  He was going by a recipe on the inside of the cupboard 
> door that said to change them if the radio had a hum.

I remember doing work experience at a local TV repair shop while I was
studying electronics.  They had this set full of gremlims that had been
plaguing them for weeks.  Their best tech had spent ages repairing one
fault after another, but it was still coming up with more things needing
fixing.  After he expressed frustration, one of the other road techs
(the ones that do the house calls) had a bash at it, literally, to
finally get rid of the thing.  Poking and prodding boards, rather
roughly, with the plastic end of his screwdriver, finding everything
that was unreliable and needing replacing.  The set that had be worked
on, off and on, over weeks, was fixed in about a quarter of an
hour.  ;-)

In his case the rough treatment wasn't ignorance, he knew his
electronics, too.  But he did know that most TV faults are down to crap
soldering, and a bit of force turns the intermittent weak points into
broken points that you can fix up.  You get all the frustrating to find
faults dealt with very quickly, then you can tinker with the odd one or
two other things left over.  The road techs would fix several sets a
day, with no test equipment.  The in-house techs might fix a few a day,
but they'd be measuring all over the place.

Of course, these days there's far less electronic component repair, and
it's often a case of working out which of the boards is faulty, and
replacing the whole board.

> I learned just how educational the oscilloscope can be, and I've never
> been without one since.  Being able to watch a circuit do its thing,
> in real time, is worth 100x what all the math is when its time to see
> why its not working as planned, although there is a place for the math
> too at times.

I think I probably bought mine when I was 17, and I've still got it.
It's got a few gremlins, but I know how *my* tools work...

They started to become a reasonable purchase price for enthusiasts back
then, this being a reasonable 25 MHz dual-channel thing, with a proper
10 by 8 cm internal graticule, rectangular screen.  Not those hokey 5 cm
round things, with massive parallax errors on the external faceplate
graticule, and just single channel.

Though I did get given a free one of those (dinky little CROs) by a
grateful client last year.  It may not be brilliant, but it's a nice
little portable servicing gadget, making it possible to drag a CRO into
some place where you usually wouldn't like to bother.

They are nearly the best tool for working on anything that's not
steady-state DC voltage circuits.  Even then they're useful tools for
finding just what crap actually is in a circuit when you're expecting
unchanging DC signals.  A multimeter just doesn't show you things like
it can.

Electronics was my first love, long before computing.  I started out
tinkering, then bought a few kits.  I've still got the first one, a ring
modulator with lots of adjustments to play with that makes your voice
sound like a Dalek from Dr Who (I must try that out on a telemarketer
one day).  Then went and studied electronics (RF, digital, video, audio,
and general electronics, with a bit of computing thrown in), but ended
up going into video production, instead of the electronics side of it.
Now I service my gear, others gear, build what I can't buy or modify
existing stuff to suit.  Lots of fun...

The lastest thing to expend my time on has been restoring a 1960s or
1970s audio mixer (haven't quited figured it out), with all discrete
components (you could keep it going until it mechanially falls apart),
multi-tapped coil passive equalisation, P&G faders, and transformer
coupling everywhere.  While playing, I discovered something I really
didn't expect - if you ground one side or the other of a floating
transformer output, the voltage across the output terminals changes.

Crude ASCII-art transformer cct:

  signal source -----  ||  ------|
                     3 || E      |
                     3 || E    VU meter on output
                     3 || E      |
                     3 || E      |
  ground  -----------  ||  ------|

You get three different meter readings depending on whether you don't
ground either output terminal, or which side of the meter you do ground.
There's about a 2dB change.  No, it's not grounded anywhere else, nor is
anything other than a passive VU meter connected to the transformer, and
I can't find any primary/secondary leakage.  The sound source is a
symmetrical sine wave.

There's always something out there to surprise you.

-- 
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