[Coco] An open letter to the coco community

Wed May 7 23:04:49 EDT 2014

On Wednesday 07 May 2014 22:25:20 CoCoList for Color Computer Enthusiasts 
did opine:

> Friends,
> I am writing this  letter to address the problem of the cost
> of producing my  converter.   Since I have retired I have the time
> to accommodate  everyone.  Also my full pension kicked in this
> month so I don't need the  money.
> 
> 
> The cost to build the units is  approximately $120 USD.
> Isn't that ridiculous?    So this is  my proposal.    I am going
> to ask $200 per unit for my  converter to those who can EASILY
> afford it.   But if you can't, I  will accept any amount you can
> comfortably manage, even if is  nothing.   What is the reason
> for this policy  change?    Because the old low frequency
> CRT''s are  disappearing fast.   I have fixed enough of the older
> TV's in my  time to realize they have a fatal design flaw and
> Gene can probably back me  up on this.    The deflection
> yoke coils are driven by the  horizontal and vertical oscillators
> They are heavily stressed by high energy  magnetic fields.
> This causes excessive vibration and heat in  the
> coils.   I have found that this combination carbonizes  and
> removes the enamel insulating the wire of the coils causing the  coils
> impedance and resistance components to drop which
> overloads the  H.O.T. (horizontal output transistor) leading
> to catastrophic  failure.   If you just replace the H.O.T. it
> immediately fails and  usually you cant see the problem because
> the damage occurs on the inside of  the coil where it makes
> sharp turns in direction.   Since the TV is  old good luck
> finding another yoke coil which is a shame because the  rest
> of the unit is in pretty good shape for its  age.

Actually Roy, I find far more output transistors have been destroyed by the 
nickel & even penny foolish driver circuits.

The thing is a switch, pure and simple, unlike the much older tube 
circuits.  It is up to the driver to maintain the transistor solidly in the 
on & saturated state until its time to turn it off, at which point the rest 
of the tuned circuit causes the retrace back to the left side of the 
screen, and the "damper diode" then supplies the rest of the reset the core 
back to zero current to get the beam back to the center of the screen.  At 
about that point, the transistor turns on again, and the beam is moved to 
the right at a rate determined by the power supply voltage and the 
inductance of the transformer.

It all works great when its nice & new.  But guess what?  The bean counters 
said they can't use a fancy pulse characterized coupling transformer to 
drive the final, costs too much money.  So they typically used a 2.2 to 4.7 
uf low voltage electrolytic capacitor.  But those things have a horrible 
failure rate when they are used at way below their voltage ratings, and the 
4.7uf they put in on the production line often measures both at half its 
nameplate capacitance, and with a considerable amount of equivalent series 
resistance after a years service.  This resistance not only creates a 
little heat because it impeed's the AC current flow but it also reduces the 
drive available to keep the transistor turned on solid.  And then it comes 
out of saturation, its own power losses rise on a curve that runs vertical, 
taking an even bigger bite out of the 300+ watts of power being 
recirculated in the properly function circuit.  So the transistor, for the 
last 5 millionths of a second, every 63.6667 millionths of a second, heats 
pretty rapidly until such time as it fails, generally blowing the epoxy off 
its body, and the resultant short circuit claims lots of the rest of the 
circuit as collateral damage long before the 75 cent fuse can react.

Generally, when I was doing that sort of repairs at the tv station, that 
particular capacitor got replaced by a huge and expensive epoxy dipped 
paper/mylar of at least 5 uf.  Then replacing the rest of the smoke 
containers that failed, generally meant that monitor was good until the CRT 
got too dim or even gassy & off color.

> Soooo.......it is imperative  something be done to address
> this problem and I am in a unique position to  invest the time
> and up front money to make a modern LCD monitor  available
> to every CoCo enthusiast.
> 
> Now my converter isn't perfect.   It  actually does too good
> of a job rendering the signal.    There  is broadband noise coming
> out of the GIME chip.    The green  signal is perfect.   The red
> signal is slightly off and the pure  blue signal is so out of line
> as to make the text difficult to  read.   The old slow CRT's did
> a lot of natural filtering and of  course Radio Shack took full
> advantage of this by ending development when it  was just good enough
> for the CM-8.  I have addressed the problem  somewhat by adding both
> input and output low pass filters to the  unit.   Its not a perfect
> solution but it does help  a  lot.
> 
So would getting the  full 5 volts into the GIME.  Those I have measured (3 
so far), because of idiot bean counters, are NOT running of the 5 volts the 
psu is making but closer to 4.85, and for the GIME, thats huge, making it 
sensitive to every light switch in the house being turned on & off.  If I 
get back to where I can dig into this some more, I am going to do an 
experiment on mine by making a slightly adjustable 5 volts to run just the 
GIME on.  Why?  Because where the schematic calls for a ferrite bead noise 
filter, the shack used 10 ohm resistors, costing the GIME around 100 
millivolts, so the output fades rapidly.  I doubled the strength of the 
monitors image by jumpering that nickel resistor.  And I suspect even the 
blue output might be 100% usable if I fed it 5.1 volts.

I said if I can get back, the keyword is "back", mine is collapsing, as I 
approach 80 yo, I now measure 5'2.5", where 50 years ago I was 5'8".

So today I bit the bullet and had 2 nerve blocks put in so maybe in a day 
or 3 I can even walk well again.  Its been touch and go for nearly 2 years 
if I had to walk more than 100 feet straight.

> Anyway , I figure with the caliber of  people in this group I will
> probably break even or close to  it.
> 
> I also figure I can turn out 3 units a week  working at a leisurely
> pace.
> 
I don't know as I'd call that leisurely. ;-)

> So there  it is friends.   My life's mission is set before me.
> I probably  won't get bored anytime soon.  :)))
> 
> Roy
> 
And as I'm fond of saying, well, it does keep me out of the bars. ;-)
> 
> --
> Coco mailing list
> Coco at maltedmedia.com
> http://five.pairlist.net/mailman/listinfo/coco

Cheers, Gene
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>
US V Castleman, SCOTUS, Mar 2014 is grounds for Impeaching SCOTUS