Atari/Amplifone Raster Flyback replacement conclusion...

  I wanted to report back here for the archives that I successfully have
my Amplifone raster deflection PCB up and running with a Well-Gardner
K7000 flyback installed in place with some fairly minor modifications.
In actuality, the largest modifications had to do with putting in a new
picture tube and needing to use the deflection coil that came with the
tube. As I posted in earlier threads, I ended up modifying the
deflection capacitor and the retrace capacitor to tweak the new
tube/coil. I had bought a used a slightly abused Amplifone deflection
coil from John (thanks John!) but it had some arcing issues when it was
actually running so I went back to the stock deflection coil.

There were a few issues I ran into...

1. HV regulation - since the amplifone HV transformer used core
saturation to regulate the anode voltage, the WG flyback essentially has
no HV regulation. I used the adjustable B+ to get the anode at 19.5Kv
and interestingly, unless the screen is a full white raster, the HV
actually stays pretty close to the adjusted voltage. When the raster is
bright, the anode voltage is lower so there isn't a safety issue. If I
really look closely under normal game conditions I can see some slight
blooming, but it is really very very slight and I don't think it is
noticeable unless you really really scrutinize it.

2. HV protection - since the original Amplifone flyback used the
focus/screen ground (high impedance) in the feedback circuit for the HV
protection and the WG flyback internally connects the the filament to
the same ground as the focus/screen, I had to do further mods since the
filament doesn't work in series with a 56K resistor. I used the anode
current limiting resistor instead of the focus/screen and tweaked the
resistor divider in the HV protection to get proper shutdown if the
anode goes above 25Kv.

I will make a formal schematic over the weekend and get it posted on my
Amplifone raster page. Considering the great information I got from Mark
about the vector HV transformer 'ultra-tuned' finickyness and the fact
that both of my amplifone raster flybacks 'rung' okay, but didn't work,
I am more than happy with this solution.

So, against all odds, I now have a oddball I,Robot *cabaret *up and
running with the original switching power supply and an original
Amplifone 19" raster monitor. The PCB is is a proto and labeled 'Ice
World'. I will post some pictures this weekend of the whole enchilada.

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Received on Sat Feb 12 02:52:46 2011