Re: Amplifone MC1....

In a message dated 2/8/00 10:31:59 PM Eastern Standard Time, jess@magenta.com
writes:

<< Hi all,
   I decided to smash up one of my Magnetic Correction Units to see what was
 inside since I didn't think it has been documented here. It came apart quite
 nice after sitting in the freezer all day. What I found was sort of puzzling
(to
 me at least) can anyone help explain this....
>>

     WoW!! I was allways to parinoid to take one apart, for fear I was
destroying a part that is difficult to replace. Ed, are you listening? Did
you ever disect the one we pulled of that damn blue board a couple years
back??
     I'm going to repeat Zonn's explanation of the MCI - this appears in
version 1.1 of my Amplifone HV faq, but is NOT on the HTML version 1.0 on
Jess' GameArchive site. (You can get version 1.1 from GameArchive, but it's
in DOC format):

http://www.gamearchive.com/video/manufacturer/atari/vector/monitors/amplifone/
amphvfq11.doc

____________________________________________________________________
 It seems fairly apparent that the HV section of the Amplifone is regulated
much
like that of the old (and maybe some new) constant voltage AC supplies that
use
ferro-resonant transformers.
     Basically the idea is that a transformer can only be driven to a maximum
voltage, after which it saturates and an increase in input voltage does not
correspond (nearly as much) to an increase in output voltage. This maximum
voltage, before saturation, is highly dependent upon the resonance frequency
of
the transformer.
     The constant voltage AC regulators use a capacitor to tune a winding of
an
isolation transformer to 60hz, causing the transforming to go into saturation.
After which voltages of around 70v to 140v input has little effect on the
output, and changes in current loading also have a much smaller effect.
Pretty
cool.
     If you look at the schematic of the Amplifone HV you will see no
feedback from
the output voltage, back into any sort of regulator. In fact the Amplifone
contains no HV regulation circuit of any kind! (With the exception of the
over
voltage regulator, see the following note.)
The 555 is simply set to a frequency, which drives (unregulated) the Q1/Q2
amplifiers, which drive the primary of the current boost transformer T1, which
drives Q3, also with no regulation!
     I have no way of knowing exactly what's inside MC1, but the only way for
this
circuit to regulate it's output voltage, given changing output currents, would
be for some type of ferro-resonant regulation to be taking place. MC1 must
be a
type of tuning coil, possibly a coil and capacitor, that along with the
inductance of T2 is used to set the resonant frequency of a ferro-resonant
regulator. (Which is probably why T2 is so expensive and hard for other
companies re-engineer.)
       These companies are probably winding the HV to give the
proper increase in voltage, but not taking into account it's inductance and Q
ratings, which in most HVTs are not nearly as critical. As a result, when it
comes time to test the transformer in the circuit, they fail at regulating the
output voltage. Just a guess mind you, I wonder if anyone could verify this?)
MC1 combined with T2 must form a resonant circuit. As the 555 is brought
closer/farther from the resonant frequency of MC1/T2 the voltage coming out is
higher/lower.
___________________________________________________________________

Thanks again, Zonn!

     Just another side note, (this is also in the faq, but I've been getting
ALOT of questions from people about this). If your Amplifone HV unit is
blowing +/- 24 regs like crazy, and killing precious BU406D's, and you are
getting an INCONSISTANT waveform at the base of the BU406D, start suspecting
that the MCI may be bad.
     The damn thing is often located very close to the 5w 50 ohm resistor
near the reg, and the high heat from the resistor can sometimes damage the
MCI. If the red MCI case looks a bit deformed from the heat, suspect a bad
"red mystery can" if you are getting the above symptoms. Try to keep the 50
ohm resistors away from the regs and MCI.
      Sorry for the ramble, but I thought it was neat that Jess decided to
disasemble the MCI. Maybe now we can figure out a way to test MCI's???

MK
    
     
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Received on Wed Feb 9 00:42:43 2000