Re: Star Wars HV unit over-voltage

Hi all,

I thought I'd post a follow up to the HV over-voltage problem I had. Last time
I told you that I simply cranked the pots to their extremes, and it came up.
 Well, this weekend while working on the deflection board, that little trick
didn't work any longer :-(. Anyway, here's how I fixed it. You may want to
bring up
http://www.gamearchive.com/video/manufacturer/atari/vector/monitors/amplifone/amplifone_hv_adjust.gif
as a reference.

Now, the first thing they say you should do is verify the +24VDC and -24VDC on
the board. Well, the values were pretty close, so I figured they were ok.
 However, as I didn't realize, the voltages are only valid if the transistors
are under a load of some kind. But since my HV was always being shut off by
the overvoltage circuitry, there was no load on the regulators.

I finally decided to disable the HV overvoltage circuitry. This can be done by
lifting the leg of diode CR1 (1N914) by R7. I put the HV probe on under the
anode cup (with the ground lead of the HV probe connected to the DAG) and fired
up the game. The HV was around 27KV, so I assumed the HV overvoltage circuit
was indeed working correctly. There was a picture, but I had to turn down the
screen adjustment to keep from burning the tube.

Now that the HV was actually running, I could take some readings of the voltage
regulators. -24VDC was only about -12VDC. The large 5W resistor across the
input and output had actually been doing the work instead of the regulator.
 Replacing the -24VDC regulator brought the HV down to 25KV, but it was still
too high for the overvoltage protection circuitry.

After verifying the rest of the devices in the circuit, I could not figure out
what was wrong. I took another look at the schematics and figured out how the
555 timer was used as some sort of loop back circuit which stepped up the HV.
 Everything in the circuit checked out, so I finally decided just to modify the
circuit.

A simple way to increase or decrease the KV was to change the value of R6 (3.9K
ohm resistor). Putting in more resistance increases the KV, while decreasing
the value decreases the voltage. I replace mine with a 2K ohm resistor, and
rechecked the HV, which was now around 20KV.

I reconnected the HV overvoltage protection circuitry and the LED was off
(yay!). Resoldering the CR1 diode brought the KV down a little (for some
reason), but I was able to adjust it back up with R7 (frequency adjust). I was
then able to set the HV over-voltage (R17) correctly.

So that was it. Maybe there's a few tips in here that Gregg Woodcock can put
in his FAQ (Pull up a leg of CR1 to disable HV overvoltage circuitry, change R6
to increase/decrease HV, whatever...)

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Mark Jenison E-mail address: jenison@cig.mot.com
Cellular Infrastructure Group Motorola--Arlington Heights, IL
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Received on Mon Oct 19 13:55:33 1998