Re: WG failures

At 12:03 PM 9/3/97 -0700, you wrote:
>
>clay said:
>"In my experience, the VAST amount of problems have been with the power
>supply section of the deflection boards. I've actually experienced very
>few failures related to the deflection stages themselves."
>
>This makes sense. According to my analog designer friend at work, the
>WG deflection amp looked to be a much more stable design than the G08
>but he had questions about the current requirements of the pass transistors
>in the regulator (since it has to supply current for both deflection amps)
>
>Zonn had tried bypassing the regulator completely on an WG, with some
>problems with vector stablity. One thing that was suggested is putting
>a pecision voltage reference at the base of the current source (Q602/702)
>to improve its stability if you were going to run it from an unregulated
>supply. Or, put 7824's and 7924 regulators in in place of the 3.3 ohm
>resistors and drive just the deflection transistors unregulated.

I should point out that by replacing the low voltage regulators with *real*
regulators the display becomes very stable.

Thing I've tried:

1) Putting 7824/7924 in place of the 3.3 ohm regulators and running the
deflection circuits unregulated. This doesn't work at all. The 7824/7924's
are designed to regulate their outputs, they wreak havoc on their inputs
which wreaks havoc with the vectors.

2) Drive the deflection board unregulated, then power the HV with a
completely seperate +/- 24 volt supply. This works relatively well, all the
noise from the HV is gone. HV noise are those little spikes that have a
tendency to show up on WG displays. They travel along the vectors sometimes
fast sometimes just standing still. They look like little square or
triangle waves superimposed on the vectors, anyone who's seen more than one
working WG must have seen these -- these completely disappear if you run the
HV off a seperate supply.

You still have vectors that wave slightly. It's not much but you can watch
them move back and forth between a pixel or two.

3) The best so far. Run the deflection and HV off a beefed up regulator. I
removed all the low voltage components (Those uprotected zener diodes and
Q100/Q101 stuff). I then used a LM317/LM337 regulator pair using the WG's
low voltage power transistors as current boosts.

This gives very stable noise free vectors. But there is a trade off between
supply voltage and HV disappation though. Higher voltages going to the
deflection allows for a faster slew rate, higher voltages going to the HV
supply allows for shorter HV life.

A look at the HV shows that a properly working HV supply has its low voltage
regulator maintaining a 32v (+/- a few volts) level measure from the
negative rail to the output of the low voltage pass transistor. So the pass
transistor must drop 16 volts! Turning up the voltages places an even
higher strain on the HV's regulator.

-------

Something *not* to try. Don't simply bypass the low voltage regulators, if
you do, you can watch the transistors in the HV regulator turn to liquid.

There is also a resistor on the neck board that would have to be changed to
lower the current through a zener diode there.

-------

The things I plan on trying.

1) Regulate the error amp only, to see how the vectors behave. Regulate the
HV with seperate regulators. The advantage would be lower heat in the low
voltage regulation section since you wouldn't be driving the yoke through
the regulators. Though this might not remove the HV noise.

If that don't work this will:

2) Regulate the deflection board through the beefed up regulators setting
them to around +/- 28 volts. Then use seperate regulators to drive the HV
supply.

Looking at the HV supply you can see that it really doesn't derive any of it
power from ground, but instead from the +/- voltage rails.

There is a 24v zener in the path of the gun voltages, this compensates for
the -24v supply, so one side of the HV regulator must be driven using a 7924
(or equivalent) to keep the gun voltages correct. Now since the HV
regulator only needs 32v (and allowing for the drop of the pass transistor)
using a 7815 would supply 39 volts, or a 7818 would supply 42 volts (to be
on the safe side). Supplying the HV with 39 or 42 volts instead of 48
should allow the HV regulator to run quite a bit cooler.

So the HV regulator will consist of a 7818/7924 regulator pair.

When this works I can add a couple of extra power transistors to the low
voltage regulation to current limit the supply, though I'm not sure that's
really needed. Using boost transistors on a 3 pin regulator defeats the
internal current limit protection of the regulator. The regulator still
protects itself, but the external pass transistor does not -- an external
current limiting transistor must be added to maintain current limiting.

Depending on how hard this is to wire up, I may have some PCBs built since I
have a lot of WG monitors I want retrofitted.

-Zonn
Received on Wed Sep 3 15:16:45 1997