Re: XY as Audio

John,

Actually, XY deflection uses transconductance amps (V in I out). So
first off, you want to use a short as a load. If you use a 100 ohm
resistor, the amp will just rail the output because it can't pump enough
current through the load. If the output is railed, you can't really
tell if the XY amp is working. By replacing the yoke with a short, the
output should sit at about zero volts.

Ok... I've got a simple way to debug XY deflection boards, however I
haven't got a chance to re-write it for WellsGardner XY monitor. I'll
try and convert it from memory...

How it works: Using a current limited +/- 15V bench power supply, two
1K resistors, and a short for a load, it is possible to re-bias the XY
amps to run at 15V. Since the power supply is current limited (to say
1.5A), nothing will "blow-up" before you have time to turn it off. With
the HV supply disconnected, X,Y,R,G,B inputs open, the yoke windings
replaced by two shorts, and the two 1K resistors installed (on P314 R604
and R606), you can measure the bias currents, and various voltages in
the amp to verify that everything is working, or find the stuff that
isn't.

Y channel -- Disconnect output transistors from X channel.

Install a 1K resistor across R604 and a sceond 1K across R606. Short
J701 pins 1 and two together. Connect J100 pins 13,14,15 to
(+15V,gnd,-15V) of the bench supply (Note: the supply MUST be current
limited to 1.5A)

Apply +/-15V power. (If the supply current limits, unplug the ouput
transistors, and check Q603 and Q604 for shorts). Don't reconnect until
the rest of the circut is working.)

Measure voltage ACROSS the following resistors:

R605 0.7V ; Diff amp current sources is working (7.6 mA)
R609 0.7V ; Output current source is working (70mA)
R611 0.7V ; Output current mirror is working (70mA)

Check VBE on Q600-Q604, all should be about 0.6V.
 
Then check the following voltages (to ground) to verify that the Y amp
is working
J600 pin 3 ~0.0V (note: for this to read zero, the output
transistors
                         must be installed and working)
J600 pin 6 ~-0.7V

Note: if pins 3 and 6 aren't close to zero then the Y amp is NOT
WORKING. Make sure the short is still installed :-)

Move the short on J701 from pins 1,2 to pins 3,5. Also move the two 1k
resistors from R604,R606 to R704, R706 and Repeat measurements above for
the X channel.

Don't worry to much about the exact voltage, because when something is
wrong, the voltage will sit a close to the rails (12 to 15V).

Once all the voltages measure OK, remove the two 1K resistors and the
two shorts. Now it's ready to try in the XY monitor.

I have fixed +20 XY deflection boards this way. Every single one worked
correctly (the first try) when I used this process. (need proof? ask
Greg Woodcock or Gaymond Lee)

-Scott

John, I've got some ideas for a WG HV replacement HV transformer... :-)
I got a batch of WG HV and Ampliphone HV units I'm trying to fix, so I
might be able to help out...

John Robertson wrote:
>
> Hmm the problem would be the same as is currently happening, except you
> would probably blow up your speakers if the outputs are bad...
> The idea here is to run the XY amps unloaded so as the output
> transistors don't croak if the drive is bad. Perhaps a resistor, say 100
> ohms might be enough of a load to test. I'm trying to find a simple way
> to power the monitors (I have about six or so) without having to have a
> running game board, nor do I want to toast the outputs by having hte
> yokes connected. Soon this will be happening. Is anyone interested in
> tested working 19" XY's? Somewhere around $250-$300US... May have my HV
> hack...
>
> Am also chatting with a TV parts wholeseller about flybacks, and we are
> looking for a cheap flyback that puts out the voltages needed, and just
> hardwire the !@!%$!% thing into a chassis. Then adjust the frequency
> down to the 15,XXX (can't remember XXX) Hz so the flyback is happy. This
> is not much different than my hack, but might be tidier if it can be
> installed in the regular HV cage. Anyone have some time that wants to
> look into this with me? That lives near Vancouver? That can handle a
> soldering iron????
>
> John :-#)#
>
> jwelser@ccwf.cc.utexas.edu wrote:
> >
> > Dumb question, and someone might've suggested this already
> > (I haven't been following this too closely) but why not, instead
> > of coming up with an audio signal to create some predictable video
> > signal, why not just use the deflection board/monitor chassis as
> > an audio amp (which is essentially what it is) and listen to the
> > outputs. If it's working properly, you should get out what you
> > put in, and you should be able to recognize it.
> >
> > The only trick is getting the right level for the input,
> > and then you'd need some sort of differential to single-ended
> > converter, because most speakers want single-ended inputs, while
> > the outputs to the yoke are differential. I can see some board
> > that plugs into the output to the yoke, which just has a diff ->
> > single ended converter (a couple of op-amps and resistors -- our
> > apps. guys put them on our eval. boards all the time at Crystal..
> > errr, I mean Cirrus Logic -- Austin :) ) and a speaker. It should
> > actually be pretty easy, and should work -- unless I'm missing
> > something.
> >
> > Of course, it doesn't test the HV, neck board, etc, but
> > it might be a real nice way to test deflection boards/chassis.
> >
> > Joe
>
> --
> John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
> Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
> mailto:jrr_at_flippers.com, web page http://www.flippers.com
> "Old pinballers never die, they just flip out."
Received on Mon Nov 9 03:17:37 1998