Testing vectorbeam monitors.

Being a bit annoyed that I have no test procedures, nor theory on how
these Vectorbeam monitors work I started playing with one today that was
giving me excessive current draw on the power lines. So I've been
working on a test procedure using a few of my shop tools and thought
others may be interested an can also expand on the process.

1) you can power up these monitors with the YOKE unplugged with little
risk to damaging any output transistors - this static test allows you to
service the VH section on the bench if you can provide the required
+/-25VDC. I use a dual regulated power supply and limit the current to
0.2A per supply and get full HV output with a known to be good chassis.

2) the deflection circuits on these monitors are basically stereo
differential transistor amplifiers so - if you unplug the LF13331 you
can feed into the inputs of the two op-amps an audio signal of -50db or
so @ 1000hz and then be able to trace the audio signal through the two
amp sections to find problems. I'm using a heathkit Audio Signal
Generator for this, you can set the output signal from 0 to 100khz, and
-50/40/30/20/10/0db adjustable.

Need to test if the feedback resistor can be lifted and see what happens
to the input differential pair, I suspect that will aid in following the
audio path.

Note too that if you use a current limiting power supply you can test
the amps by unplugging both the 13331 AND the output transistors - with
no input signal there is little risk (is any) of damaging the driver
transistors, as long as you limit the current ot around 200ma max at +/-
25VDC.

I'd love to find a service manual on this sort of audio amp, but hunting
online today didn't lead to anything useful other than theory - and tha
twas more time than I wanted to spend on troubleshooting these.

I'll try and report more tomorrow as I dig further into this chassis
which currently is pegging the am-meters on the -27 supply without any
obviously shorted devices.

John :-#)#

-- 
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9 
Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, VideoGames)
                 www.flippers.com 	
"Old pinballers never die, they just flip out"
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