Re: Testing vectorbeam monitors.

John Robertson wrote:
> Grant Thienemann wrote:
>> This might be helpful for you!
>>
>> http://www.outerworldarcade.com/arcade/cinematronics/manuals/cine_faq_v096.pdf
>>
>> -Grant
>>
>
> Yes, however there is little on troubleshooting the actual amplifier
> when the problem is not the output transistors! At the moment I have
> amplifiers that appear to be unbalanced and I don't (as yet) know why.
> What I am trying to do is set up a process for troubleshooting the X &
> Y differential amps that is fairly easy, and by treating them as audio
> amps I think this will do the job. I don't do a lot of analog amp work
> these days and am getting rusty...usually these deflection boards are
> a snap to fix.
>
> Today I will be taking typical voltages on the various transistors
> with a zero volt input - this should produce a zero volt deflection
> and work from there.
>
> The annoying thing is this amp is almost identical to one that was
> published back in the late 60s in an electronics magazine (I'm pretty
> sure it was Popular Electronics Brute 70) as a high power
> transistorized stereo amplifier - I built the darn thing back then for
> a family friend, but can't find pdfs of the article(s) (two issues?)
> as I seem to recall that they covered troubleshooting the differential
> amp...more google searching I guess is in store for me.
>
> Similar amps:
>
> http://www.hilberink.nl/amps/amps4.htm
>
> http://michaelgellis.tripod.com/audioamp.html
>
> John :-#)#
>> On Fri, Feb 1, 2013 at 3:14 AM, John Robertson <pinball@telus.net> wrote:
>>
>>> 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 :-#)#
>>>
>>>

As a followup found an interesting book on transistor theory:
Fundamentals of Linear Electronics: Integrated and Discrete
 By James F. Cox (2001) by the following search terms in google
"troubleshooting the differential amp transistor". Reading pages 216 to
221 (missing pg 281 - darn!) is very helpful - rounds out the
explanation in the theory section of the vector pdf discussed above.

The book is running $85 used, so I don't think I'm going to treat myself
just yet with a copy (new is $220!)...and no library near me has a copy:

http://www.worldcat.org/title/fundamentals-of-linear-electronics-integrated-and-discrete/oclc/247983125

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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