Re: Testing vectorbeam monitors.

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 :-#)#
>>
>> --
>> 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"
>>
>> ---------------------------------------------------------------------------
>> ** Unsubscribe, subscribe, or view the archives at http://www.vectorlist.org
>> ** Please direct other questions, comments, or problems to chris@westnet.com
>>
> ---------------------------------------------------------------------------
> ** Unsubscribe, subscribe, or view the archives at http://www.vectorlist.org
> ** Please direct other questions, comments, or problems to chris@westnet.com
>
>

---------------------------------------------------------------------------
** Unsubscribe, subscribe, or view the archives at http://www.vectorlist.org
** Please direct other questions, comments, or problems to chris@westnet.com
Received on Fri Feb 1 13:03:21 2013