Re: BZ board shows right half of screen only..please help..

From: Marc Alexander <marc_at_wolfems.com.au>
Date: Fri Jan 12 2001 - 02:17:35 EST

Some of this may seem silly but I just have to ask...

Are you sure the X and Y are outputting positive and negative on both
the channels?

There's a few things I'd do, in something like this order (and I've
probably forgotten something!)

If you don't have a 2 channel CRO then this will be a bit harder, see the
simpler tests further down.

***
Be sure the X and Y outputs _really_ are swinging both positive _and_ negative
for the entire screen frame..
And that the Z output does show positive action for a whole screen cycle, and not
disappear, that should show for sure if it is the X, Y or Z...

If it is the X or Y output, you should see no action in one of the
quadrants of output (ie. no waveform in one polarity direction on one of the outputs)

Connect the oscilloscope in XY mode to the XY outputs, that should immediately
show you if the X and Y deflection is happening for the whole frame also.

If that is ok...
Then you could swap the Z channel into the Y input to the CRO and you will
see the Z brightness versus the X, if you get only half of a line across the screen
then you can see that the Z is not outputting for that side of the X deflection.
And then the same with the X...

...other tests...
If the Z is going down for just one quadrant, it may be a problem in the Z BLANK
circuitry, LM319's @ D12, and B12, TL084 @ C11, this outputs /WNDWBLNK based on
action in all axis.

You could rig /WNDWBLK high, so the Z BLANK circuitry is disabled, this will
bypass the Z BLANK circuitry and let you see if it is at fault.
On the 74S86 at B7, you'll need to disconnect pin 12 from the board, and hold it high with
a resistor, anything from 470R to 2K2 (this is just for short cct. protection)

And check for action on all the DVX and DVY inputs on the X and Y DACS, especially
the most significant one(bit), it may well control the positive and negative direction
and if stuck, would cause one polarity of an axis to be dead.

Well I'm reading this back and it's a jumbled mess! Hope it helps anyway :)

Marc

----- Original Message -----
From: <MKDUD@aol.com>
To: <vectorlist@synthcom.com>
Sent: Friday, January 12, 2001 4:53 PM
Subject: Re: VECTOR: BZ board shows right half of screen only..please help..

> In a message dated 1/11/01 11:49:09 AM Eastern Standard Time, mrp@sonic.net
> writes:
>
> << Nope, you might be missing the -15V that drives the X DAC though....
> Or the X DAC might be blown. I don't know the BZ board very well, so
> I'm not sure what else is the the analog section of this board, but it
> might be in some sort of a problem with an amplifier or voltage follower
> in the X section.
>
> Do you have an osciliscope? Can you trace the output of the X DAC to where
> it hits the edge connector to see where you lose -X?
> >>
>
> Crazy thing is, is that I'm not losing the signal anywhere, at least not that
> I can see. Maybe I have my new scope set incorrectly??
> I did socket and replace both TL082's. One CRAZY thing I found today was that
> I had a glass .1uf cap (C79, goes across collector and emitter of Q4
> Darlington 2N6044) was practically shorted and was reading 7 ohms. I then
> found that all of these transistors were bad Q4, Q5, and Q6 (probably
> caused by C79). I can see the bad cap blowing the darlingtons, but could this
> have also affected Q8 (3904) in the Z amp? This is down the same ground line
> from Q4. I haven't pulled Q8 yet, but I'll test it tommorrow.
> Any other ideas?
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Received on Fri Jan 12 02:38:25 2001

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