Re: Analog Vecor Generator

Mendel Pearl wrote:

> Hi there,
>
> I am trying to repair my beloved Tempest board.
>
> The problem is that the 'Y' output does not function, and the 'X' output is
> distorted.
>
> So, the first thing I did was to check if the DAC's get their data, and they do
> get it both on all data lines. Then I checked waveforms on the outputs, in the
> crosshair (the large 'plus' sign) test mode. Am I right both the output should
> look like a sawtooth?
>
> When I examine the current outputs of the DACS, I found that the working, but
> distorted 'X' output was nice virtual ground, and after the current/voltage
> convertor I get a waveform. BUT, in the middle of the sawtooth there is a big
> triangular 'dip'. Could this be beause the DAC is bad, or is it more likely the
> DAC is getting wrong data? You can spot this dip in any test mode.
>

Hope for wrong data. Fix the other output first and then see if this is still bad.

>
> For the 'Y' current DAC output, which is not working at all, I measure a 5 volt
> p-p random-like sawtooth (which does not resemble the one you should get AFTER
> the current/voltage convertor) in stead of a virtual ground.
> So my guess is the DAC's bad, but I am not sure, because I cannot explain what
> the DAC08 is exactly doing (this one generates a 'BIPOLAR' signal, a current
> added to that of the X-Y DACs).
> AM6012 DAC's cost about $30 here in Holland, so I want to make sure before
> buying and blowing them again. :)
>

If you don't have the virtual ground it probably ISN'T the DAC, the opamp is
probably bad. It's the output of the opamp, fed back through a resistor inside the
DAC, that gives the virtual ground. If the opamp is bad and has no output there is
no feedback and you get exactly as you describe.

>
> I tried to find exact information on this AVG circuit on the net, but I cannot
> find a really good description of it's priciple of operation, and why the analog
> multipliers MC1495 are in the circuit.
>

The multipliers are a form of pincushion correction needed by certain monitors.

>
> Any expert help on troubleshooting this AVG would be VERY appreciated :)
>
> Thanks for reading,
>
> Mendel Pearl
Received on Wed Nov 3 04:04:31 1999