Analog Vecor Generator

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.

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. :)

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.

Any expert help on troubleshooting this AVG would be VERY appreciated :)

Thanks for reading,

Mendel Pearl
Received on Wed Nov 3 03:48:12 1999