Re: Anyone have any troubleshooting advice?

Tony Jones wrote:

>>Probably a stupid question, but did you check ALL pins, especially the
>>power supply pins?
>>
>>
>
>Yes, I checked all pins on the DACs and the analog multipliers.
>Voltages are good.
>
>On the AM6012s, 5.27v on pin 20 (v+), -15.01 on pin 17 (v-).
>On a DC scale, I get 0v on pins 14/15 (VR+/VR-) for both DACs.
>
>On test screen 4 (crosshatch), read 0.4V on pin 18 (IO) of the Y DAC (F10),
>reads zero volts on the same pin of the X DAC (A10). On the scope, this shows
>up as a noise like waveform.
>
>

Pin 18 of the DAC should be zero. If it isn't, the opamp it connects to
is bad, in this case the TL082 at D11. I suppose it is possible that
the LF13201 at E11 could be bad, but at .4 volts the opamp output should
be clipping hard to one of the power supply rails, but your problem
seems to be more of a "only swings one way" thing. 99% chance of opamp
failure.

>>From looking at the schematics, pin 18 is what feeds into D11/E11 which
>eventually feeds into the multipliers, so I was *assuming* pin 18 was the
>analog output of the DAC.
>

It is a CURRENT output. If the opamp is working it will buck the
current to maintain 0 volts (otherwise known as a virtual ground) at
that point.

>
>But where I'm getting confused is that pin 18 also splits off into the BIP
>circuit, from the schematic, it almost looks like the BIP circuit feeds
>into the DAC (as an input) on pin 18.
>

The BIP circuit injects a CURRENT offset into that node. The current
from the BIP circuit sums with the current from the DAC and the opamp.
 The opamp tries its best to keep the total current (and the total
voltage) at that node zero.

>
>Before I started looking at the digital inputs to the Y DAC I figured I needed
>to understand what pin 18 is and get some basic explanation as to how this
>part of the circuit works.
>
>The following is a JPEG of what the screen running the crosshatch test pattern
>http://www.tonyjones.com/temp/tempestbady.jpg
>
>thanks!
>
>Tony
>
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Received on Sun Feb 9 16:38:09 2003