xls 0volt problem

[nighteyes]

Hi there,

 

I'm having a problem with an xls we have as a spare. We recently bought a fatfrog(very,very happy :=) ). Only having a spare table that doesn't really work, isn't really a spare.

 

The problem is that when i turn the desk on, the step indicator shows 0V. according to the manual this would mean there is a problem with the 0v line from the ic's. If i then press go, the desk operates fine, except that all the channels are turned on, and stay on. I have checked the 0v internally and i can't find anything wrong with it. Maybe i'm checking the wrong line?

 

Also, maybe it would help if i knew what the desk actually checks in order to make this diagnosis of 0v. It might help tracing the problem internally.

[Brian]

On power up the ADC measures a number of internal voltages...

 

1. The output of the DAC

2. The battery

3. The ADC reference voltage

4. The 0v line

 

The ADC reference voltage should give a reading of full scale and the 0v line of '0'.

 

Given all your channels come on and stay on it tells us that something in the ADC circuitry is wrong, it's always reading full scale.

 

Options are the ADC itself ( IC39 ZN439 ) and its reference feeds or the analogue multiplexers and their control logic.

 

All the analogue lines are fed to 8 4051 analogue multiplexers on the main board (ic2,ic3,ic11,ic12,ic16,ic17,ic22,ic23) which feed the adc.

 

A quick test would be to check the reference to the ADC - pin 11 on ic39 should be 2.55volts dc. If it's not check pin 10, that should be the same.

 

Further tests will probably need a 'scope to check out control signals.

[nighteyes]

i actually do own a scope. Having some understanding about logic circuits, i tried making some sense of the latches,op-amp, and multiplexers, but without knowing what im supposed to see, it is kinda hard. Thanks ever so much for the info. i'm gonna try it today!