If you doubt the operation of a 277, replace it from another on the board.
If the results are the same, then check the resistances of R229 and
R230 by opening one leg of the resistor. If those are correct and it
does'nt work we'll ask Stefan
On Fri, Jan 29, 2010 at 8:03 PM, Anton Kan <antonxiii@...> wrote:
> Hi there,
> I've recently started up working on my v1.1.0 ModularEEG again, and have hit
> the same stumbling block. I have built both boards myself, and am looking at
> the output on Electric Guru.
> The problem is that when I turn the screws on the trimpot, the frequency
> changes instead of the amplitude. This occurs when a channel is connected to
> U_cal and Cal_GND, but also when it is unconnected it allows me to change
> the frequency of an irregular square wave (or at least it looks square
> because it hits the top and bottom of the graph on Electric Guru). The
> amplitude does not change whatever I do.
> I've spent many days now trying to find what's wrong, it's very frustrating
> considering I am so close to completion.
> The digital board seems fine, when the analog board is not connected
> electric guru shows a flat line at 0. I am running the P2 firmware.
> Now on the analog board:
> When the trimpot has a low resistance (measured between leg 1 of P202/3 and
> VGND) the frequency is very short, when I turn the screw and the resistance
> increases, the frequency rises. The amplitude is always at maximum (between
> 0 and 1023) on electric guru.
> In the testing tips it recommended measuring the voltages between AGND and
> some IC pins on the analog board. There were problems here.
> Checkpoint Voltage
> VGND +2V correct
> IC201 pin 1 +2V correct
> IC202/3 pin 6 +2V correct
> IC205/6 pin 1 +2V correct
> IC205/6 pin 7 WRONG! was 4V instead of 2V. (This could explain it maxing
> out electric guru...)
> The resistors seem to check out fine. But measuring the resistances between
> various pins of the IC didn't always produce the results that were described
> on previous emails (although that was for v0.4 whereas I have v1.1.0 so I
> don't know if it's relevant). Would any other readings be helpful?
> Now, my suspicion is that it is some of the ICs that are faulty, but really,
> I don't know and it's highly frustrating.
> Does anyone else think this is the case? Has anyone had this problem before?
> Any help would be appreciated, thank you.
> Anton Kan
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