Ok Dave, thanks so much!!Ok,
So I have been looking at the schematics in a bit more detail and would like you to take some oscilloscope measurements on some further points.
HOWEVER, before you take ANY measurements, you need to identify WHERE to take them at...
IC303 pin 1 [Y].
IC303 pin 14 [X].
At the junction of R424 (1.5k) and R421 (47k) [Y].
At the junction of R423 (1.5k) and R420 (47k) [X].
IC401 pin 8 [Y].
IC401 pin 7 [X].
Under NO CIRCUMSTANCES make the following measurements until we have discussed them!
IC401 pin 10 [Y].
IC410 pin 5 [X].
All these are to be taken when running the default Mindstorms game.
All of these reading will be both positive and negative - so your oscilloscope needs to be set-up with 0V level being in the middle of the screen.
You should start off at the highest voltage/division for each measurement and slowly reduce it to get a sensible reading on the oscilloscope screen.
We need to identify a suitable oscilloscope timebase - and leave it fixed at that for all measurements.
As the measurements are in pairs - once you have adjusted the Y scale on the oscilloscope for the [Y] reading - leave it the same for the [X] reading.
What we are looking for are differences in the voltage levels between the [Y] (good) and [X] (bad) side.
I would expect to see (at some point) you will observe either all negative or all positive voltages for the [X] side readings - and this should be round about where we have the fault...
Wow! Great job Dave!! Thanks so much....i'll check this!I have been having a look at various documents today and I think we can 'short-circuit' the fault finding exercise in favour of going straight to the potential culprit.
The most likely culprit is IC401 (LM379) Dual 6W Audio Amplifier used to drive the scan coils.
First (with the power OFF) check for continuity (with your multimeter) from the NEGATIVE side of capacitor C411 (1000 uF) to pins 3, 4, 11 and 12 of IC401.
We are expecting very low resistances. Any open circuits could mean PCB track damage.
Note: In the measurements below, the Voltage shouldn't exceed +/- 9V (under normal circumstances) but it is always wise to start off with a high V/div setting and work your way down.
With the power ON (and suitable safety precautions to avoid contacting any high voltages around the CRT), use your oscilloscope to measure the voltage waveform on pins 8 [Y direction] and 7 [X direction]. The voltage should be oscillating both above zero and below zero. Note the indication on the oscilloscope of Vmax and Vmin. You will need to choose a suitable timebase to see some vectors being drawn (i.e. the voltages varying that you are measuring).
If both of the pins you are measuring (7 and 8 of IC401) indicate both positive and negative voltages (with respect to 0V/GND of course) then everything up to the amplifier IC401 should be fine.
Next, measure the output pins of IC401 at 10 [Y direction] and 5 [X direction].
Again, you should observe the voltage going both positive and negative (relative to 0V/GND). I would like to bet that one pin (pin 5 - X direction) will either always be positive OR negative.
If this is the case, IC401 (LM379) is most likely faulty.
I removed heatsink and im trying to check pin 7 and 8 but i can't see voltages of more or less 9vThe metal protection is the heatsink that keeps the IC cool. This IC is the highest stressed component on the board (as it drives the scan coils) hence it has the highest probability of failure...
If you can’t measure directly on the pins of the IC - the pins of the IC are connected to other components. Find out where the pins of the IC go to (by looking at the schematic and following the PCB tracks).