Zero Point SDX LED problem

[euphyllia]

I've now built 2 ZPSDX, but I can't get the modulation rate LED to work on either of them.  I have removed the LEDs to check that they are working, which they are, but once they have been soldered in place, they never illuminate.
All other functions work.
I did a search, but couldn't find anyone else with the same problem.
Please help!
Cheers
David.

[jjanssen1]

Hi,
are you sure that you have your led cathodes going to the right pad for the leds
Hope this helps
Joshua

[muddyfox]

Never actually populated my zpsdx but a shot in the dark... with the led out, can you verify there's voltage on the pads at any point?

That, and the orientation. 

[euphyllia]

OK, so I changed both LEDs, and now the voltages read on one board, 0.3V, on the other 1.85V, both 3mm LEDs.
The one with the higher voltage is working, but is always ON, i.e. it is not pulsing with the modulation rate, and doesn't change with the mod rate increasing or decreasing.
I'll have another look at the one where there is insufficient voltage, but in the meantime, any guesses as to why the LED works, but doesn't 'flash'?
Also, could someone explain via the schematic, how the LED turns on/off?  I can see the mod part of the schematic and how changing the rate effects the LED, but my newbie brain can't work out how the LED turns on/off.
Thanks so much
David

[euphyllia]

Alright, thanks for the voltage suggestion.
One TL062 was faulty, so replacing that seemed to work.
The other board, where the LED stays permanently on, pin 1 and 7 voltages on TL062 stay stationary at 5.6V, and do not fluctuate up and down between 0.5-7V (depending on rate).  It doesn't seem to be a faulty IC, as I swapped out the one that works.
Any suggestions where to look?
Thanks