Problem with R18 and offset calibration

[samu]

Hi,

I think (I´m almost sure) that I´m doing something wrong, but I can´t realize what.

The problem is that R18 potentiometer doesn´t have any effect on the signal on TP4. Following the calibration instructions (thanks again Carl), there is nothing to do with the components, I mean, all of them installed.

Measuring the circuit those are the results: the inputs of the preamp should be equal, and so them are, but the output is VB+5.

I´m using Lab power supplies to generate 10, 5 and 15 volts. I tried to disconnect TP3, to connect in- to in+ and to connect in- to VB but R18 doesn´t have any effect.

I suppose the out of IC6 should be VB when there is no pulse on the coil, shouldn´t it?

I don´t know what else to do!!

Thanks

[Kevco]

Samu,

Make sure you do not have the MOSFET switching first. Removing R5 (Carl's proceedure), or the MOSFET, or IC5 will stop the switching.

Check the voltages (against ground: the +12 V battery ground)!!

TP2, TP3, and pin 3 of IC6 should all be 0.0 volts. Both inputs to IC6 should be equal, and at 0.0V (as wired), for TP4 to be 0.000V.

Also check:

Pin 4 of IC6 should be -5.0V.

The voltages on both sides of R17, R18, and R19.
R17: One side should be +5V, other side should be near +1.6V
R18: Also one side near +1.6V, other side near -1.6V.
R19: One side should be -5V, other side should be near -1.6V.

I had problems soldering with the new RoHS components. They required higher soldering temperatures than I was expecting. I had many bad solder connections! I never had that many before RoHS!

You may find some of those voltages wrong, if a bad solder connection is causing your problem. If a voltage is wrong, check the circuit near that point, to find bad solder connections.

Did you clean solder flux?

Kevco