Problems with building an ESR meter

[Simoc]

I built an ESR meter according to this link:
http://www.ludens.cl/Electron/esr/esr.html

As I completed building its electronics, it worked. Then I made a
casing for it and installed it there. When that was almost ready, I
tested it, and it still worked. But there wasn't yet a place for a
battery in the case, I made it now (had to detach the circuit board
then), and replaced the board. Then I tested it -- and it...didn't
work!

I took all the guts out from the casing, and measured _every_ component
I can with a DMM, tested the op-amp by using as comparator, and
measured _every_ connection, got 0 ohms and no shorts in wrong places.
So virtually only possibly broken component is the transformer. But I
don't think.

So what could have gone wrong? As I'm pretty unfamiliar with using an
op-amp as an oscillator, so could someone here explain how is it done
in that circuit? And how could I debug the circuit by measuring
voltages on some points on the circuit with power on, to find out where
is the problem? I just have measured that op-amp gets the necessary 5V
on Vcc.

Thanks in advance

 

[Simoc]

Some addition:

BTW, is it possible to somehow test the circuitry without the xformer
to see if it's faulty? by shorting those two non-common leads going to
its primary and secondary, or putting some resistor between them? (As
according to the text, the purpose of the transformer is to step down
the voltage and provide low-impedance output for being suitable for cap
testing, so I thought if I could test the circuit without the xformer
by measuring resistors etc other than caps by it.)

Still thankfully in advance, Simoc