Re: Op-amp mystery

On 9/10/2024 2:31 AM, Liz Tuddenham wrote:

It's always the stupid things that cause the most trouble:
 I was working on a fairly straightforward experimental audio circuit
using TL074 op-amps laid out on Veroboard, which is prone to inter-track
shorts unless you are very careful with your soldering.   The gain of a
virtual-earth stage was too high, so I altered some resistor values to
cut it down.
 As this was a stereo system, I experimented with one channel first and
then switched off and replaced the equivalent resistors in the second
channel (one of the other op-amps in the same chip).
 The input signal was a 6 Kc/s sinwave at about -10 dBu.  After the
modification, the volume control, which was in a feedback loop, didnt
seem to make much difference.  Then I realised that applying the signal
to the left channel gave a distorted version of it in the right channel
and vice-versa.  Obviously I had shorted two tracks somewhere - but
where?
 Things got even worse when I realised that the input signal was
appearing on the virtual earth input of each op-amp, this definitely
can't happen!  I decided it was time to switch off the power supply and
go and ponder the problem over a cup of tea.
 That was the point where I discovered I hadn't switched the power supply
on in the first place.

Too funny!
Reminds me of a remote display/control device I made for one
of our products in the 70's.  All designed with CMOS and other
low power technologies as it had to sit at the end of a long
cable (e.g., for use on a flying bridge).
I got the prototype board assembled and debugged properly.
Then, discovered the power switch (an alternate acting pushbutton)
had been OFF the whole time!  The circuit was able to run off
power derived from the data signals.
Hmmm... now, how do I make it LOOK like it's "off" vs. "on"?