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From: Don Y <blockedofcourse@foo.invalid>
Newsgroups: sci.electronics.design
Subject: Re: Op-amp mystery
Date: Tue, 10 Sep 2024 05:27:28 -0700
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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"?