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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 24th Aug 2017, 6:45 pm   #1
Biggles
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Default A simple mistake?

I have been building and repairing circuitry for over forty years, and somewhat arrogantly thought that I was competent. But recently, I fell into a daft trap while constructing a simple transistor based phono pre-amp. After building a circuit copied in the most part from a commercial design, but using veroboard, all was ready to power up and test. So far so good. I connected a low level audio signal to the input of the first channel (it was a stereo pre-amp) and scoped the output after checking the DC conditions. All perfect and working. Right, connect up the other channel. Very low output. DC conditions all perfect. Spent about three evenings trying to work out why the output was low. Tearing my hair out. Now here's the funny bit. I eventually twigged that the low output channel was in fact the correctly operating one. The "duff" channel had an o/c in the feedback circuit and the gain was sky high. Oh dear. On fixing the fault all is good. So, always look for the obvious, a lesson I have still to learn.
Alan.
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Old 24th Aug 2017, 7:01 pm   #2
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Default Re: A simple mistake?

Reminds me of the time I replaced the scratchy RF/IF gain-control pot on a HF receiver.

It was a 2-gangs-on-one-knob type (like a stereo volume-control) - one gang controlling the RF stages, the second gang the IF, with different 'laws' to take into account the different valves in the front-end and IF chains.

Yours truly managed to transpose the ends-of-the-pot-track wires (which were all black!) to one gang, so with the control fully anticlockwise the RF gain was minimum but the IF gain max, and when fully-clockwise the RF gain was at max but the IF gain was at minimum.

So at no point did the receiver have enough overall gain to receive anything!
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Old 24th Aug 2017, 7:07 pm   #3
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Default Re: A simple mistake?

I'm sure you are not alone: I've certainly been there - in fact, more often than I care to confess! Although enduring such experiences is indeed extremely irritating, in retrospect and in subsequent quieter moments, such events have taught me the value of humility: none of us are perfect.

To err is human; to be perfect is divine.
Experience is a bad teacher: gives you the exam first and the lesson afterwards.

Al.
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Old 24th Aug 2017, 7:10 pm   #4
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Question Re: A simple mistake?

G6Tanuki: But surely with the shaft of that control at its mid-point, you would have heard something, no?

Al.
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Old 24th Aug 2017, 7:13 pm   #5
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Default Re: A simple mistake?

No, the 'laws' of the 2 pots conspired against it.

Turning the control changed the quality of background-noise level a bit but nothing was received (I was only using a bit of wire as an antenna).
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Old 24th Aug 2017, 7:24 pm   #6
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Arrow Re: A simple mistake?

Ah; I see! So if the two pots were reverse log. (most likely) then when in the mid-point, there would still be considerable attenuation.

Al.
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Old 24th Aug 2017, 7:45 pm   #7
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Default Re: A simple mistake?

Yep. Normally, even when using a 'normal' amateur HF antenna [dipole] the RF gain needs to be wound up to more-than-half-way. I guess the first half of the control was designed to satisfy the gain-control needs of those who could afford Rhombics or similar!
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Old 24th Aug 2017, 7:59 pm   #8
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Default Re: A simple mistake?

Quote:
Originally Posted by Skywave View Post
To err is human...
... but to make a real c*ck-up you need a computer!
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Old 24th Aug 2017, 8:48 pm   #9
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Default Re: A simple mistake?

I had the same problem with a Rank Arena tuner amp, looking for low one channel, after several hours, the penny dropped. It was in fact high one channel, If I have one of these to repair I replace all the light grey electrolytic capacitors. Mick.
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Old 24th Aug 2017, 10:43 pm   #10
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Default Re: A simple mistake?

Also remember that the person who has never made a mistake has never learned or made anything.

Joe
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Old 24th Aug 2017, 11:19 pm   #11
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Default Re: A simple mistake?

And the only thing harder than never making a mistake is living with a partner who never makes mistakes!
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Old 24th Aug 2017, 11:56 pm   #12
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Default Re: A simple mistake?

Almost 40 years ago, I tried my hand at CTV work. I was asked to look at one (a Siemens I think) that had been stored in a (damp) cellar for 6 months or so. I had a scope (a Russian CI5Y I think). The colours were obviously amiss, and I scoped the waveforms on the CRT base plate. One was the "odd one out", so I checked as much as I could in that channel. Eventually, getting nowhere, I consulted what literature I had (I think I had one of the Patchett books) and it clicked, I was looking at the good channel, the other two were faulty.
There have been many more over the years!
Les.
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Old 25th Aug 2017, 8:10 pm   #13
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Default Re: A simple mistake?

The human brain has two 'systems' that run concurrently. The first, termed 'System 1' by some psychologists, makes snap decisions and jumps to conclusions very quickly based on often flimsy evidence, and is probably responsible for keeping us alive. However, it is often wrong. 'System 2' is slower, more analytical, but consumes a lot more energy. It takes some conscious effort to use 'System 2' and override 'System 1's snap decisions.

'System 1' jumps to the conclusion "one channel is low" because, based on the majority of evidence available to the brain, that is the highest probability. 'System 2' sits idling away until the evidence that one channel is actually high - a relatively unlikely occurrence - becomes overwhelming, after maybe several hours of fruitless fault-finding in the channel that 'System 1' incorrectly blamed in the first place!

This is how we've evolved. It takes some mental effort to go against several million years worth of evolution. But that's what the best diagnostic engineers do, and it takes lots of practice. Even the most experienced fall into the trap.

Thanks for sharing!
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Last edited by Phil G4SPZ; 25th Aug 2017 at 8:12 pm. Reason: Spelling
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Old 25th Aug 2017, 10:48 pm   #14
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Default Re: A simple mistake?

Been there, done that on more than one occasion. One particular incident springs to mind, an item that I have repaired hundreds of with very common stock faults showing that same symptoms as usual. Did all the usual stuff to no avail. Spent hours with the schematics, DMM and scope and still no further (convinced the fault was one one of the 4 PCB's in the unit). The other guy I work with wandered over, had a quick look and 10 minutes later fully functional. Fault on one of the other boards....
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Old 26th Aug 2017, 12:12 pm   #15
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Arrow Re: A simple mistake?

Quote:
Originally Posted by Phil G4SPZ View Post
The human brain has two 'systems' that run concurrently. (Etc.)
Thanks for that post, Phil: I found it to be very interesting. It goes quite a long way to explaining a phenomenon that I've gradually noticed in myself as I have grown older. Briefly, when I get an idea of how to do something (repair / modify / create), I am now aware that my first 'idea' is never the best one. (Sometimes, retrospectively, it becomes clear that it's downright stupid! ) So now I don't rush in. I might write the essence of that idea on a scrap of paper and then stop, look at what I've written and think about it for a couple of minutes: then go away and do something completely different. It's similar to solving crossword puzzles. You get stuck on a clue, leave it and walk away. Then later - which may be a few days - when doing something else, bing! The 'light' comes on! And you've solved it.

So my personal motto in such situations now is "Your first idea is never the best one: leave it. Just have faith . . . and wait!"

Al.
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Old 26th Aug 2017, 1:05 pm   #16
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Default Re: A simple mistake?

This thread is a great story and it is based on our notions of symmetry. Typically there is some system, whether it be a dual channel amplifier (or maybe three channels), or maybe a differential amplifier or some mirror image circuit arrangement that has "gone out of balance". Then we are left wondering which is the abnormal side.

An absolutely classic case of this dilemma crops in Neurology with the pupil size of the eye. (The size is set by the activity of the parasympathetic and sympathetic arms of the nervous system acting in opposition on a sphincter muscle and radial dilator muscles of the iris). A patient, or a relative of the patient, notices that one pupil has a larger diameter than the other....but is the bigger one abnormal, or is the smaller one the abnormal pupil ? Many doctors have chased their tails blaming the wrong pupil.

Fortunately there are tests that can sort it out, but it is an interesting example of the same type of problem.

Last edited by Argus25; 26th Aug 2017 at 1:11 pm.
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Old 26th Aug 2017, 5:06 pm   #17
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Default Re: A simple mistake?

Quote:
Originally Posted by Skywave View Post
Thanks for that post, Phil: I found it to be very interesting.
Hi Al. Yes, it's something I've also noticed. Once you realise what's happening, it's quite enlightening! The concept of System 1 and System 2 is outlined in detail in the excellent 2011 book, "Thinking, Fast and Slow" by Daniel Kahneman (Penguin Books, ISBN: 978-0374275631). I only have the Kindle edition or I'd pass it on to you. It's based on several decades of modern psychological research, but it makes a very good read without being overly academic.

I think the research confirms why we all tend to jump to the first conclusion that appears to fit the evidence. It seems that we mentally replace a difficult, complex or detailed question with a simpler one, then we answer that.
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Last edited by Phil G4SPZ; 26th Aug 2017 at 5:09 pm. Reason: Addendum
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Old 26th Aug 2017, 7:38 pm   #18
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Default Re: A simple mistake?

I remember spending ages trying to figure out why a VCR my brother had would no longer work properly. Something had happened and after working on it I couldn't get any video signal out of the thing. Only once I got hold of the schematics and started scoping the signals inside did I find I'd knocked a pot turning some video gain or similar down enough to give no picture. Main problem was that was months between cause and effect so he was singularly unimpressed.
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Old 26th Aug 2017, 7:58 pm   #19
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Default Re: A simple mistake?

Another mistake - not one I made but one I fault-found and got well paid for. . .

A demonstration-lab at an event with one of the top CPU-manufacturers, where there was a bunch of digital electronics being shown. I was doing the videoconferencing-side, taking the three venue-cameras and a feed from the various bits of lab-gear so we could do split-screen presentations of the results of various bits of DSP and streaming the results to a few dozen international sites.

To summarize, someone had not wanted to cut a length of coax from the drum so had put a plug on the 'pigtail' from the centre and one on the few metres of cable unrolled at the free-end.

This was hidden in a cupboard under the demo-bench.

100 Metres of coiled-up coax is quite some delay-line which explains why things didn't go well until I got involved.

Last edited by G6Tanuki; 26th Aug 2017 at 8:06 pm.
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Old 26th Aug 2017, 11:28 pm   #20
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Quote:
Originally Posted by Phil G4SPZ View Post
The concept of System 1 and System 2 is outlined in detail in the excellent 2011 book, "Thinking, Fast and Slow" by Daniel Kahneman (Penguin Books, ISBN: 978-0374275631)
Thanks for that, Phil: I'll see if I can acquire a (printed) copy. I've always been fascinated by the enormous diversity of humankind and its motivations: perhaps I should have trained to become a psychologist; I know my son has.

I also wonder to what extent the degree of difference is (if any) between man and woman. Perhaps the existence of System 1 harks back to our ancient ancestors: the requirement instantly to choose between fight of flight: a decision, which if wrong, could have disastrous consequences. Whatever, I'm sure that by the time I've read that book, much will be revealed to me.

Al.
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