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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 4th Sep 2020, 6:54 am   #1
Diabolical Artificer
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Default Grey beards and boffins; what do see when you see a circuit.

I was looking at this thread here - https://www.vintage-radio.net/forum/...d.php?t=170736 and wondered what David being our resident boffin and grey beard see's when he looks at a schematic or circuit.

When I look at a schematic I look at it in terms of voltage and am trying to look more in terms of current, but I'm obviously missing a trick by not looking in terms of Z, which still confuses me. My brain see's high Z and thinks oho! that'll be hard to drive, when obviously it won't. It's the old high Z = low I, low Z high I dichotomy mind zapper conundrum.

Anyway, back to the original Q; what do our experienced members see and look for when looking at a schematic? I'd like to improve my reading of them so am looking for clues.

Andy.
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Old 4th Sep 2020, 7:09 am   #2
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Default Re: Grey beards and boffins; what do see when you see a circuit.

I am going grey but am not a "boffin".
Being a big picture person I go top down, I try to mentally create the block diagram first, I try to recognise the circuit functions, e.g. in an amplifier, preamp, phase splitter, drivers, output, feedback loop etc.
I then dive a little deeper looking at how each of these functions is realised, e.g. for the phase splitter, concertina or long tailed pair.
I then look for anomalies, has the designer been very clever or cut any corners.
I am afraid that voltages and currents come last.

Peter
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Old 4th Sep 2020, 7:23 am   #3
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Default Re: Grey beards and boffins; what do see when you see a circuit.

In my case, it depends a lot on why I'm looking at a schematic.

If it's for a repair, then finding the correct area and taking note of voltages and how one would expect it to work.

When just general browsing, it's more of a "this section does this, that section does something else and what's that bit on the end".

As per Peter, voltages, currents, waveform etc are a distant end of the line need to know thing.

But mostly I just see a page or more of connected components that do a job (hopefully), although badly drawn schematics tend to infuriate me somewhat - I'm talking commercial type stuff, not back of the envelope stuff we all do at times.
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Old 4th Sep 2020, 8:32 am   #4
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Default Re: Grey beards and boffins; what do see when you see a circuit.

I think a lot of it is down to experience. Some schematics just look horrible from the start, or, as in the case of the example starting this thread, there just being something instantly odd or unusual.

A tidy mind producing a tidy schematic and tidy final construction will, in my opinion, give a more reliable product. It's almost like buggy software - It may be the bees knees for sorting a problem but further down the line there will be maintenance issues.

From this - It does depend quite a bit on why I am looking. If it is just for interest and the diagram is messy I need very good reason to even continue. In general though it is probably either a top down approach looking at the circuit then the modules etc, or a signal flow approach. Sometimes though I am looking at a schematic just to see how a certain component is being used or subcircuit type has been designed and being utilised. Really it varies quite a bit.
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Old 4th Sep 2020, 11:15 am   #5
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Default Re: Grey beards and boffins; what do see when you see a circuit.

Quote:
Originally Posted by Diabolical Artificer View Post
Anyway, back to the original Q; what do our experienced members see and look for when looking at a schematic? I'd like to improve my reading of them so am looking for clues.

Andy.
Recognizing circuit function(s)

ie: that snippet in the thread you referenced to, the function of those FET's can only be one thing, switches, you can tell they are switches because they are in series with the input of the following amplifier stage, once that's figured out you can then figure out what they are switching and how the switches are activated, eg: download the manual from the link below and scoot down to manual page 84 remembering that the gate voltage on the FET needs to be sufficiently -ve with respect to the source to turn the FET off...just like a valve:

https://elektrotanya.com/fostex_g_16.../download.html

Lawrence.
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Old 4th Sep 2020, 11:48 am   #6
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Default Re: Grey beards and boffins; what do see when you see a circuit.

I was a designer, working in R&D, in the same building where the products were made and tested. If I drew a diagram that was hard to understand, I condemned myself to spending the rest of my life repeatedly explaining it to different people. So I drew neat diagrams. Not neat as in looking good to an artistic eye, looking balanced on the paper, following the rule of thirds and that sort of crap, no neat as in uncluttered and clearly showing signal flows and showing control flows as different to signal flows. Signals flow left to right, feedback right to left. If it means fitting the diagram onto N sheets then so be it.

Take the time to write a nice helpful circuit description. Write it in layers. What the overall thing does, what goes in and out. Then divide it up into sensible sections and describe those. Maybe add nominal voltages/RF power levels. Perhaps sketch or photograph waveforms and spectra.

Looking at that that Fostex snippet, I've no idea how I did it, I just looked and thought "They couldn't have.... they couldn't be so cretinously stupid!" but they were. And what I saw can explain all the reputation that unit has acquired. Why did it take so long to get noticed?

I'm a cynical designer. I never took the kings shilling and moved to management (the financial incentive is a lot more than a shilling!) I stayed in engineering and built up a lot more experience than anyone less stupid than me (they'd have moved to management!) would have stuck around to get. Having been around the block a few times, I know how much bad design is out there. Someone with the repairman's perspective would use the logic "Well, it worked once" but some things like this example are so badly designed that they shouldn't really work and that those that did, did so purely through luck.

Sit down, look at the circuit. The FET gates need to be a long way negative to turn the FETs off. The diodes do that OK. But when you want one on, the diodes block any attempt to being the gate voltage back to near the source voltage. So now you can bask in the feeling of wonderment.... How the hell did they expect that to work?

When you learn to read (whether writing or Morse code) you start spelling things out in letters and assemble the words. With practice you learn whole words as entities and you just recognise whole words. The same thing happens looking at circuitry. It's just practice. Experience and immersion will get you there. In fact with experience and immersion, you won't be able to prevent it happening.

I saw two FETs oriented as analogue switches into the + input of an opamp, saw the diodes on the gates and Oh-Oh... where are the pullback resistors? GOTCHA! quick as that.

I do miss things!

I do sometimes make assumptions and get locked in perpetual circles. The issue is recognising it, and breaking the circle. If you've eliminated ALL the possible causes, it must be one of the 'impossible' ones. Simples!


David
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Old 4th Sep 2020, 11:52 am   #7
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Default Re: Grey beards and boffins; what do see when you see a circuit.

I could never get on with a beard

Hair short at the moment. Normally it's a bit long and looks a bit like a cross between a dandelion and a very surprised Einstein.

No moustache, either.

I'm not very clear in my avatar, but I am the one on top! Crossing the main bridge over the Tweed at Peebles. Corrie, underneath is also grey. Coincidence.

David
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Old 4th Sep 2020, 12:05 pm   #8
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Default Re: Grey beards and boffins; what do see when you see a circuit.

I'm certainly not a boffin, I wish I was. Not having any technical background/training I find looking at a schematic rather mindbogglingly! It doesn't help me much, all the lines and symbols don't relate to what I see on the underside of a chassis.
Despite this I seem to get by, I've fixed numerous old radio sets now, but understanding a schematic I'm sure would considerably widen my knowledge.
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Old 4th Sep 2020, 12:12 pm   #9
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Default Re: Grey beards and boffins; what do see when you see a circuit.

What you see in a circuit diagram very much depends on your skills and experience. I try to divide the circuit into functional blocks to try to work out what's going on. I don't bother much about voltages and currents unless I'm trying to fix a problem, and I know next to nothing about circuit impedances.

The point is, I'm not a designer and make no pretence of being one. I just nick other peoples' designs and hope they know what they're doing
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Old 4th Sep 2020, 12:50 pm   #10
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Default Re: Grey beards and boffins; what do see when you see a circuit.

One of the great analogue designers was Jim Williams of Analog Devices. His technical notes were absolutely superb - clear, easy to understand, sketches and photographs of scope traces.

He learnt his craft at University. He used to play a game with his colleagues. One of them would introduce a fault in a piece of test gear (lift a resistor, snip a lead - that sort of thing). The challenge was to find the fault.

Alas Jim Williams died in 2011 aged 63 from a stroke, and the equally great Bob Pease died on his way back from Williams memorial ceremony in a car crash.

But there is a superb article about pranks perpetrated by the likes of Williams and Bob Widlar (of current mirror fame), and technical pranks perpetrated against them, here https://www.edn.com/pranking-bosses-...d-competitors/

Craig
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Old 4th Sep 2020, 2:08 pm   #11
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Default Re: Grey beards and boffins; what do see when you see a circuit.

I see the flow and function, almost seeing the electrons doing their bit. Capacitors are spiky, inductors smooth. Almost got it from day one, I have always loved looking at circuits and imagining them working. Then again I am quite odd!
 
Old 4th Sep 2020, 2:13 pm   #12
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Default Re: Grey beards and boffins; what do see when you see a circuit.

I'm grey-haired, but certainly not a boffin. Buffoon is more like it...

But like several other here I start by trying to find the various blocks. 'That's the local oscillator'. 'That's an address decoder'. 'That's a state machine producing the memory timing signals'. 'That's the phase splitter driving that push-pull out stage'. (not all in the same device of course). I only start thinking about voltages and currents (or relative signal timing, or..) when I am looking at the block I am interested in.
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Old 4th Sep 2020, 3:14 pm   #13
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Default Re: Grey beards and boffins; what do see when you see a circuit.

Smoke usually
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Old 4th Sep 2020, 3:29 pm   #14
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Default Re: Grey beards and boffins; what do see when you see a circuit.

Nice circuit diagrams separate the blocks for you, and don't try to put everything onto one page, whatever the cost.

As far as blocks go, there are a few basic circuits like common emitter amplifier, common base amplifier, emitter follower... then there are FET and valve variants on these basic themes.

THere are inverting and non-inverting opamps, voltage followers, virtual ground summing amplifiers.

Analogue switches using FETs, bipolars or diodes.

Into the RF world you get block filters, tuned amplifiers, broadband amplifiers. When you look in a bit more detail you see class A, B, C bias.

You have switch mode PSU topologies. Buck, Boost, Flyback, forwards converter.

You just pick up the basics as you come across them, then you start to pick up the variants on those themes. It isn't something you can cram, it's something you absorb, and then you start to see the variants on those themes. You don't need to plan a hierarchy of them in your head, their derivative nature gives you one ready-built.

I've watched Andy pick up new circuit structures and ideas and find his way around them. He's making decent progress and going at a rate he's comfortable with. If he just keeps on going, he'll get wherever he wants to go.

Universities teach circuitry and their analyses. If you're OK they last in your head long enough to get through that year's exams. Then you forget them as you load up on the next year's burden. Later on when you're doing things for a living, something rings a bell. You're not sure but you've seen something like this before. If you're lucky, you remember enough to know where to go looking to play catch-up. After you've done something useful with it, you'll remember it. You also really, really remember your whoopsies. Everybody makes them!

In the case of the Fostex circuit, there's a large group right now learning from someone else's whoopsie.

Learn by your experiences.
Learn by your errors, but learn by someone else's if possible.

Those 'Bad Circuits' are one of thr best bits in Horowitz and Hill 'The Art of Electronics'

David
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Old 4th Sep 2020, 3:47 pm   #15
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Default Re: Grey beards and boffins; what do see when you see a circuit.

Quote:
Originally Posted by Radio Wrangler View Post

Those 'Bad Circuits' are one of thr best bits in Horowitz and Hill 'The Art of Electronics'

David
I agree, and it's a pity they don't appear (at all) in the 3rd edition of said book
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Old 4th Sep 2020, 4:19 pm   #16
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Default Re: Grey beards and boffins; what do see when you see a circuit.

Bad circuits have moved here: https://artofelectronics.net/bad-circuits/
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Old 4th Sep 2020, 4:28 pm   #17
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Default Re: Grey beards and boffins; what do see when you see a circuit.

Valve circuits have for me a useful clarity in that the valve symbol is instantly recognisable as the basic amplifying block in the circuit. After all, it's amplification that's at the heart of electronics. The valve may of course be sitting in a circuit that uses its gain to oscillate, or uses particular electrodes in an RF mixer circuit, but, as long as the signal flow is shown in the conventional left to right form, it's not usually too difficult to work out what's going on.

Likewise, op amp circuitry shows its gain blocks pretty clearly. The op amp symbol even conveniently shows us which way the signal is going. And a look at the feedback network connected to the inverting input gives a strong clue as to circuit purpose, whether just an amplifier, and integrator, or precision rectifier. So far, so recognisable.

However, where I can have a problem interpreting the mind of the designer is in discrete transistor circuitry. Here I do empathise with the original question. Some annotation on the circuit diagram would often be helpful in sorting out the intended transistor functions. When transistor circuits were first being designed in the 1950s, the designer's mind was still based on valves, so sorting out the functions of sequential stages is usually straightforward. Later, however, transistors began to arrive in two opposite flavours - not just PNP, but also NPN. Then clever designers started to use a transistor as a constant current source instead of using a resistor. Likewise, 'cascode' circuits became common, not to mention Vbe multipliers in power amplifiers and transistors as stable voltage sources.

Integrated circuit designers of course then found that a transistor on the chip is much cheaper than a resistor, so have used prodigious creativity to avoid designing with anything much other than transistors. Direct coupling between stages became feasible using both NPN and PNP devices, usefully eliminating capacitors, but enabling one fault to propagate right through the circuit, taking out any number of devices like the collapsing of a house of cards.

So instead of the major circuit blocks being clearly noticeable by looking for the active devices, a discrete transistor circuit diagram can resemble a maze of interconnections between numerous transistors. A clear description of the designer's intention is virtually essential for a complete understanding of the functionality. The role of the technical writer continues to be important in interpreting the designer's linguistic obscurities to the service technician working under the rack with boards sticking out on extenders and where the test prod just slipped and changed the fault symptoms.

Martin
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Old 4th Sep 2020, 6:14 pm   #18
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Default Re: Grey beards and boffins; what do see when you see a circuit.

I've spent a fair bit, maybe too much, of my life drawing circuit diagrams, often for manufacturing, training and even publication. Good layout and making standard circuits stand out helps as well as following the best guidelines - avoid close parallel lines, avoid lines crossing and show clear joints. Challenge yourself to have as few crossing lines as possible. The old US standard of having 'B' supply lines at the bottom of the diagram leads to many crossing lines and, to me anyway, is much less clear than the British/European style with the HT at the top. However, Tektronix set a very high standard in their tube equipment manuals.

I find a true circuit diagram much easier to follow than a diagram which tries to place components in their mechanical location - that usually leads to many crossing lines and a great loss of clarity. Try laying out a star-delta motor starting circuit as a circuit diagram and then as a wiring diagram. The first shows the function clearly and helps understanding, the second is almost impossible to understand but possibly easier to wire up.

One pet hate, currently causing me consdierable trouble is wiring diagrams which show only terminals with no information about what is inside the "box". (They're very popular with central heating equipment manufacturers, amongst others). How anyone can follow the circuit logic to find faults I cannot imagine. It also leads to the mindset that there is only one way a system can be designed and any variation is "wrong" no matter its technical merits.

PMM
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Old 4th Sep 2020, 7:17 pm   #19
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Default Re: Grey beards and boffins; what do see when you see a circuit.

Thank you all for your thoughtful posts. first off "grey beards and boffins" was just shorthand for more experienced/ been at it a few years etc.

The point about chopping a schematic into functional blocks is a good one. I was also quite surprised that most if not all of you don't see a circuit in terms of voltage and current. I was almost certain that a lot of you would be thinking in terms of the Thevenin or Norton's equivalent circuit of some such.

I'm also aware that we're all coming at a problem from different angles, some as repairers, some as designers or as a combination of the two.

When I look at a circuit board, PCB or hand wired circuit in a device it's often difficult to see where all the connections go, I may see a collection of components and see a bridge rectifier made up of individual diodes, or see resistors and low value capacitors in conjunction and see a filter, but a schematic to my mind shows the relationship of each building block to each other, that and signal flow and a few other things. A schematic from a repair POV also obviously helps you locate components that may be otherwise obscured on a densely packed PCB.

As I said in my original post as regards reading schematics I hoped to gain an insight, a few tips from the more experienced. I found this sentance intriguing " Capacitors are spiky, inductors smooth. " could you elucidate please "merlin"?

The best circuit description I've ever come across was in a Tascam 38 R2R manual. the full schematics were shown, and then each individual function was clearly explained, IE "when the play button is pressed, Q1 switches on, turning Q2 off, which in turn also turns off Q5. IC3 pin 6 then goes high, enabling....." etc, hope you get the picture. Each description was accompanied by a small schematic, showing voltages etc. I think I gave it to Michael Maurice, wish I'd have kept it, it was an education in itself.

If there's one aspect of schematics I hate and find difficult, it's multiple wafer switches. Often all that's shown are numerous digits floating around in space, in a circular format joined by dotted lines with no explanation of their inter correlations.

Thanks again for your IP, Andy.
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Old 4th Sep 2020, 8:25 pm   #20
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Default Re: Grey beards and boffins; what do see when you see a circuit.

It is the way I see currents to through them, almost in colour (blue for caps.).
 
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