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Tech Chat Reading circuit diagrams

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Cathovisor
(@cathovisor)
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Posted by: @mfd70

One thing to remember when reading voltages in vintage electronics, which often catches me out, is the Fluke true RMS meter (or equivalent) you are using would be more accurate and have a higher input impedance than anything in existence outside a research lab back then. Voltages can be out by quite a bit from the stated values in the manual and not effect the operation of the circuit. Old British TV and monitor manuals from as recently as the eighties would state that the voltage readings were measured with an AVO 8 and we always had a "good" AVO in the workshop for voltage checks.

Not necessarily: on the 1000V DC range an AVO has an input resistance of 20MΩ, whilst the Fluke will be 10MΩ. This sounds a lot but it can have a bearing on those circuits (like voltage multipliers) that have a high source impedance to start with. Where the modern Fluke wins is at low voltage measurements - however, a good service manual will often specify what voltage range the measurements were taken at where an AVO was used, even more so when the AVO 7 (500Ω/V) was in common use. This is the reason why I have an AVO 7 to hand for old sets, it saves me having to do the mental adjustments.

What I would say to any new engineer using a digital meter is not to obsess about readings down to the third digit after the decimal point - unless you're measuring a precision voltage reference it's really not necessary.

 

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Posted : 07/01/2022 2:50 pm
Sundog
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@cathovisor Yes especially when many of the circuits we work on have 20% tolerance components.

I take my hat off to the designers who made the stuff work well enough with those components. Of course better tolerances were available but cost was and still is a major factor.

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Posted : 07/01/2022 7:36 pm
sideband
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Posted by: @red_to_black

the key is picking out the salient points and recognisable circuit blocks, not to try to assimilate the whole diagram, especially on complex systems such as TVs and VCRs which to be fair has used some fairly exotic circuitry over the years.

That is one of the most relevant comments! I remember back in the early days when I started in the trade. I could read circuit diagrams but had trouble with some of the more complex TV circuits. On one occasion I was trying to find the video amplifier and the sync separator in the circuit of a Ferguson 405 line TV. Probably didn't help that I was using one of the Newnes R & TV books and the circuit was spread over several pages. The chief engineer came over to help and stressed the importance of finding the parts of the circuit you DO know.....in this case find the CRT first....easy. From there trace the circuit from the control grid of the CRT (grid modulation in those days) back (over the page).....through a capacitor and choke to the anode of....the video amplifier (in this case a PCF80). From the video amplifier you can follow the circuit down to the sync separator. With a little bit of explanation, he made it easy and I was soon able to break circuits down into manageable chunks. 

It's more difficult for beginners these days as the mentoring isn't available.

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Posted : 08/01/2022 10:38 am
19Seventie
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Posted by: @red_to_black

the key is picking out the salient points and recognisable circuit blocks, not to try to assimilate the whole diagram

This is very much an area that confuses me, I do tend to look at the whole thing rather than specified areas, but that's usually because I'm not quite sure where one section ends and another begins. That being said, I think I have a GEC 2028 circuit diagram which uses colour blocks to specify each section, I should spend some time studying that to get an idea of what to expect in each section. 

Posted by: @sideband

It's more difficult for beginners these days as the mentoring isn't available.

This is what I've struggled with, I've had brilliant guidance in other terms with fault-finding and such from many people on this forum and on UKVRRR too. Everything I know is owed to those who have helped me, but understandably explaining circuit diagrams over text and a few pictures at a time isn't an easy feat. 

My knowledge on ohms law has increased massively over the past couple of days too, which I think will help a lot next time I have to read a diagram. We'll see where we get with that!

Thanks all for the help, I really appreciate it!
'70

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Topic starter Posted : 09/01/2022 8:29 pm
crustytv
(@crustytv)
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Posted by: @19seventie

This is very much an area that confuses me, [..] I'm not quite sure where one section ends and another begins. That being said, I think I have a GEC 2028 circuit diagram which uses colour blocks to specify each section, I should spend some time studying that to get an idea of what to expect in each section. 

You should also have a look at Thorn circuit diagrams, especially the 3000 series through to the TX series. To me, these are some of the finest examples of how circuit diagrams should be done.

To assist you in your endeavours, I have fast tracked your account to have full access to the Forum Data library. Please familiarise yourself with rule #5 pertaining to Data library conduct.

You will not only find multiple examples of period service data, but a good many books, like the patchett series, used as part of the City & guilds training for Television engineers.

p.s.

You may also, at some point in the future, find the training exams the TV engineers had to take, of some use or at least some interest. They date from 1969 though to 1983

https://www.radios-tv.co.uk/vintage-electronics-blog-forum/city-guilds-radio-television/

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Posted : 09/01/2022 8:47 pm
19Seventie
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Hi crusty, thank you so much for granting me early access, I really appreciate that. I've just had a quick nose at the Thorn 3000 circuit diagram and I see what you mean, that looks brilliant. I think that may be one of the clearest diagrams I've seen to date. I've had a re-read of rule 5 and agree to all T&Cs. 

I'll certainly have a look at the books and other documents related too, and I think I'll sit some of those tests myself a few times to see where my strong points are and weak points and go from there. Who'd have thought anyone would want to sit and look at exam papers after leaving school, yet here we are! 

I know what I'm going to be doing for the next few days when I get chance!

Thanks again, really appreciate the help
'70

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Topic starter Posted : 10/01/2022 12:44 am
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