View Single Post
Old 1st Aug 2011, 12:47 am   #7
Kat Manton
Retired Dormant Member
 
Kat Manton's Avatar
 
Join Date: Jul 2005
Location: West Yorkshire, UK.
Posts: 1,700
Default Re: circuit diagrams.

Quote:
Originally Posted by bargoedboy View Post
Hi, do you guys read a circuit just by looking at the piece of equipment concerned, or do you actually need a circuit diagram?
I can't speak for any of the guys, but this is an example of what one of the girls gets up to: Philips PM5100 L.F. Generator

The day I took the cover off, I'd drawn a diagram of one of the PCBs.

Going by the post dates, within six days of taking the cover off, I'd traced, drawn (in CAD software) and posted a complete circuit diagram.

The problem with this when you're paying someone else to repair an item is it takes time. If they're charging a reasonable hourly rate, you could easily end up paying £250 plus for reverse-engineering and £25 to have the fault repaired. It isn't commercially viable; why many repairers won't touch something if they can't get or aren't provided with a diagram.

I do this for fun; it doesn't matter how long it takes to get something working if I'm keeping it. To get something back for the time and effort I put in I'd have to sell items for considerably more than anyone would pay for them. Would anyone pay £300 for a scruffy MW/LW radio..?

The way to avoid either rejection or large bills from repairers is to learn to do it yourself. Major components (valves, transformers, big electrolytics, pots, sockets) are easily identified. Valve function is fairly obvious from their type and physical location on the chassis. Valve pin-outs can be found on the 'net. So you can draw all these bits in roughly the right place, then trace out connections from the valve electrodes etc. you've identified to passive components (resistors, capacitors) and draw those too. It's easy when you've done it a few times and have looked at diagrams of other similar equipment; you know roughly what to expect. There aren't zillions of ways to wire up 4 x EL34 and a transformer to make an output stage, for example.

This will be simple; it's just an amplifier. Amplifiers aren't complicated as the function they perform isn't complicated. Turning an audio signal into a bigger audio signal is a lot simpler than extracting one tiny radio-frequency signal from other signals and noise then turning it into sound and moving pictures.

There might be more valves in it than most but when most of them are used as straightforward voltage amplifiers, it's just a lot of simple things connected together, therefore overall it's simple.

Quote:
Originally Posted by G8HQP Dave View Post
Some people can look at an item, draw a diagram in their head, then fault trace on that. Ordinary people have to write it down.
Ah, cool; I'm not ordinary... I think I've known this for years...

Kat
Kat Manton is offline