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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 30th Jul 2011, 9:03 pm   #1
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Default circuit diagrams.

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 get a diagram for an old Valve amp of mine, and most guys won't work on it unless they have diagram.

I know enough to not kill myself but not enough to diagnose the amp for faults.
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Old 30th Jul 2011, 10:00 pm   #2
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Default Re: circuit diagrams.

The simplest circuits can be worked out just by looking but a proper circuit diagram is always a big help. Given sufficient time and perseverance it's perfectly possible to trace out circuits which are as simple as most valve amps are (this is much tougher for, say, the more sophisticated military equipment though). The hardest items might be potted transformers (plain tins with a bunch of leads coming out) but even these can be diagnosed with experience and a bit of basic measurement. And if you should come across any components you don't recognise then I would bet that the expertise available on this forum would be second to none (and I do mean none) in helping you identify them. One thing you don't get when you trace the circuit yourself though is the very handy operating voltages which are often included on the manufacturer's version.

Any info about the amp (make and model would be best) would give us a good start and a few pictures, inside and out, would be great too. If you decide in the end not to trace it yourself then you can probably find folks on here who will sort the amp out for you for a very reasonable sum.


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Old 30th Jul 2011, 10:48 pm   #3
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Default Re: circuit diagrams.

A well-drawn circuit diagram is a great help.

The average 5-valve superhet is quite possible to repair without (although when a dozen or more wires terminate on a wave-change switch, I find myself hoping that the fault is somewhere else). And as GrimJosef comments, wires disappearing into a tin box can be baffling - especially if originating from a metallised valve that the pinout details aren't available for (pardon the grammar).

Of course, if the Phantom Fiddler has got there first and made a few modifications, then without the 'proper' circuit diagram, the challenge is very much increased!
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Old 31st Jul 2011, 11:44 am   #4
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Default Re: circuit diagrams.

The amp in question is a bit more complicated than most.
Its a Little rock 50 made by White amps of sunderland back in late 70`s possibly early 80`s.
It has 4 EL84`s
and 4 ECC83`s and one ECC81 (12AT7) possibly the phase inverter or reverb driver.
As luck would have it one of the guys on here has been kind enough to take a look at amp for me, Mrthermionics as we only live about 5miles from each other.
But here is a couple of pics anyway.
this is where a resistor had burnt out and I was just about to replace it on Filter cap board.
I have since changed all the filter caps and small cap shown along with resistor.
and this is the main board, I am a little worried regarding solid wires being used as i would have thought Vibration in a loud amp may cause dry joints.
Thanks all.
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Old 31st Jul 2011, 12:28 pm   #5
G8HQP Dave
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Default Re: circuit diagrams.

If a circuit diagram is not available, reverse engineer it from the actual item as far as possible. When fault tracing, staring at a diagram is better than staring at the item (except where the faut is obvious). 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.
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Old 31st Jul 2011, 1:24 pm   #6
neon indicator
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Default Re: circuit diagrams.

Solid wires is fine.
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Old 31st Jul 2011, 11:47 pm   #7
Kat Manton
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Default Re: circuit diagrams.

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.

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...

Voider of Warranties.
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Old 31st Jul 2011, 11:58 pm   #8
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Default Re: circuit diagrams.

It very much depends what you're working on. With a bit of experience, a 1950s AM superhet radio can usually be completely restored without a circuit diagram because the designs are all so similar. More complex stuff is much more difficult. One approach is to trace out the circuit, but this can be very hard work and there's always the danger of making a mistake. It's always better to source an authoritative circuit diagram if at all possible.
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Old 1st Aug 2011, 2:55 am   #9
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Default Re: circuit diagrams.

First check the tubes.
Look up the diagram of each tube.

Most of the time anode-screen troubleshooting can fix almost any tube amp.
First check voltages on empty sockets. A missing voltage in one channel and not the other has it down to the stage. You really don't need to trace an entire circuit except where you find a voltage missing or way too high.

Get some tube extenders (or spend hours carefully wrapping small wires around one pin at a time of each tube).

Similar channel tubes should have the same anode voltages and screen voltages.
The tubes with wires running to the output transformer are the output tubes and should have the highest voltages.
The grid voltage should never be positive in reference to the cathode.
The input tubes use less voltage than the output tubes.
These general principles can keep something running without needing to recreate the entire diagram.
Good Luck
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Old 1st Aug 2011, 9:53 am   #10
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Default Re: circuit diagrams.

I find it much easier to reverse engineer a circuit which is "wired up" than to do the same for a circuit which uses a printed circuit board (PCB) I'm continually turning the board over to look at components and tracks. Shining a bright light on the back doesn't work very well. Photographing the track side and reversing the image can help.
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Keep the soldering iron hot.
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Old 1st Aug 2011, 6:54 pm   #11
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Default Re: circuit diagrams.

Graham, the way I do it is this. Draw a rough outline of the board and components, I guess you could use a digital camera now, shows when I did the last pcb
Then annotate each component i.e. R1 R2 C1 etc then identify each wire end as being either N S E or W i.e. North, South etc.
Compile a list which might go something like
continue until you have all nodes tabled then write alongside each where they go e.g.
R1-N C1-N TR1c
R1-S + supply
R2-E R3-N TR1e
R2-W 0V
When the list is complete you would have actually checked each node more than once,e.g. Checking TR1e should have R2-E and R3-N against it.
The process is a bit long winded, good for winter nights though in front of the telly but the end result enables you to draw a circuit diagram. I actually place the components down in the same orientation so R2 would be horizontal with its right hand leg (East) going to R3-N and TR1e. Hope this description gives you some idea how I do it but be prepared to redraw the circuit a few times as you find the circuit layout opens out, if you know what I mean. When I have arrived at the best draw I then sit down in front of the PC and do a 'proper' one to save or print.
With regards to using a circuit diagram for repair, yes I always endeavour to obtain one if I can. Knowing the fault, the circuit allows me to analyse and come up with the best test sequence. My best ever was looking at a Philips G8 that had intermittent Green. I viewed the circuits and arrived at the conclusion that it could be a Tantalum perhaps? on the RGB board. I changed what I considered to be the most likely culprit and presto! Green now perfect, a fault found just by analysing the circuit diagram. Priceless!
Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way!
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