An Intimidating Experience

[Nowhere-Man]

Suggest that you read about and understand Ohm's lawa and Kirchoff's laws.

http://www.bbc.co.uk/schools/gcsebitesiz...rev3.shtml
http://www.electronics-tutorials.ws/dcci...dcp_4.html

Kirchoff's most widely known theory is the junction current, whereby current coming into a circuit or crossroads has to part the circuit at equal value.

[ppppenguin]

Suggest that you read about and understand Ohm's lawa and Kirchoff's laws.

http://www.bbc.co.uk/schools/gcsebitesiz...rev3.shtml
http://www.electronics-tutorials.ws/dcci...dcp_4.html

Kirchoff's most widely known theory is the junction current, whereby current coming into a circuit or crossroads has to part the circuit at equal value.

Sorry, your explanation doesn't quite make sense, even as English**, let alone engineering. I thnk you're trying to explain KCL, which simply states that the sum of currents flowing into a junction is always zero. It can also be stated as the sum of currents entering a junction must be equal to the sum of currents leaving that junction.

If you apply KCL, KVL and ohm's law to any linear circuit (and many non-linear ones) your should understand what's happening. A certain caution is needed if non-linear devices are included as ohm's law can only be applied at the particular voltage across the non-linear device and current though it. A filament lamp is not linear (its resistance rises with temperature) but ohms law can still be usefully applied. That same non linearity also makes it useful in a lamp limiter. At low currents the resistance is low, so the radio etc will operate more or less normally. If the radio is faulty and tries to take a high current the resistance of the lamp will rise and thus protect the radio.

Once you've really understood KCL, KVL and ohms law you tend to use them instinctively, without even reailising you're using them. That comes with experience.

**With apologies if English isn't your first language.

[Nowhere-Man]

Suggest that you read about and understand Ohm's lawa and Kirchoff's laws.

http://www.bbc.co.uk/schools/gcsebitesiz...rev3.shtml
http://www.electronics-tutorials.ws/dcci...dcp_4.html

Kirchoff's most widely known theory is the junction current, whereby current coming into a circuit or crossroads has to part the circuit at equal value.

Sorry, your explanation doesn't quite make sense, even as English**, let alone engineering. I thnk you're trying to explain KCL, which simply states that the sum of currents flowing into a junction is always zero. It can also be stated as the sum of currents entering a junction must be equal to the sum of currents leaving that junction.

If you apply KCL, KVL and ohm's law to any linear circuit (and many non-linear ones) your should understand what's happening. A certain caution is needed if non-linear devices are included as ohm's law can only be applied at the particular voltage across the non-linear device and current though it. A filament lamp is not linear (its resistance rises with temperature) but ohms law can still be usefully applied. That same non linearity also makes it useful in a lamp limiter. At low currents the resistance is low, so the radio etc will operate more or less normally. If the radio is faulty and tries to take a high current the resistance of the lamp will rise and thus protect the radio.

Once you've really understood KCL, KVL and ohms law you tend to use them instinctively, without even reailising you're using them. That comes with experience.

**With apologies if English isn't your first language.

I think I was trying to say where you have a milliamp value which enters a circuit junction, say, 0.2 Amp plus 0.5 Amp coming into the junction, the rule is you can't have a higher value leaving the same junction on the way out. It's ages ago since I needed to use it, but put simply I'm aware current doesn't change in series resistances which is why the current stays the same in a series heater circuit but voltage can drop along a chain. For example, in an ACDC set a large resistor is used to drop voltage for HT and also for the heater chain voltage. This needs to be worked out in case where a line dropper is broken and the resistances may need to be worked out but, again, it was ages ago since I needed to do this.

It got more complex when capacitors were used but I totally forgot the maths to that.

I just simplify it and think of voltage dividers.

In this specific case, if a 100 watt resistor was fixed value and not a lamp, I'd look at the first resistance as being a bit above 500 Ohm and then the set itself is going to have a resistance too. Not quite sure what that is by memory although I think typical current is usually 170 milliamps So, the voltage is going to be divided between 2 resistances.

Anyway if it's a 100 watt lamp and the lamp burns bright then current must be rising in the set while voltage drops. Presumably if the lamp is dull then there ought not to be excessive current draw in the set.

I have heard of lamps being used as resistors but mainly in cases to drop voltage in a line chord. Or the term "barreter" rings a vague bell.

[Nowhere-Man]

I should add I looked into current limiters and will be using this when I try to start the set. Not sure why Chas Miller doesn't ever refer to either current limiters or variacs in the book he wrote but, all in all, a current limiter seems like a good idea.

I will be checking neutral and line resistances first to check for any short circuit. Lots of other basic tests too, around heater circuit, dropper and so on.

The dodgy cap, I remarked earlier was smoking, specifically goes across chassis ground to the anode of the rectifier valve. However, there is a 0.005 I could also do with changing.

maybe a dose of whisky before plugging in?

[Nowhere-Man]

This job is now more or less wrapped up so am enclosing a pic of the chassis as it is now. I found soldering these film capacitors a bit of a pain at times due to the very short leads so think next time I'll try and get hold of long leaded capacitors such as Spragues that seem far more convenient to use.

The mains filter capacitor is rated for mains safety and that is now soldered between the rectifier anode and ground. Hopefully this will now be far safer than the old half-melted wax Hunts.

I did a test today between neutral and live and I found the resistance to be one and a half K. Not sure why the plug fuse was a 3 amp? I'll have a look what resistance is quoted in my service manual.

Very soon it will be ready for testing.

In one case I tried soldering multi-strand to lengthen one of the capacitors where the leads were too short. I then taped around most of the connection to eliminate any chance of a short. I'll probably use a few cable ties as well to keep the wires snug.

[Refugee]

I use these for restoring valve kit and they can often be slipped inside the shell of the old wax ones to make the set look original.

http://uk.farnell.com/vishay-roederstein...dp/1166877

[tiv48]

or if you don't mind buying from china
http://www.ebay.co.uk/itm/10pcs-yellow-l...ltDomain_0&hash=item257810df62
Rob T
PS other values available

[Nowhere-Man]

I use these for restoring valve kit and they can often be slipped inside the shell of the old wax ones to make the set look original.

http://uk.farnell.com/vishay-roederstein...dp/1166877

The wax paper caps were so bad I'd not have really been able to even stuff them. I didn't know the set was as old as 1947. Actually it came from a lovely house and belonged to a lady whose husband had died and I think she just wanted to sell what she didn't need. So, I winded up with it and, as she had plugged it in, I tested it back home and managed to get one station.

Only recently did I become aware it was 1947 and many wax caps were sticky to touch as there were signs of them melting.

The new caps are very high voltage and all from Maplins. However, I dislike sometimes having to try and solder wire to the leads to span across soldered points which is why I prefer long leads.

Maplins have offered me a inline fuse at about 500 Milliamps if I want to buy one. Actually I was looking into how much current would need to be drawn to be harmful to the valves. Many ACDC sets hover around 0.17 Amp so not sure if 500 milliamps would be low enough.

[Nowhere-Man]

or if you don't mind buying from china
http://www.ebay.co.uk/itm/10pcs-yellow-l...ltDomain_0&hash=item257810df62
Rob T
PS other values available

Thanks. I think next time I will get some of my caps specially made for radio friendly work but will also continue to shop at Maplins as I found the staff were terrific. Always very helpful and patient and maybe a bit amused I have bought up nearly all their higher voltage stock.

[Nowhere-Man]

Am logging off shortly. Weekend may be spent making a very simple current limiter. I'll test it by plugging in a lamp and see if it dims due to the voltage drop across the eliminator bulb.