Ferranti 545 power supply

[stuart_morgan_64]

I need some explaining re the series power supply circuit markings. On the dropper resistor I would assume the 16, 17, 20, 18 are the value of the resistance in ohms. The varite on the dropper is 20 ohm? The varite in parallel with the dial lamp is 25 Ohm? It reads 800. Can these be replaced with a normal resistor.

I assume the 270, 250 is the voltage input select, whats the 746?

How does a Varite differ from a resistor.

Thanks.

[Nuvistor]

The 16,17 etc are the reference numbers for the components list. The 270, 250,746 are the resistors values in ohms.

The varite is a negative temperature coefficient resistor, that is high value when cold and low value when hot. This limits the switch on surge from cold, current through the varite heats it up and the resistance falls.

[ms660]

Originally Posted by stuart_morgan_64

I need some explaining re the series power supply circuit markings. On the dropper resistor I would assume the 16, 17, 20, 18 are the value of the resistance in ohms. The varite on the dropper is 20 ohm? The varite in parallel with the dial lamp is 25 Ohm? It reads 800. Can these be replaced with a normal resistor.

I assume the 270, 250 is the voltage input select, whats the 746?

How does a Varite differ from a resistor.

Thanks.

So far as I can make out the resistance values are on the right, the circuit references are on the left, so, 270, 250,746 are the resistors values in ohms.

Varite is another name for a thermistor.

The resistance of a Varite changes with temperature, if cold at switch on it's resistance is comparatively high to its hot resistance, from cold some current flows which results in heat, the heat produced causes it's resistance to reduce, causing more current to flow causing more heat etc etc.

EDIT: Post crossed.

Lawrence.

[Sideband]

...so in a nutshell, you can't replace the 'varite' with an ordinary resistor. Unless it's physically broken it should be fine. The one in parallel with the lamp won't do much unless the lamp has failed. It's there primarily to keep the radio operating when the lamp fails.

[Boater Sam]

Looks like there is nothing out of spec unless bits of the dropper are open circuit.

[stuart_morgan_64]

Hya, thanks for all the replies. The one on the voltage dropper has broken, just two end caps left. What could I use as a substitute? Thanks

[ms660]

Possibly a VA1010 if you can find one, if not then search for a modern equivalent, something around 200 ohms at 100ma and in the kohms at 25 degrees C.

Lawrence.

[stuart_morgan_64]

I have been looking and searching a little more, easier at home with a PC and not a phone screen. Most of what I am finding is ones rated at 3A or so, so I assume will never warm up enough to drop resistance.

[Nuvistor]

You could try the wanted post section of the forum or look for a scrap chassis that uses a Varite .