Trouble finding replacement for 1960s resistor

[thejamesrankin]

Hi,

I'm having trouble finding a replacement for a failed resistor from the 1960s that reduces the power to a small motor driving a timer. Stamped on the resistor is "4k ohms" and the number "114". I know very little about resistors but I thought I'd try here first as you guys know your stuff. The power coming into the resistor is 240v AC.

I've attached a couple pictures that may help. It's definitely on its last legs as it powers the motor for about 6 seconds then cuts out and the resistor smokes.

Regards,

James

[Boater Sam]

That appears to be a wire wound resistor, if it's smoking hot then the motor is drawing too much current, the resistor is not the problem.

[Diabolical Artificer]

I think those are 7w resistors. Measure its resistance - no power and the voltage across it and post your result.

Andy.

[thejamesrankin]

Hi Sam,

The motor works fine on full voltage (it's a dual voltage motor) and the timing mechanism it drives is free running. Can you decipher what resistor I should look for just in case I need a spare?

Thanks,

James

[Restoration73]

The resistor may run warm or even hot to the touch when used.
http://uk.farnell.com/welwyn/w22-3k9...3k9/dp/9504958

[ITAM805]

It looks like 4k, this one should do the job 4k7

[thejamesrankin]

Hi guys, thanks for your answers! Out of curiosity how do you figure out it's 7 watts? What does the number "114" stand for?

James

[AC/HL]

Rosenthal is the Manufacturer, 114 is probably a type number, the Internet is not very forthcoming unusually.

[G6Tanuki]

Those type of resistor do run rather warm!

One question: has the equipment been out-of-service for some time? If so what you're seeing may well be steam rather than smoke: the formers used in some similar power-resistors are slightly hygroscopic and absorb moisture from the atmosphere over time. Then when power's reapplied the heat boils-off the water as visible steam!

[In the case of the mains dropper-resistor in an Eddystone 840A that had spent years in an outdoor unheated shed, the steaming was rather spectacular and had me reaching for the fire-extinguisher!]

[RF Burn]

My 10 pen'rth:

If the resistor is getting hot then this almost certainly means that the resistor is GOOD.
Wirewound resistors almost invariably (unless the cement coating is damaged and the turns shorted together) fail open circuit, hence no current will flow, no heat will be dissipated and the motor will not run for even 6 seconds as the OP states.

The value of the resistor is 4K Ohms, the nearest modern value will be 3K9 which is close enough for 99.99% of electrical applications, however I do not believe that the power rating is 7 Watts, the resistor looks to be too long for that. Can the OP please measure the length of the resistor as it could be as high as 14 Watt rated (the manufacturer's marking 114 could be significant).

There is almost certainly a problem with the motor, the first suspicion is that it has developed shorted turns, anyway it will need repairing (rewinding), or replacing.

By the way the W22 resistors recommended as replacements are not rated for mains voltage use. (Limiting Voltage 200V)

[Diabolical Artificer]

The reason I said 7w is that I have the same or similar ones here, about 2" long, 1/4" width. The nearest Welwyn I have with known marked wattage is 7w. If the R in question is bigger then 7w is wrong

I suggested measuring the voltage drop then you can work out what current is being drawn using ohm's law and thus work out if the resistor is being over run.

Andy