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Old 4th Dec 2019, 2:30 pm   #1
stuart_morgan_64
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Default Grundig 7070W hot resistor.

I have this chassis from a radiogram, replaced electrolytics and caps exceot the pf range. Its on the bench with only 6 ohm speaker connected to the sockets in middle of the transformer. R43 1.3K which comes off of the bottom of the audio TX is getting very warm quickly. What is likely cause? Replaced with 1.2K 5W. HT around 220V

Any sugggestions please. Thank you
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Old 4th Dec 2019, 2:43 pm   #2
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Default Re: Grindig 7070W hot resistor

It's the HT feed to everything except the EL84 anode, so it's reasonable to expect it to get quite hot. Does the set work normally otherwise?
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Old 4th Dec 2019, 2:45 pm   #3
stuart_morgan_64
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Default Re: Grindig 7070W hot resistor

Its is working yes, but very hot very quick, whole unit on variac is drawing around 20ma, but within 30s to 1 min to hot too touch, i may be worrying for no reason
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Old 4th Dec 2019, 3:28 pm   #4
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Default Re: Grundig 7070W hot resistor.

All seems ok at the moment.
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Old 4th Dec 2019, 3:31 pm   #5
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Default Re: Grundig 7070W hot resistor.

If the resistor is overheating it will start to smoke and smell. It's normal for power resistors to be too hot to touch. They are dumping energy as heat.
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Old 4th Dec 2019, 3:46 pm   #6
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Default Re: Grundig 7070W hot resistor.

If you want to be sure that there are no problems with excessive HT current, why not measure it? It may not be given in the circuit but you can work out roughly what it should be by looking at the valve data. I doubt that the rest of the valves plus the screen grid of the output valve amounts to more than around 40mA so the resistor should be running at just under 2 watts. Ohms law for power (I sqrd R) so (0.04A <squared> x 1200 ohms) = 1.92 watts.....just a rough idea. Alternatively, measure the voltage across the resistor, square it and divide it by the resistance (P=E squared / R)
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Old 4th Dec 2019, 3:56 pm   #7
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Default Re: Grundig 7070W hot resistor.

If you check the voltage being dropped across the resistor and divide that by 1,200 (Ohms) you'll get the current in Amps (which will be much less than an amp of course). Then if you multiply the voltage drop by the current, it will tell you how many Watts is being dissipated in the resistor. 5 Watt flameproof wirewound ceramic resistors typically have an upper temperature limit of 150C - very hot indeed! EG:

https://www.esr.co.uk/components/pro...-resistors.htm

As has been said, it's the HT load resistor across the reservoir and smoothing cap, so has to work hard.
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Old 4th Dec 2019, 8:22 pm   #8
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Default Re: Grundig 7070W hot resistor.

According to a schematic I'm looking at, on FM, the voltage given for the main HT at the reservoir capacitor is 273 volts, the screen grid voltage on the output valve is given as 220 volts, that's a voltage difference of 53 volts, the resistance of the hum cancelling section of the output transformer will be small compared to the 1.3k resistor so can more or less be ignored, that leaves power dissipated in the 1.3k resistor as Vsquared/R which equals 53squared/1300 which equals 2.24 watts approx. which is within the rating specified in the schematic and well within the rating of the 5 watt resistor under normal conditions.

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