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Old 4th Aug 2020, 2:47 pm   #1
vinrads
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Default I found this component on a German Juke box amp

This amplifier is similar to the Rock Ola ones I am used to working on , but a different layout , and it uses an ECL82 instead of a 6CY7 in the AVC circuit ,I have often wondered what this component was it looks like a diode, in the cct for the Rock Ola it shows a resistor symbol with a ring around it ,A very strange cct you will agree , I am thinking it is a voltage dependent resistor , one reads 3.56 k both ways , the other o/c the socket on the left is where the volume control plugs in , your thoughts please , Mick.
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Old 4th Aug 2020, 2:58 pm   #2
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Default Re: I found this component on a German Juke box amp

I am wondering if it's a thermistor .

Difficult to see but it looks as if there is a little bead inside.

Try warming it up whilst measuring the resistance.

Cheers

Mike T
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Old 4th Aug 2020, 3:12 pm   #3
vinrads
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Default Re: I found this component on a German Juke box amp

Ah ,yes Mike it does act like a thermistor going down to 250 ohms when I poke it with the soldering iron , the question is what does it do in the cct , and I need to replace the o/c one, Mick
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Old 4th Aug 2020, 5:19 pm   #4
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Default Re: I found this component on a German Juke box amp

I believe it's a thermistor used in the AVC circuit.
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Old 5th Aug 2020, 1:44 am   #5
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Default Re: I found this component on a German Juke box amp

It looks like 4K7 at room temperature.
It looks like it is meant to increase the negative feedback as the signal increases.
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Old 5th Aug 2020, 8:20 am   #6
vinrads
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Default Re: I found this component on a German Juke box amp

I agree, it is in the alc circuit but why is a temperature sensitive device is used in this way what would affect its temperature ? current flowing through it maybe . the problem is I need to replace the faulty one any suggestions please .Mick.
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Old 5th Aug 2020, 9:56 am   #7
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Default Re: I found this component on a German Juke box amp

Exactly so. As the volume increases, so the drive to the thermistor increases, reducing its resistance and thus increasing negative feedback to reduce amplifier gain and maintain volume roughly constant.

Similar tricks can be done using a small light bulb though its TCR will be positive so the circuit is arranged differently.
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Old 8th Aug 2020, 11:41 pm   #8
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Default Re: I found this component on a German Juke box amp

You could try a modern incarnation of an NTC 4k7 thermistor - some through-hole ones here at Farnell: https://uk.farnell.com/c/circuit-pro...g=through-hole
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