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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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4th Aug 2020, 2:47 pm | #1 |
Dekatron
Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
Posts: 3,763
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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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4th Aug 2020, 2:58 pm | #2 |
Moderator
Join Date: Dec 2010
Location: Exeter, Devon and Poole, Dorset UK.
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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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4th Aug 2020, 3:12 pm | #3 |
Dekatron
Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
Posts: 3,763
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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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4th Aug 2020, 5:19 pm | #4 |
Heptode
Join Date: Jan 2008
Location: Sheffield, South Yorkshire, UK.
Posts: 656
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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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5th Aug 2020, 1:44 am | #5 |
Dekatron
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,554
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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. |
5th Aug 2020, 8:20 am | #6 |
Dekatron
Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
Posts: 3,763
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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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5th Aug 2020, 9:56 am | #7 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,535
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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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8th Aug 2020, 11:41 pm | #8 |
Nonode
Join Date: May 2016
Location: Cornwall, UK.
Posts: 2,336
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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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