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VMat 13th November 2012 05:12 PM

"Sensitive DC" relay coil

I'm going to replace a relay in my amp (that same Technics I've been talking about recently). I've found an apparently suitable replacement, but it has a "sensitive DC" coil. Can you explain to me what's the difference between that "sensitive" coil and a regular one? The only difference I could find in the catalog is that the "sensitive" coil has a higher resistance (and a corresponding lower current under nominal voltage). Not sure if that's relevant, other than making me change the resistor in series with the coil to adjust the voltage/current accordingly.


Mooly 13th November 2012 06:12 PM

That would be my understanding of a "sensitive" relay too.

cliffforrest 13th November 2012 06:44 PM


you are 100% correct.

I think the term comes from early electronics where a "sensitive" relay could be driven directly from a sensor without the need for an amplifier or switch. This was a big deal when any sort of gain involved vacuum tubes.

As you suggest, a series resistor could be used, often not necessary since over-driving a relay coil is no big deal - but do the math!


VMat 13th November 2012 09:31 PM

OK, great!

About the resistor: it's already there, they feed the 24V coil out of 40~45V. The coil in the relay I found is about twice more resistive, so I need to double the series resistor as well. I may try a 48V coil and drop the resistor, if I can find such a relay.

Funny design. If it were today, I guess it wouldn't get any "green" awards for wasting ~0.5Wh to drive a single relay...

Thank you, guys!

VMat 18th November 2012 02:16 PM

Just to let you know: replaced the relay with a Finder 4052.7024 ("sensitive" type). Had to change the series resistor and add some pieces of wire (about 1" long) to extend the terminals and make them fit to the existing holes in the board - drove me nuts for a few minutes, but in the end, it worked well.

The old relay was bad, indeed. I opened it, and I can see one of the poles is closing much sooner than the other. The coil is probably not strong enough to make the second pole close properly, and that's why the right channel sometimes would not play at low volume levels.

Apart from that, there was another curious defect. Sometimes I still wouldn't hear one of the channels after I put the bottom cover on. It only happened to one of the inputs. The resistance between that input and the chassis was zero, so at some point the bottom cover should be touching the circuit. After excluding some candidates, I finally looked at this small piece of metal which holds the heatsink. It's screwed to the bottom cover, and touches the copper side of the main board exactly where the input signal tracks are (going from the RCA terminals to the input selection switches). The metal thing has been moved around a few times now, and scratched the green insulation on the board. A little piece of paper there, and... voilá! It's working now.

The amp has been sitting in a closet for years. I'm glad I restored it. Will do the job in my home office very nicely. At a minimum, it's much better than the mini-system I've sold recently.

Thanks again for your help!

Mooly 18th November 2012 04:11 PM

Thanks for the update on all that, great to hear you have sorted it.

(Sometimes if a part won't fit then it can be worth re-drilling the PCB).

cliffforrest 18th November 2012 04:40 PM

+1 :up:

VMat 18th November 2012 09:24 PM


Originally Posted by Mooly (
Thanks for the update on all that, great to hear you have sorted it.

Just another way so say "thanks". :)

I'll feel bad if I just grab some info and don't get back with the results.


Originally Posted by Mooly (
(Sometimes if a part won't fit then it can be worth re-drilling the PCB).

Oh, I thought I should do that at first, but it's 8 pins, and there are several tracks running between and around them, so I gave up...

All the best!

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