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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 23rd Sep 2022, 7:22 am   #1
llama
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Default Stylophone dud speaker

A recent family conversation had "Stylophone" mentioned so, knowing I had at least one of them, I popped a PP3 in it. No sound.

I plugged an amplifier into the line out socket and everything worked as it should, but no sound from the speaker. It was open-circuit. It's clearly marked 75 Ohms - I guess that means super-thin voice-coil wire so more vulnerable.

I downloaded the circuit from the Internet, having determined that I'd got the PUJT version. The audio output stage is a single transistor driving both the line-out socket and the 75 Ohm speaker.

I don't think I own a 75 Ohm speaker but found an identical footprint 8 Ohm speaker that'd come out of an old PC. I suppose I could have re-designed the output stage to directly drive this speaker but lack the skills to do so - at least in a sensible time-frame. So I thought of using perhaps a dual or quad op-amp all paralleled-up to drive it, taking advantage of the class B output stages. In the end I thought a single 741 might do it so quickly knocked-up a voltage-follower and fed the speaker via a 100 Ohm resistor to limit output current. This worked but was too quiet. So I gradually reduced the series resistor and ended up with just a wire.

A bit of tidying-up and the boxed up Stylophone now works to my satisfaction and possible use by the grandchildren.
Graham
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Old 24th Sep 2022, 10:41 am   #2
Paul Stenning
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Default Re: Stylophone dud speaker

This is a good solution using parts that were to hand.

Another possible solution could have been a small output transformer from an old pocket transistor radio. Although not intended for the standing current of a single transistor output stage, it would probably work OK.
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Old 24th Sep 2022, 1:49 pm   #3
paulsherwin
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Default Re: Stylophone dud speaker

An LM386 could also be used. These can work with a very low component count if the application isn't very demanding.
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