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Old 10th March 2013, 01:08 PM   #11
johnm is offline johnm  United Kingdom
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Old 11th March 2013, 07:57 AM   #12
Mooly is online now Mooly  United Kingdom
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My first thought on seeing the circuit is that you need LED's that will light with either polarity of applied voltage. That neon can be driven either way depending on which transistor is on and which is off, so there must be some significance in the circuit being configured that way.

You could use a small bridge rectifier with LEDS connected in series across the bridge output (the - and + terminals) and connect the AC input pins across where the neon goes. Change the two resistors to something like 22K 1watt to limit the LED current or leave the resistors as they are and add a single series resistor to the LED change.

As I say, I haven't studied it in detail but something like that would be my first approach. (with any established thread we wouldn't normally make changes or delete )
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Old 11th March 2013, 12:48 PM   #13
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Hey Mooly - good to hear from you again! Still indebted to you for your help with my Sony TA-F6B saga a year or so back

Thanks for the suggestion regarding the strobe, but the reason I choose the other design is that it's already been tried/tested here (post no.5), and it's also been designed to fit into the space originally occupied by the neon bulb mounting device:

Restauration Technics SP-10 MKII • LS3/5a le forum

He revised his layout & circuit values slightly here in post no.48:

Restauration Technics SP-10 MKII • LS3/5a le forum

I was just curious why the LED on the far right was at odds with the other two. By design, or just a mistake? I would ask the fellow myself, but as you can see the website is in French, and I know about as much French as I do LED and strobe design alas

I have all the parts, but just wanted to clarify that end LED orientation. I've also noticed he uses one current limiting resistor for 3 LEDS which I thought was 'not the done thing'... but then again it obviously works!

Cheers,

- John

P.S. The circuit for this area of the SP10 is shown below.
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File Type: jpg SP-10 Logic copy.jpg (794.3 KB, 68 views)

Last edited by johnm; 11th March 2013 at 12:53 PM.
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Old 11th March 2013, 02:05 PM   #14
Mooly is online now Mooly  United Kingdom
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That LED has to be the wrong way round and yes, using separate resistors for each LED would be better. The trouble with putting the LEDs in parallel is that it relies on all three being identical in characteristic. If they are not then the current won't split equally and one will be brighter than the others. The 5 volts that the circuit runs off isn't high enough to put them in series either given that each LED would need around 2.2v ish ? (for an orange LED).

That small circuit is just using a transistor as a switch to turn the LEDs on when the "signal" line goes high. What puzzles me is that the neon is driven from two different signals matrixed together by the two driver transistors. Although the waveforms are shown identical the timing or phase can't be (or the neon wouldn't light as both transistors would be on or off together and so the neon would never see a voltage difference across its terminals). Maybe it was done to get the best visual display from using a neon.
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Old 11th March 2013, 05:55 PM   #15
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Thanks Mooly. If I wanted to use three resistors would I just use a standard LED resistor calculator, or does that transistor change things?

If not it looks like 150R (using 3 x standard 20mA, 2V leds, at 5V) is the standard value. I might reduce that to around 110-120R just so I get a nice bright strobe light:

http://led.linear1.org/led.wiz

Surprised there aren't more posts on the net regarding these LED mods as many have SP-10s and those strobes are all on their way out now after 30+ years of use. Compared to the Garrard 301/401s or Thorens 124s where you can buy just about any spare part even today.
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Old 11th March 2013, 06:23 PM   #16
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The transistor doesn't change anything because its operated as a switch and so the volt drop across it when on is negligable.

Best way for choosing LED series resistors is to try them on test. Modern LEDs are hugely improved over those of a few years back and can be really brilliant on just a milliamp or so. Try one with a series resistor across a 9 volt battery.
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Old 11th March 2013, 08:45 PM   #17
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Quote:
Originally Posted by Mooly View Post
The transistor doesn't change anything because its operated as a switch and so the volt drop across it when on is negligable.

Best way for choosing LED series resistors is to try them on test. Modern LEDs are hugely improved over those of a few years back and can be really brilliant on just a milliamp or so. Try one with a series resistor across a 9 volt battery.
That's a cunning type plan - thanks Mooly! I have a 9v battery so will give that a whirl
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Old 11th March 2013, 09:12 PM   #18
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LEDs can handle a LOT more power when strobing for a short duration and a small duty cycle, so the battery test doesn't tell that much useful information.

Also, I didn't look at the circuit or posts, but the circuit may only be illuminating the LED half as often as with the neon (the LED being a diode); it would still stop motion. But you might get a much better picture triggering on either side of the AC waveform, flashing twice as often. And if you used one LED in each direction each would only flash half the time, and you might be able to pump up the power even more.

Avoid LEDs with phosphors unless you want to experiment a lot, some can be considerably slower to die out. For instance, some colors are ultraviolet LEDs with phosphors to create the desired visible color.
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