Electro Harmonix Zipper power supply adapter circuit question

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industrialarts

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Oct 1, 2020
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91
I have an EH Zipper - original, not Tube Zipper - that I got working but the way the schematic shows the AC adapter (really a DC adapter, it takes 9 volts center positive on an 1/8" jack) makes no sense. I believe the schematic is just not finished but the board seems to be made like the schemo. I don't see how the transistor is biased without the battery being installed, which makes no sense. I checked a bunch of other EH pedals from that same era and no two power diagrams match, the closest is a Soul Preacher pedal which has a 15k resistor between collector and base, which doesn't make sense either.

Can anybody straighten me out on what's supposed to be going on here? And what function does the transistor serve? Other pedals with single supplies seem happy with just a resistor divider for the reference voltage.

Thanks

Mark Springer
Vintasonic

PS - the transistor that is in place is a BC239 not the original 2n5088. Someone else was in here before me and subbed it, but I don't see how that would make a difference. Yes, I know they are based different but I have it installed correctly
 

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radardoug

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Oct 10, 2009
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You mean Q4? It doesn't need bias. On battery, the base emitter junction acts as a diode. On ac, it acts as an emitter follower biased to 9 volts. It does mean you need a battery in it even when running on ac.
An odd circuit but it will work.
 

industrialarts

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Joined
Oct 1, 2020
Messages
91
Thanks for your reply, that's exactly what I was thinking but it seemed so stupid I thought there was some technical magic beyond my skills going on. So it works as designed.

Still, what is the point of the transistor? Is it their to act as a pass transistor to limit the input voltage in case of overvoltage? The output would be held to the battery voltage (minus diode drop) I guess.
 
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