[sdiy] Small amplifier circuit using AC power supply

[Takuya KATAYAMA]

Hello Jon and the Lists!

Jon, you're misunderstanding the circuit.
It works with DC SUPPLY, NOT AC.
Yes, an AC to DC conversion circuit is on the board.

Look at the board carefully, from AC 12V input to a 4-terminal device
(called bridge diodes) is connected, isn't it?
Or, instead of 4-terminal device, 1 or 4 diode(s).
It converts AC to DC.

I think the problem is a big capacitor rating 25V 470 - 2200 uF
following the diode(s) is out of order.
Replace the diodes and the capacitor with similer one, maybe the board
works again.

=======================================================
Takuya "PATA" Katayama at Muse Music Synthesizer Lab. Japan


jdarby at lplizard.com wrote:

>Howdy!
>   I am searching all around for information on how to design a small
>transistor amplifier using a 12 volt AC power supply so I can repay my
>friend for a favor by repairing her early 70s Welcome Back Kotter record
>player. Finding information about DC transistor amplifiers has been
>overwhelmingly easy, every one of my electronics books describes using a DC
>power supply to amplify an AC signal, which would be fine, except I would
>like to utilize the 12v AC power supply (A shared 120v/12v transformer that
>drives the record player's motor and the 12v side drives the amplifer) and
>potentiometer with attached on/off switch for simplicity's sake and not
>having an additional 9-volt battery and switch. One side of the AC power on
>the circuit board follows a similar routing to what you would expect a
>ground trace would look like, worming its way all the way around the circuit
>board with many components connected to it. I just can't understand the
>circuit any further without some help. I thought replacing the 25-year old
>caps and resistors would save me a hill of work since when I got it it
>seemed to work for a few seconds until it warmed up and then faded away
>scratchily, but the problem will not go away and I am not confident enough
>with transistors to replace the ones in the circuit with my Radio Shack
>purchased transistors (not certain of capatibilty even after I figure out if
>the originals are NPN or PNP). So, if anyone can suggest maybe an online
>resource I can brush up on using AC power supplies with transistor amplifier
>circuits I would appreciate it! I would like to apply it later to adding an
>on-board amplifier to an AC theremin circuit. Thank you very much!
>
>Jon Darby
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