Analog 120v three channel light organ

Thread Starter

Cyclicz

Joined Apr 17, 2017
62
A number of years back I had gotten a kit for an old style three channel light organ. The circuit looks like this:

Recently the high voltage rated audio input transformer released some smoke and I have yet to find a replacement part. Is it possible to simply up the audio in voltage using an aplifier of sorts and just bypass the transformer? I’m not sure if the transformer does anything besides up the voltage so that the SCRs trigger. (My thought is that it might also isolate) If I’m unable to bypass the transformer is there another way I could get this thing working again without going and finding a new kit?
Any feedback is appreciated.
 

#12

Joined Nov 30, 2010
18,223
The transformer is obviously an isolation method to keep the wall outlet voltage from getting into the amplifier. If an amplifier sends 25 watts to an 8 ohm speaker, that's about 14 volts RMS or 20 volts peak. The SCR doesn't need 20 volts, but it needs some current. I calculate 20 volts divided by 4.7K as about 4 ma which won't fire most SCRs. Therefore, I suspect the transformer does increase the voltage, but the place to start is the part number of the SCRs to find how much current they need to fire. "106" is not any part number than I know about. Please try to read the label on the SCR.
 

AnalogKid

Joined Aug 1, 2013
10,126
C106 is an older, very common line of SCR's made by Teccor, Motorola, GE, etc. Lotsa package variations, generally similar specs.

The transformer is ***CRITICAL** to safety isolation. Do not operate this circuit without one.

Because the gate is so sensitive, the transformer could have a turns ratio of 1:1 and probably still work, even after the signal is attenuated by the three band filters. The big problem is the line isolation. I strongly doubt that the original transformer was UL rated for this application. Mouser used to have a line of small audio transformers with a few hundred volts rated isolation (but no UL rating).

ak
 

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#12

Joined Nov 30, 2010
18,223
Thanks AK. 200 ua is entirely within the range I didn't expect.
This thing should start firing at about 1/3 watt RMS to the 8 ohm speakers with a 1:1 transformer.
 

Thread Starter

Cyclicz

Joined Apr 17, 2017
62
Thanks AK for the information. I'll most likely find a different route to find a replacement as the new transformer would most likely just blow in another couple years. (And man, you know your stuff, it is, in fact, a C106-D series.)
 

Alec_t

Joined Sep 17, 2013
13,154
A standard miniature mains transformer (for, say, 24VAC) used in reverse could work. Might need a series resistor on the input winding to ensure the winding impedance exceeds the audio source impedance, and perhaps attenuation of the output.
 

AnalogKid

Joined Aug 1, 2013
10,126
Even a small line transformer would be quite a bit larger than the space allotted on the pc board, plus the bandwidth is very small. However, it would be start, and a way to see if the rest of the circuit still is alive.

Triad makes a small pcb transformer with audio bandwidth and 500 V isolation (but no UL cert), but they're $15 in ones. You get what you pay for - it is an excellent part.

TS: please post a photo of the board showing the SCRs and the transformer location.

ak
 

bertus

Joined Apr 5, 2008
22,096
Hello,

Looking at the two pictures in the openings post, it looks like thet the transformer has cross connections:

cyclicz_PCB_filled.png

Bertus
 
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