Color organ problem

Shominy

Lunatic Member
There is not an appropriate forum to place this because it is not audio and not DIY, so... :dunno:
Poking around online yielded nothing and I hope that someone can assist.

My little vintage color organ all of a sudden began outputting a hum through the input speaker line wires. It is audible through my system on R (it, and the R speaker are together), and was initially puzzled because that hum was heard after all audio equip was turned off, then unplugged.

It was at that point I noticed the the organ's lights are fully lit all of the time now. and that it's new output hum is truly there- checked by connecting only it directly to a speaker. Receiver is fine.

As a side note, my Phonics Digitrack DAC (iMac to receiver input) died at the same time. I cannot imagine a correlation, but maybe?

Any ideas? A failed component in the color organ is my guess but upon inspection, nothing 'blown' and having left evidence of failure.
 
This is a best guess without seeing the system.
The left and right channels work the same way. Let's look at just one. The organ input is usually the amplifier speaker line. Inside the organ has 3 filters to break the music into high, mid, and low frequency bands which in turn runs the 3 color light strings. There is a power supply to power the works. Normally the signal goes only one way--- from the amp into the organ. It sounds like the power supply filter noise is coming through the busted filter section going wrong direction into the speaker. You say the lights are on is good indication of busted filter.
 
I hooked my vintage color organ to a very small amplifier with a microphone so I wouldn’t risk what you are describing. Better safe than sorry with these ancient (but fun) toys!
 
There are a lot of color organ circuits but the classic one is very simple. There is a small transformer at the input that couples audio to some passive filters (for various frequencies) with a volume pot/control for each frequency. The filtered audio is then used to trigger thyristors that switch 120V bulbs on and off. Some schematics are here.

If this is what you have, then the thyristor would seem to be the likely point of failure. I suggest you try removing all the bulbs of one color (one frequency band) at a time to see if that stops the hum. If it works then very likely that thyristor is bad.

Note: circuit is likely connected directly to the AC line. Use appropriate caution.
 
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Thanks, guys.
Sorry- just got around to imaging. The metal "ballast" is spot welded shut.
Single pot seen in top image for light intensity/sensitivity for all of them.

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There are a lot of color organ circuits but the classic one is very simple. There is a small transformer at the input that couples audio to some passive filters (for various frequencies) with a volume pot/control for each frequency. The filtered audio is then used to trigger thyristors that switch 120V bulbs on and off. Some schematics are here.

If this is what you have, then the thyristor would seem to be the likely point of failure. I suggest you try removing all the bulbs of one color (one frequency band) at a time to see if that stops the hum. If it works then very likely that thyristor is bad.

Note: circuit is likely connected directly to the AC line. Use appropriate caution.

Thanks for the link for schematics. Delicious :).
 
Thanks, guys.
Sorry- just got around to imaging. The metal "ballast" is spot welded shut.
Single pot seen in top image for light intensity/sensitivity for all of them.

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View attachment 2576137
View attachment 2576138

Is the middle photo all there is under the hood? It looks like it is only one channel though the lights are in series in different colors.
If that is all the hardware then the circuit is in the can. It is likely a very simple circuit. The can is to protect the innocence.
Depending on how handy you are and experience. It may be better hand over to a tech. To go further need be unplug from the wall all the time and look inside the can (can opener).
 
Is the middle photo all there is under the hood? It looks like it is only one channel though the lights are in series in different colors.
If that is all the hardware then the circuit is in the can. It is likely a very simple circuit. The can is to protect the innocence.
Depending on how handy you are and experience. It may be better hand over to a tech. To go further need be unplug from the wall all the time and look inside the can (can opener).
Yes, single channel. After opening the can, here is what I saw.
The transistor does have the smell of death on it, so you may have been right. The unsoldered lead was my doing in loosening it.

Now, I cannot find a Motorola SCR833 transistor reference anywhere for sourcing it, or a suitable replacement. Any ideas?

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@audiotemp also.
 
The ones I've worked on used SCRs not transistors. Based on the part number yours must be SCRs also. Most any SCR rated at least 200V and however much current your lamps draw should work.
 
The ones I've worked on used SCRs not transistors. Based on the part number yours must be SCRs also. Most any SCR rated at least 200V and however much current your lamps draw should work.
Thank you. What is an SCR?

EDIT- got it. Rectifier.
Do you have a source for something appropriate in this use?
 
An SCR is a 4 layer (P-N-P-N or N-P-N-P) semiconductor device having only 3 pins because one of the middle layers has no external connetion. It's a switching type of device, which means that you trigger it and then it stays "on" until the current drops to zero. It conducts current in one direction only and this is why your circuit has 4 diodes in a bridge to convert the incoming 60Hz AC to 120Hz pulsating DC.

The BTA12 is a TRIAC, which is like two SCRs of opposite polarity (one P-N-P-N and one N-P-N-P) in parallel, making a device that conducts current in both directions (although in really it's a bit more complicated than that). Both SCRs and TRIACs fall into the general category of being "thyristors".

I think at the time color organs became out, TRIACs may have still been relatively expensive while SCRs were relatively cheap---hence the SCR + diode bridge in your circuit.
 
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I'm not expert on SCR choices but I think something like ST TS820-600T may be what you want. This may have a more sensitive gate than what's in there, though.

Note that you need to check the diodes in your bridge to make sure that one of those has not failed.

I'm not sure what kind of capacitors those are, maybe you need to check those also.
 
Thank you for the detailed information!

I ended up buying a Motorola 2N4442 SCR 200V 8A Silicon Controlled Rectifier Thyristor on auction site - sole reason is that I did not have to buy a lot. Should this work?

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Also, will L and R pins be the same orientation as the original as long as I replace with print side up?
 
I have been trying to understand SCRs. Without the gate connected in my situation, what is the point of the SCR/Thyristor being there? Seems to me it is acting like a diode with no behavior options.

AC in through the diode bridge to ∩∩∩ rectified.

Then what happens?
 
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I have been trying to understand SCRs. Without the gate connected in my situation, what is the point of the SCR/Thyristor being there? Seems to me it is acting like a diode with no behavior options.

AC in through the diode bridge to ∩∩∩ rectified.

Then what happens?
You have a P-N-P-N device, i.e. three junctions in series, first PN, then NP, and then PN again. With positive voltage applied across the device, the two P-N junctions will be forward biased but the NP junction will block any current flow.

One way to look at this device is as to imagine an initial PN diode followed by an NPN transistor and another way is as an initial PNP transistor followed by a PN diode. You can actually make a similar circuit using an NPN and a PNP transistor, with the base of each transistor connected to the collector of the other (and with the emitters forming the input and output). To get current to flow in this circuit, you need to get one of the transistors turned on. So, connect a "gate" to the NPN's base and use it to apply a forward bias to the NPN. It will turn on and, assuming proper voltage is applied emitter to emitter, current will flow end to end.

Since the end-to-end current is flowing though the PNP transistor's emitter-base junction, that transistor will be turned on as well. In fact, there's a positive feedback process that will turn both transistors fully on. At that point current input through the "gate" is no longer needed since the PNP transistor supplies current to bias the NPN and vice versa. Only when current flow stops due to lack of applied voltage to the device does it reset to "off".

P.S. There are SCRs with differing amounts of gate current needed to switch the device on. The 2N4442 seems to need 7mA typical and 30mA max, which seems high. Other "sensitive gate" devices require a lot less.
 
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