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ADC Sound shaper One Woes

HotRodStereo

HotRodStereo

New Member
I am very new to all of this and still learning, so be gentle with me.

I am working on this Sound Shaper One EQ, I opened it up and R122 was smoked, so I replaced it with a 1K ohm resistor as per the diagram. I powered it back up and after about 10 seconds the magic smoke appeared again so back to the drawing board. Since it was in line i replaced the 100uf 25v capacitor, and after about 10 seconds of power, pop went the weasel.

I have a variac and a DBT, I didn't use either because it was late, i was tired, and i like making stupid decisions.

My question (beyond what do i do next) is why oh why is there even a capacitor in the circuit at all? Is my power switch shorting?
 

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What other symptoms were there before you opened it up?

As far as I can tell, all that resistor does is restrict current going thru the "on" LED. I suspect the cap is to rectify the signal to keep the LED from blinking.

One way it can smoke would be if the voltage on it were too high. That appears to come directly from a tap on the transformer.
 
RVT1K said:
What other symptoms were there before you opened it up?

As far as I can tell, all that resistor does is restrict current going thru the "on" LED. I suspect the cap is to rectify the signal to keep the LED from blinking.

One way it can smoke would be if the voltage on it were too high. That appears to come directly from a tap on the transformer.
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All i did was a cursory test before i opened it up on the inputs and outputs and it seemed to work fine on the audio channels at least, I cant remember if the LED was on before i replaced the resistor or not, I will see what the voltage from the transformer looks like!
 
Did you ever get this sorted out? I have a sound shaper one with the same issue. I went through and recapped the unit after just checking the circuit with that resistor. Replaced that diode and no change. Still smoked that resistor. I’m intrigued. There was so much heat on that resistor that it melted the clip from my DVM lol
 
Forresthicks said:
Did you ever get this sorted out? I have a sound shaper one with the same issue. I went through and recapped the unit after just checking the circuit with that resistor. Replaced that diode and no change. Still smoked that resistor. I’m intrigued. There was so much heat on that resistor that it melted the clip from my DVM lol
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I haven't figured it out yet tbh, the best lead here was an issue at the transformer supplying too much current. Its on my backlog of repairs rn since it makes more sense to fix things worth money first, but i have a feeling simply removing the wires to the LED is how i will "fix" it.
 
Resistor is current limiter to the on LED. So, either voltage is too high, a short in wire, or marginal design to begin with. I would expect they set it for about 20 mA, which is about 400 milliwatts for the resistor, which is close to its max rating and will make it run quite hot. The voltage across the cap should be about 22 volts. Measure voltage across the cap, not to ground. Make sure the other side of the LED is attached to points 19 (or 17) on the power supply board, and not grounded. The solution may be as simple as going to a higher value resistor. (1.5k to 2k), or use a higher wattage (1 watt) resistor.
My 2 cents
 
Shorted LED, perhaps ?
Can`t say I every recall encountering one shorted over my decades of servicing electronics that have used LEDs.
Open yes, but not shorted.
It`s possible from a strong power line surge/spike for a LED to short, after all it is a diode with low reverse voltage ratings !
Just a thought.
 
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