CA3080 question

[Willy]

Hi all

I have a Marshall ED-1 compressor that is not working when it´s ON.

I noticed with the audio probe that the signal goes to +In and -In in the CA3080 IC but there is no signal at the Out pin in the same IC.

Does this mean that the IC is bad? Thanks in advance. Excuse my English

Willy

[Jay Doyle]

It may, is there any way to post the voltages on the different pins of the chip?

It sounds like it may be though, you can easily fry those chips if there is more than 2ma into the Iabc pin.

[Willy]

Thanks for answering!

This are the voltages
Pin  Volt
1     0
2     3.63
3     3.63
4     0
5     0.002
6     3.56
7     8.15
8     0

[R.G.]

The Iabc input should look like a diode drop to V-. Yours is sitting at nearly ground, not 0.4-0.7V, so it's likely that whatever is supposed to be feeding current to that point is not doing so. The voltages look OK on the chip. Check out what should be feeding the Iabc node (pin 5).

[Jay Doyle]

I would tend to think that the chip is fine and that the current isn't getting to the Iabc pin.

I'm taking a guess here, but I would bet that your Marshall is a close copy of the Ross/Dyna circuit.

If your pedal just 'stopped' working one day then my guess would be to check the solder connections to the 'sustain' pot or whatever it is, that sounds like the culprit to me because that is the part that feeds the Iabc port and if that fails and doesn't supply a voltage to the Iabc port, the chip doesn't amplify, it shuts off.

[Willy]

Yes! It goes (pin 5) to a 27k resistor, then to a 1M pot and then to the (I think) emiter of a C550C transistor (that´s what I read). How do I check it? Is this the same that BC550?

[Jay Doyle]

Yes! It goes (pin 5) to a 27k resistor, then to a 1M pot and then to the (I think) emiter of a C550C transistor (that´s what I read). How do I check it? Is this the same that BC550?

DId this pedal just one day stop working? Or did it degrade over time? The first it could be anything, the second, I would be almost certain it is the pot.

Check the continuity on the pot. Check that 1Meg pot, and yes it goes to the emitter of the C550. I would venture to guess that the transistor is just fine. Check the solder connections on the wires to the pot and the pot itself.

I'd try reheating and dabbing a tad more solder onto all of the pots wire connections both at the pot and on the board.

[Willy]

It stop working from one day to another. I tried changing the transistor but didn´t solve the problem.

I´ll try the solders option or changing the ic. I´ll let you know! Thanks

Willy

[Paul Perry (Frostwave)]

I'm completely unfamiliar with the circuit, but I agree with RG that it is likelier to be something wrong with whatever is sending the control voltage to the resistor to pin5.
Just be very careful measuring around the 3080, if you short pin5 to +, it's all over.

[R.G.]

Yes! It goes (pin 5) to a 27k resistor, then to a 1M pot and then to the (I think) emiter of a C550C transistor (that´s what I read). How do I check it? Is this the same that BC550?

This is easy then. Take your voltmeter and read the voltage on each side of that 27K resistor and each end of the 1M pot. With no signal in, one end of the 1M resistor and the emitter (yes, it is) of the C550C (that's a BC550C device) must be nearly the power supply, perhaps 0.8V less than your battery voltage. Of the other two pins on the 550C, one should be *exactly* the power supply. That's the collector. The last one is the base and it should be connected to the power supply by a resistor, and have a largish cap, 10uF or so to ground.

Possible failures are shorts to ground or near ground at the base, emitter, 1M pot, 27K resistor, pin 5; or opens from power supply to base resistor, or base resistor to base, or base-emitter, or base-collector, or 1M pot, or 27K resistor.

If you can give us the voltages on the base resistor, transistor, 1M pot and 27K resistor, we can tell you what's probably wrong.