Fixing an old Amiga sampler – And a failed attempt to

To check the Opamp circuit. Without any audio.

Pin 3 should be around 2.5 V because of the two 1k resistors that divide the 5V power in half. If not that the resistors are not installed correctly or do not make contact.

The Opamp should be in balance to amplify an analog signal, which means that pin 2 needs to be 2.5 V too. The Opams just ampllifies the slightest difference between pin 2 and 3. If the voltage on pin 2 is higher than pin 3 the output is low. Id the voltage on pin 2 is lower than pin 3 then the output is high. If the output is between 0 and 5 volt the that is only possible if the voltage op pin 2 is about the same as the voltage on pin 3. If in balance the voltage on pin 2 shall be 2.5 V as well.

Since pin 2 only gets DC from pin 1 also pin 1 needs to be 2.5 V.

I think your output was 5 V which cannot be okay. If the output is 5 V then the voltage on in 2 is also 5 V but if the voltage on pin 2 is above the voltage of pin 3 then the output should be 0 and not 5. So it either does not have the feedback via the resistor or the voltage on pin 3 is not 2.5 V.

Once these voltages are okay any signal will attempt to raise or lower the voltage on pin 2 and the output with go the other way to compensate and keep pin 2 at 2.5 V. so you will measure almost no signal on pin 2 because the output cancels out the input signal on pin 2. But in doing so the output now has the same signal as the input but with reversed polarity.

The amount of feedback is the potentiometer, when set to 1k the amplification is 1x. The amplification is the ratio between the input resistor (1k) and the feedback resistor, for example if the feedback resistance is 2k then the amplfication is 2k/1k = 2 times. This is because the output has to have double the voltage of the input to keep the voltage on pin 2 unchanged (because the opamp keeps the pin 2 voltage to be the same as the pin 3 voltage until it clips to either max or minimum output voltage).

Now you also see why the + op the cap needs to be on the Opams side, that pin has 2.5V while this input signal averages to 0. So the Opamp side has the higher voltage.

You use a mono jack on a modern cassette player. I'm almost sure they used a stereo socket on the cassette player and shorted left and right. They do this so you can use stereo headphones and not only have sound in the left ear. Your mono jack will short the right channel to ground and with that short the complete signal. So you will have no audio input at all this way, this will never work. Use a stereo jack with left and right shorted on a player like this.

Our old friend the broken flat ribbon cable?🤨

I still use various trackers to this day, mostly for demos before going into the studio. Great to see they are still used and loved. And I love the 66º North t-shirt! When I lived in London I’d go to Iceland fairly regularly and would always stock up on the 66º North stuff. I think the manufacturer of the sampler has sanded off the information on the IC so it can’t easily be duplicated.