Sound Skulptor forums
Build help and troubleshooting => MP566-II Tube microphone preamplifier with HPF => Topic started by: guenter1910 on July 29, 2026, 09:58:29 AM
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Hi all, just finished the MP566-II and everything seems to work ok, but LD2 does not light green when a signal is processed (it stays dark). When saturating LD2 does light red, so this way of LD2 is working. Testing the LED shows that both directions (red and green) are ok. What I have seen is that U4.2 Pin 7 stays at -16 V whether a signal is applied or not. Does this mean that the Op Amp is dead - at least the one for the green path of LD2? I have also checked the connections and Diode-orientation of the R's, C's and D's that are components in this part of the circuitry and this also seems to be fine.
Any help is appreciated!
Thank you in advance,
Guenter
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Check the bi-color LED with your DMM in diode test mode. It should light up in one color, then the other when you reverse the test leads.
Check that you receive the peak voltage of the input signal on U4 pin5, the same as pin3.
Check the DC voltage on U4 pin6. It should be a few millivolts above 0.
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Voltage on U4 Pin 3 and 5 are the same
DMM Diode Test shows that both sides (red and green) are working.
What I see on Pin 6 of U4: with Gain at 0 and Out at 0: 3,26V. Turning Out up to max the Voltage on Pin 6 stays at 3,26 V. When backing off the Out a bit the Voltage goes up to 15V, the red LED is on, the Voltage fades down and the green LED comes on for a while and fades to off. Voltage gets down to 3,26 V again.
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What I see on Pin 6 of U4: with Gain at 0 and Out at 0: 3,26V.
The issue is here.
Please report the DC volts (against 0V) on U4 pins 1, 2, 3, 5, 6, 7, also 4 and 11.
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U4 against 0V
Pin 1 -16,54V
Pin 2 3,59V
Pin 3 10mV approximately
Pin 5 10mV approximately
Pin 6 3,26V
Pin 7 -16,55V
Pin 4 16,62V
Pin 11 -16,55V
Both Out and Gain at 0, no Signal applied
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Check the value of R47.
The value is 1Meg (brown, black, black, yellow, brown) but when measured in circuit, it should give you about 12k.
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it gives also 1 Meg in the circuit
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You have a broken connection.
Check that R47-Right is connected to
- R46-R
- R45-L
- U4-pin2
Check that R47-Left is connected to
- D11-L
- R48-R
- U4-pin6
If one link is missing, repair it with some wire.
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the connections are all ok
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what should I check next?
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Check that R46-L is connected to the 0V. Check on the resistor leads, not the PCB pad.
Reflow the solder joints of R45, R46, R47, R48.
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After reflowing the solder joints it seems to work now. This is a bit of a surprise to me because measuring the connections alway showed that everything is fine. Thank you for your help!