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MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« Last post by JPK on Today at 07:48:11 PM »

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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MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« Last post by JPK on Today at 07:37:53 PM »

and than the signal fades away quite rapidly.
Try to follow the signal on TP1, TP4, TP5, TP6.
Check what happens between the initial working state and the final faded away state.
Also cjheck if something happens with DC.

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MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« Last post by guenter1910 on July 30, 2026, 07:46:55 PM »

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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MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« Last post by guenter1910 on July 30, 2026, 07:37:30 PM »

XLR Pin 2 and 3 on the cable 29,4 Ohm, on the 500 Rack Slot 28,94 Ohm
Pin 2 on the cable to 0V on the board 22 MOhm
Pin 3 on the cable to 0V on the board 22 MOhm
both slowly increasing

Mic Cable is ok. I checked with a different cable - same behaviour 

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MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« Last post by JPK on July 30, 2026, 06:57:36 PM »

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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MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« Last post by JPK on July 30, 2026, 06:51:56 PM »

Check the resistance value between:
- XLR pin2 and pin3, 
- XLR pin2 and 0V,
- XLR pin3 and 0V.
Also check your mic cable.

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MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« Last post by guenter1910 on July 30, 2026, 02:16:15 PM »

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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MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« Last post by guenter1910 on July 30, 2026, 01:54:25 PM »

Phantom Power on Pin 15 is stable (49,45 V), Voltage on R3 is 49,45V on one and 47,90 V on the other side, C1 populated again.

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MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« Last post by JPK on July 30, 2026, 08:44:57 AM »

but the partslist with C1 on is crossed out
Make sure you are using the latest parts list. C1 must be populated.
Does the 48 V rail (on the finger connector, pin 15) remain stable when the voltage on the XLR drops?
What is the phantom voltage level on each side of R3?

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MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« Last post by JPK on July 30, 2026, 08:34:01 AM »

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.