Sound Skulptor forums
Build help and troubleshooting => MP566 Tube microphone preamplifier => Topic started by: JPK on January 19, 2022, 09:38:56 AM
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Check the usual suspects:
- Check you didn't leave the polarity switch on the center position (=mute),
- Forgotten solder joint,
- Bad solder joint (reflow suspicious joints),
- Reversed components (electrolytics, diodes, transistors, IC's). In case of IC's, probably destroyed, better replace,
- Flipped IC's or transistors,
- Trim all the component leads and test pins as short as possible. Nothing must touch the chassis plate.
Check the DI input
If the DI input works then the issue may be in the switching between DI and XLR. Check Q1 (2N7000) . This component is sensitive to static electricity and is better handled without your shoes on (body grounded).
Try to follow the signal
Send a constant 0.5VAC sine signal to the input. It must be measured between pins 2&3 of the input XLR or pins 8 & 10 of the extender. Check the level with the preamp connected because the input impedance of the preamp may pull the signal down.
Connect the voltmeter black probe to the 0V pin.
- Set Pot IN to 10, Pot OUT to 0, Gain switch to MID:
you should read 3VAC on TP1. - Set Pot IN to 10, Pot OUT to 10, Gain switch to LOW:
you should read 2.25VAC on TP3 and 7.2VAC on TP4.