Sound Skulptor forums

Build help and troubleshooting => MP573 Microphone preamplifier => Topic started by: JPK on December 16, 2020, 06:41:09 PM

Title: In case your MP573 doesn't work as expected
Post by: JPK on December 16, 2020, 06:41:09 PM

Check the usual suspects:

• Check you didn't forget the jumper on CN1,
• 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 DC voltages:
First check the supply voltages as described in the setup guide.
Next, without any input signal, connect the voltmeter black probe to TP2 and note the DC voltages you get at the points shown on the attached picture. Compare with the expected results (more or less 10%).

(https://www.soundskulptor.com/prodimages/debug/mp573b-volts.jpg)

Check the DI input
Check you didn't forget to remove the JMP3 jumper.
Check you didn't swap the 2 transistors on the DI board.
If the DI input works then the issue may be in the switching between DI and XLR. Check the 2N7000 on the DI board. This component is sensitive to static electricity and is better handled without your shoes on (body grounded).

Try to follow the signal
Send a good constant sine signal on input, like 0.5VAC measured between pins 2&3 of the input XLR. 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 TP2 and trace the signal on:

• Input transformer pin 7: 0.9VAC,
• TP1: 4.8VAC (Gain=40, TRIM=Min),
• output transformer yellow wire: 8.4VAC (Gain=40, TRIM=Min).

The reading should follow the input signal level.
The place where the signal drops will give you hints on where to look for.

In case you need to remove the input transformer follow these steps

• Remove the 4 screws on the Carnhill transformer (2 in front and 2 in the back).
• Remove solder on the 7 pads of the back daughter board and remove it. Use a desoldering pump.
• Remove solder on the 9 pads connecting the transformer in/outs on the front daughter board and remove the transformer. Be careful when removing the front daughter board not to pull the transformer front plate with it because it is not attached anymore

To verify the transformers DC resistances
CARNHILL VTB1847 (unplugged) :

• Yellow-Orange : 7.6 ohms
• Red-Brown : 6.4 ohms
• Blue-Green : 21 ohms
• Violet-Black : 21 ohms

CARNHILL VTB9045 :

• 2-4 : 24 ohms
• 3-5 : 24 ohms
• 7-9 : 144 ohms
• 8-10 : 142 ohms