[Kev4272]

Hi, I recently built two MP573 but I have the exact same problem on both of them. There is a 100 Hz hum and no mic input signal. The DI works fine though. At first, I thought that the problem was with the input transformer, so I redid all the solder on it, but the problem is still there. I also checked all of the components and they all seemed at the right place and on the good side.

[JPK]

Try replacing the 2N7000 on the DI board.
This component is very sensitive to static electricity and should be handled with your body grounded (removing your shoes).

[Kev4272]

I am not sure that would be it, because the problem is still there even when the DI board isn't plugged into the main PCB board.

[JPK]

Is it still there with the jumper JMP3 in?

[Kev4272]

Yes it is

[JPK]

Do you hear the relay clicking when you insert a jack into the DI socket?
Same question when you switch the impedance between 1200 and line?
Same question when you switch the impedance between 1200 and 300?
Did you insert the relays in the right direction?
It could also be an issue with the solder joints on the relays.
You will need to remove the input transformer. Follow the instructions here.
Make sure you have cut the pins of the 90° headers that attaches the transformer PCB's short enough.
Also take this opportunity to check the DC resistances of the input transformer coils.

[Kev4272]

I hear a click between the 1200 and the line but not with the 300 and the DI. Also, I checked my relays, and they are all in the right direction.

[JPK]

The DI relay is RLY1,
The 300 Relay is RLY3. It is activated by the impedance switch. Check the solder joints on this one.
Do you hear the DI relay clicking when you insert JMP3?

[Kev4272]

Alright, so I redid all the soldering on the three RLY but the issue is still there on both of the preamp. I don't hear the clicking when I insert JMP3.

[JPK]

Check R28 (value & solder joints).
Check JMP3 solder joints.
Check the resistance of the coil of RLY1, across D5 (with JMP3 removed). You should get around 2800 ohms.
Check the direction of D5.

[Kev4272]

I redid the soldering on the JMP3, the R28 and the D5 just to be sure. The R28 is 1.2K and I'm getting 2.9k for the RLY1 across D5.

[JPK]

RLY1 seems to be dead.

[Kev4272]

But if RLY1 is dead, wouldn't the DI not work because its is RLY? Because right now the DI is working perfectly on both of the modules.

[JPK]

It seems to stay in the rest state which connects the DI input.
You can do another check:
Measure the resistance between JMP3 top pin (jumper removed) and V+ : 4k?.
V+ is taken on the test pin V+.
Measure the resistance between JMP3 bottom pin (jumper removed) and V- : 0?.
V- is taken on the anode of D4.

[Kev4272]

I am getting nothing for the both of them.

[JPK]

V+ is taken on the test pin V+.
V- is taken on the anode of D4.

[Kev4272]

Thank you, I was doing it wrong ahah. So I redid It and now I have 4k? for the V+ and 0? for the V-

[JPK]

So it seems to be the relay.

[Kev4272]

Alright, can I order them from you guys?

[JPK]

Send me an email https://www.soundskulptor.com/en/contact.php.