Sound Skulptor forums

Build help and troubleshooting => MP573 Microphone preamplifier => Topic started by: Kev4272 on February 07, 2021, 05:31:01 PM

Title: 100 Hz Hum And No Input
Post by: Kev4272 on February 07, 2021, 05:31:01 PM

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.

Title: Re: 100 Hz Hum And No Input
Post by: JPK on February 08, 2021, 11:15:38 AM

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).

Title: Re: 100 Hz Hum And No Input
Post by: Kev4272 on February 08, 2021, 07:05:28 PM

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.

Title: Re: 100 Hz Hum And No Input
Post by: JPK on February 08, 2021, 07:24:00 PM

Is it still there with the jumper JMP3 in?

Title: Re: 100 Hz Hum And No Input
Post by: Kev4272 on February 08, 2021, 07:32:31 PM

Yes it is

Title: Re: 100 Hz Hum And No Input
Post by: JPK on February 09, 2021, 08:21:14 AM

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 (https://www.soundskulptor.com/forum/index.php?topic=5.msg5#msg5).
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.

Title: Re: 100 Hz Hum And No Input
Post by: Kev4272 on February 09, 2021, 09:07:53 AM

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.

Title: Re: 100 Hz Hum And No Input
Post by: JPK on February 09, 2021, 03:13:57 PM

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?

Title: Re: 100 Hz Hum And No Input
Post by: Kev4272 on February 09, 2021, 09:04:07 PM

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.

Title: Re: 100 Hz Hum And No Input
Post by: JPK on February 10, 2021, 10:13:05 AM

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.

Title: Re: 100 Hz Hum And No Input
Post by: Kev4272 on February 10, 2021, 03:52:07 PM

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.

Title: Re: 100 Hz Hum And No Input
Post by: JPK on February 10, 2021, 04:10:54 PM

RLY1 seems to be dead.

Title: Re: 100 Hz Hum And No Input
Post by: Kev4272 on February 10, 2021, 04:29:53 PM

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.

Title: Re: 100 Hz Hum And No Input
Post by: JPK on February 10, 2021, 05:53:15 PM

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.

Title: Re: 100 Hz Hum And No Input
Post by: Kev4272 on February 11, 2021, 09:58:32 AM

I am getting nothing for the both of them.

Title: Re: 100 Hz Hum And No Input
Post by: JPK on February 11, 2021, 10:14:19 AM

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

Title: Re: 100 Hz Hum And No Input
Post by: Kev4272 on February 11, 2021, 09:48:33 PM

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-

Title: Re: 100 Hz Hum And No Input
Post by: JPK on February 12, 2021, 11:21:26 AM

So it seems to be the relay.

Title: Re: 100 Hz Hum And No Input
Post by: Kev4272 on February 12, 2021, 05:59:09 PM

Alright, can I order them from you guys?

Title: Re: 100 Hz Hum And No Input
Post by: JPK on February 14, 2021, 08:54:33 AM

Send me an email https://www.soundskulptor.com/en/content/6-contact