[atre594]

Hello, I just bought the MP573 kit about a week ago and I just finished building it. I was happy to see that it measured as intended with all the setup guide tests. 

I was able to get the unit working and it sounded good for about 2 min and after a while it just Started to create a buzz and then cut out.

I turned off the unit and checked my work. Seemed fine, so when I turned it on it worked perfectly fine again but after a few minutes the problem arose again. 

Thoughts? I’m new to electronics and I’m building this for educational reasons. I may be doing something wrong that is simple. Any help would be appreciated!

[JPK]

Were you able to set the bias at 3V on TP2/TP4 ?
Are the 3 pins IC's U1 or U2 getting hot?
Is the thermal relay PTC1 getting hot?
Check that you do not have a short circuit between any 2 adjacent wires on the output transformer PCB.

[atre594]

Thanks for the response.

Yes I was able to get 3 volts with TP4/TP2. 

U1, U2 and PTC1 don’t seem to be getting hot.

I have triple checked the output transformer and I redone any suspicious solder but it’s still having the same issues. 

I have noticed that the unit works fine when I turn it on and off again. As soon as i start switching the impedences and phase it eventually starts to sound buzzy and ultimately cut out. 

I also noticed this when redoing the setup guide test:

Using the multimeter I tested the ohms on the card slot at -V, OV, +V. 

-V & OV read: OL 
OV & +V read: 8.4M Ohms 

It said that it needs to be greater than 1K Ohm but I wasn’t sure if this was perhaps not normal. 

Again any help would be greatly appreciated! 

[JPK]

Can you monitor the DC supply voltage (24V) between tp2 and V+ and check if it goes down when the pre cuts out?
If it doesn't :
Send a constant sine signal to the input and monitor the AC voltage on TP1, against TP2.  Check if it goes down when the pre cuts out.
If it doesn't :
Monitor the AC voltage on the yellow wire of the output TX, against TP2.  Check if it goes down when the pre cuts out.
This should tell you which stage is responsible for the cut.

[atre594]

Ok so after testing:

When monitoring 24 dc volts. After the cut the DCV did not change it stayed at 24.0.

Next: I set up a 1Khz sine wave to send through the unit and When monitoring AC volts between TP1 & TP2. The voltage went down when the signal cut out.  

Looks like this may be the issue. 


Other things I noticed:

1st: 
When listening to the microphone and sine wave (separately) I noticed when I flipped the phase the signal would favor one side in my headphones. So when I had it on POL it was left heavy and when I had it on REV it was right heavy. I referenced another 500 preamp I had so that I could make sure it wasn’t my headphone amplifier and indeed it wasn’t. Looks like I may have wired something wrong?

2nd:
 I was hearing a buzz when touching the stepped pot. Didn’t sound like 60hz but I may be wrong. I also noticed when I (accidentally) touched c23 the same buzz but much louder. Not sure what it means just thought I should mention it.

Thanks again for you help!

[JPK]

Redo the second check (monitor the AC voltage on TP1, against TP2) with the gain switch on 10 or 20dB: the first gain stage is bypassed in this case. If it doesn't cut, the issue is in the first stage. you will need to do the DC voltages tests .

[atre594]

Hello after performing the last test you mentioned it still cut out. I had the unit at 10db with a sine wave playing and measuring at ac voltage.

Not sure what to do from here

[JPK]

Does it cuts out 100% or does it attenuate by 30dB like if the line relay was switching in?
After cutting, does the the DI input becomes active?
Do the test directly on the input transformer pin 7 (against TP2) in order to remove any post transformer circuitry from the equation.

[atre594]

So after doing the test from tp2 to pin 7. It cuts out as well. 

Before sending signal the ACV reading is .008 and during the test it’s reads .725 on the 20db setting on the pot. After it cuts out it returns to the 0.008. 

[JPK]

Can you do the same test on the transformer primary : between pins 2 and 5 ?

[atre594]

Ok after testing input transformer pins 2 and 5 it does the same as the last test. Reads .008 ACV without signal, then .193 ACV with sine wave, and .008 ACV when it cuts out


Also I noticed that Q6 gets extremely hot very quickly. I know it uses the heat syncs but it stood out to me how hot it was 

[JPK]

The last components to check are the 3 relays: DI, Line and Impedance.
You didn't reply to this question: After cutting, does the the DI input becomes active?
Can you hear one relay clicking when it cuts out?

Also I noticed that Q6 gets extremely hot very quickly.
If you were able to set the bias at 3V, then the heat should be normal: Hot to the finger but not causing harm.

[atre594]

The last components to check are the 3 relays: DI, Line and Impedance

You didn't reply to this question: After cutting, does the the DI input becomes active?
Can you hear one relay clicking when it cuts out?

I don’t hear the relay switch over. 
I haven’t had the DI PCB in the whole time. I just have the jumper in.

I put the DI PCB in and removed the jumper. It also cut out. When I plug a cable into the input of the di I do hear the relay switch over. 

the unit is quite noisy when I move the output transformer cables it buzzing quite loudly. Same also happens when I touch U1 & U2