[slothproductions]

Hello

I'm making the general power check on my MP573 build.
Without the output transformer connected the reading is 24.27 Volts and with the transformer connected, it says 24.20 Volts.

Is the last reading acceptable, even if it's just 0.07 Volts lower than the first?

With the output transformer connected, I checked the DC voltages, and some of the values are deviating from the intended ones:

A: 2.5V
B: 4.0V
C: 22.3V
D: 13.3V
E: 2.0V
F: 12.7V
G: 0.9V
H: 2.6 (deviating)
I: 0.03V
J: 2.0 (deviating)
K: 23.3V (deviating)
L: 1.5V (deviating)

OUTPUT TRANSFORMER RESISTANCE MEASUREMENTS:
Yellow - Orange: 7.8 ohms
Red - Brown: 6.8 ohms
Blue - Green: 20.8 ohms
Violet - Black: 20.8 ohms

INPUT TRANSFORMER RESISTANCE MEASUREMENTS:
2-4: 24.8 ohms
3-5: 24.7 ohms
7-9: 133 ohms 
8-10: 133 ohms

Do you have any idea where to look to fix this problem?

[JPK]

Check your solder joints on Q4, Q5, R41...R45.

[slothproductions]

I've now made a visual inspection of Q4, Q5 and R41...R45, but I cannot see anything suspicious. I also did a continuity test with my DMM on the mentioned joints, but it seems like there is a continuous flow.

I've set the bias to 3.0V without any problems and connected a dynamic mic. Following things happen:

• After powering on there is a good clean signal, but after approx. 45 seconds, the signal disappears. Could this be a sign of a bad joint(s)? Seems like it happens when heat is generated. I've tried powering on the lunchbox, waited some time, then powering on again, and the signal is back for about 45 seconds, until it disappears again.
• LED is not turning red, when the signal is close to clipping, only green.

I tested the DI, and there doesn't seem to be any problem there. Signal is stable besides that setting to "LINE" (DI connected) does not output any sound (is it normal?)

Additional info:

• Gain switch is working
• "Trim" pot is working
• Impedance switch is working (beside the aforementioned LINE setting when connected to DI)
• Polarity switch is working in all three positions
• 48V is working with condenser mic (but the same problem with disappearing input signal happens)

[JPK]

but after approx. 45 seconds, the signal disappears

Does U1 or U2 get hot?
Same question for PTC1?

[slothproductions]

U1, U2 and PTC1 doesn't get hot at all. Tried testing for some time now. 

I noticed that the time it takes for the input signal to disappear depends on how long I wait, until I turn it back on; the longer I wait, the longer time there is input signal. 3 minutes is the most after having it turned off the whole night.
- so its actually not around 45 seconds every time, but it fluctuates.

The signal disappears by first having small fade outs and then suddenly it disappears completely.

[JPK]

Can you please recheck the DC voltages A to L once the sound is gone?

[slothproductions]

Yes:

A: 2v5
B: 4v1
C: 22v3
D: 13v3
E: 2v0
F: 12v7
G: 0v9
H: 4v1
I: 0v3
J: 3v5
K: 22v5
L: 3v0

[JPK]

The voltages are spot on, so it is not a supply issue.
Now Try to follow the signal, before and after the disappearance of sound to find out at which stage it breaks.

Send a good constant sine signal on input, like 0.5VAC measured between pins 2&3 of the input XLR.
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).

[slothproductions]

Im quite new to this DIY world (but I'm keeping a steady learning curve!). How do I send a sine signal on input? Do I need a signal generator or function generator and an oscilloscope for this test? Maybe this is a newbie question, if so - sorry!

[JPK]

Do I need a signal generator or function generator
Yes. Or you can use your DAW and loop a sine signal or use a sine generator pluggin.

and an oscilloscope for this test?
A scope makes things easier but your DMM can do the job.

[slothproductions]

Okay, I've now sent a sine signal with a generator plugin into the preamp. I only managed to send an input signal of 0.1VAC measured between pins 2&3 of the input XLR.

Measurements before the disappearance of sound:

• Input transformer pin 7: 0.59VAC
• TP1: 3.31VAC
• Output transformer yellow wire: 6.24VAC

Measurements after the disappearance of sound:

• Input transformer pin 7: 0.00VAC
• TP1: 0.00VAC
• Output transformer yellow wire: 0.00VAC

They all went to zero. The voltage starts to drop very slowly, and suddenly it drops quite fast.

[JPK]

Can you do the same test on the input transformer primary, between pins 2 & 5 ?

[slothproductions]

Yes. With an input signal of 0.1VAC, I get the following between pins 2 & 5:

Before sound disappears:
0.14VAC

After sound disappears:
0.00VAC

It also drops to zero.

[JPK]

It could be one of the relays.
Is it the same in 300, 1200, line?
Is it the same from the DI input?

[slothproductions]

It could be one of the relays.

Actually, I don't know why I forgot to mention this, but as you can see in the attached picture, I accidentally touched the corner of RLY1 with the iron in the beginning of the build: https://drive.google.com/file/d/1Hkm66048bO6PcwcqzHrrULHdgh4NNYaw/view?usp=drive_link

Do you think this could cause the problem?

Is it the same in 300, 1200, line?

Yes. It's the same for 300, 1200 and line; they all drops to zero on the measurement points (incl. pin 2 & 5 on the primary). Except for line where the measurement on output transformer yellow wire keeps showing 0.020VAC after the sound has disappeared.

Is it the same from the DI input?

DI works fine - the sound doesn't disappear!

[JPK]

I accidentally touched the corner of RLY1 with the iron
This is most likely the cause of the failure.

You will need to remove the input transformer to change the relay. follow the guide here.
The relays are:
AZ850-24 or TQ2-24V or EA2-24NJ.

[slothproductions]

AZ850-24

Does this one match then?:

https://www.conradelektronik.dk/da/p/zettler-electronics-az850-24-printrelae-24-v-dc-1-a-2-x-omskifter-1-stk-507436.html?utm_source=google&utm_medium=surfaces&utm_campaign=shopping-feed&utm_content=free-google-shopping-clicks&utm_term=507436&gad_source=1&gclid=CjwKCAiAuNGuBhAkEiwAGId4aqwkE-ukmo-26AyVCPdMXlnMnhqiPyRikROEoGhgsfsoh5REAGXvOxoCpFkQAvD_BwE

I've noticed that on the relays from the MP573-kit it says "2A/30VDC", while it says "1 A at 30 VDC resistive" in the datasheet for the relay linked above.

Is this a problem? 

[JPK]

the conrad relay is the correct one despite their photo which does not correspond

Is this a problem?
No.

[slothproductions]

Okay, thank you. I will replace the relay when received and return with an update.

[slothproductions]

YES! I replaced RLY1, and the unit is now working as expected! Sounds amazing!!

Thank you so much for your help JPK - what a great service!!

If your having more time for this one:

The LED is turning red at the same time as the signal clips in my DAW. But shouldn't it do it -3 dB before clipping?

[JPK]

The red LED switching level depends on R48, R49, R50, R51, R53, R54.
Check if these resistors are in their correct spot.

[slothproductions]

I've checked, and they are all in the correct spots. But I guess the joints in these places could be better - could this influence the reactivity of the LED?

[JPK]

Sure. A bad solder joint can completely compromise operation.

[slothproductions]

Okay, I will try fix it. 

Anyway, I see this thread as solved. Once again: thanks for the help on this one - means a lot!!

[Gavin1393]

Havin similar issues with my MP573 at Setup time.

Had no issues with the short circuit check that I could tell.

Power of 24.3 with the Output transformer connected and disconnected....

When looking to set up and calibrate the bias of Q6 my DMM started at around 20v odd and slowly dropped to around 1v but still wouldn't settle. Around 1.3 - 0.7v seemed the norm.

I've looked for obvious soldering issues on the PCB but see nothing. I found that RELAY 3 on the underside of the PCB looked like it may have overheated and suffered some heat damaged ...

I'm at a loss to understand what to do next?