Sound Skulptor forums
Build help and troubleshooting => MP573 Microphone preamplifier => Topic started by: slothproductions on February 04, 2024, 04:00:27 PM
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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?
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Check your solder joints on Q4, Q5, R41...R45.
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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)
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but after approx. 45 seconds, the signal disappears
Does U1 or U2 get hot?
Same question for PTC1?
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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.
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Can you please recheck the DC voltages A to L once the sound is gone?
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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
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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).
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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!
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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.
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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.
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Can you do the same test on the input transformer primary, between pins 2 & 5 ?
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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.
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It could be one of the relays.
Is it the same in 300, 1200, line?
Is it the same from the DI input?
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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!
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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 (https://www.soundskulptor.com/forum/index.php?topic=5.msg5#msg5).
The relays are:
AZ850-24 or TQ2-24V or EA2-24NJ.
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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?
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the conrad relay is the correct one despite their photo which does not correspond
Is this a problem?
No.
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Okay, thank you. I will replace the relay when received and return with an update.
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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?
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The red LED switching level depends on R48, R49, R50, R51, R53, R54.
Check if these resistors are in their correct spot.
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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?
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Sure. A bad solder joint can completely compromise operation.
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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!!
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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?
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If I was you, I would try replace the suspected relay. At least thats what fixed my problem :)
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I did read up on the relays and how if they dont work they get rather hot...so hopefully that is all that was wrong. Have ordered 3 in case...
Thanks mate! I'm hoping this will fix the issue. Otherwise @JPK help will be appreciated!
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Can you do the measurements shown here (https://www.soundskulptor.com/forum/index.php?topic=5.msg5#msg5) : DC voltages, and output transformer check?
We will add a new relay to your new order.
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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.
ALL CHECKED[/list]
Check the DC voltages:
First check the supply voltages as described in the setup guide.
short circuit check v- an OV 6.5M ohm
V+ and OV 6.9M ohm
Removed DI and unplugged CN2
Connect the (-) lead to TP2 and the (+) lead to V+. 24.39v
Connect Output transformer 24.37v
Measuring Q6... Started at about 3v and plummeted to o.6v before i turned off the device. Same issue as before.
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%).
A 2v6
b 4v1
c 22.5
d 13.5
e 2.01
f 12.85
g 0.1 !!
h 24v !!
I 4.1 !!
j 23.5 !!
K 24.34 !!
L 0.1 !!
@JPK - All goes pear shaped from G....
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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
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The transformers are OK.
There is something wrong around Q4, Q5, Q6.
Check all the solder joints around there.
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In lesser company I might have argued that there was nothing wrong with my soldering, since I'd used Flux on every joint, perfected my technique...but there it was Q6 and its three pins, two attached to the board and one, like the Wright brothers, had lifted off the board. OR, I just soldered it to mid-air...
Soldered it correctly and sheepishly considered all the other joints in the area, and they are looked good.
Pulled out the DMM... and got near perfect results for the voltage test. Adjusted the P2 and got 3.000v after a good few turns with the blue tool JPK sent me.
Plugged in the Mic's and SOUND!Glorious SOUND!
Did all the other checks and all seems delightfully positive!
Thanks JPK
Appreciate the guidance - as always, you were spot on with where to look! I think it took me all of 30 seconds to see the problem once I knew where to look!
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(https://fonts.gstatic.com/s/e/notoemoji/15.0/1f44d/32.png)