Sound Skulptor forums
Build help and troubleshooting => MP573 Microphone preamplifier => Topic started by: EduardoMota on February 06, 2022, 08:52:53 PM
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Hello,
I bought the MP573 last week but I had some problems in the setup process.
During the setup tests, we couldn't complete step 3 "General power check". We are getting +16 V between TP2 and V+ instead of the expected 24.2V.
- We have the jumper on CN1,
- Polarity switch is not on mute position
- Rechecked all solder joints
- Rechecked for reversed components (electrolytics, diodes, transistors, IC's),
- Trimed all the component leads and test pins as short as possible. Nothing is touching.
Here are the measured voltages on the test points suggested in the forum.
- A: 1,8V
- B: 3,055V
- C: 14,77V
- D: 8,49V
- E: 1,22V
- F: 7,83V
- G: 0.01V
- H: 1,18V
- I: 0V
- J: 0,43V
- K: 0,5V
- L: 0V
The values are all below the expected. I guess the problem is related to the fact that we don't have those 24V in the circuit.
After looking at the schematic, is not clear to me where does the 24V voltage come from. Also, why is there a 24V difference between TP2 and V+ since TP2 is connected to the 0V point (assuming V+ is the supply rails voltage it should be a 16V difference?)?
We would much appreciate if you can help us.
Best,
Eduardo Mota
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TP2 is NOT connected to 0V.
Make sure it is the case with an ohmeter.
The 0V is the ground of the lunchbox. In the MP573, the local ground (TP2) is shifted down by -9V by U2. This is how we get 24V between TP2 and V+ (15+9 = 24V).
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Ok, I guess I was mistaken by this part of the circuit (see attached schematic image).
TP2 is not connected to the 0V point in the circuit but my voltage is 0V at TP2. Do you think there is a problem with U2? Is this common? Attached there is an image of our circuit in case it helps with anything.
(https://i.imgur.com/dwAITZp.jpg)(https://i.imgur.com/qn8hSNl.jpg)
(https://i.imgur.com/wjBgSG3.png)
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but my voltage is 0V at TP2
Verify you don't have a short circuit between TP2 and 0V.
Check the voltage (against 0V)
on the anode of D4 (should be -15V),
on the case of U1 (should be -15V),
on the case of U2 (should be -15V).
The jumper on CN1 is on the wrong pin.
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Regarding the short circuit between TP2 and 0V. I just confirmed and it is not short circuited, the actual voltage difference is -0,74V.
The voltage (against oV):
on the anode of D4 is -14,72V
on the case of U1 is -13,47V
on the case of U2 is -0,7V
The jumper on CN1 is on the wrong position in the photo because we were trying if that was the problem at the time but we have already corrected its position.
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Do you think the problem might be a faulty U2?
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Is U1 getting quickly very hot after powering?
Same question for U2.
Can you measure the ohmic resistance
- between TP2 and 0V,
- between TP2 and V+ ?
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Hi, we just measured what you requested.
- U1 is getting quickly very hot after powering indeed
- U2 is moderately hot (I believe it is a normal temperature for a voltage regulator)
When measuring the ohmic resistance between the points you asked we got different values depending on the position of the multimeter probes. Also the values were not stable in some cases so here is the range.
- between TP2 and 0V - Black probe on TP2 = 10-80 k ohm
- Black probe on 0V = 1-10 M ohm
- between TP2 and V+ - Black probe on TP2 = 30,8 k ohm
- Black probe on V+ = 6,75 M ohm
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The current limiter U1 sees too much current
Can you measure the ohmic resistance
- between the case of U2 and 0V,
- between the case of U2 and V+ ?
Do you also get TP2 near 0V when the output transformer is disconnected?
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All the measurements so far are with the transformer disconnected therefore yes the voltage at TP2 is close to 0V (it is around -0,74V).
Here are the ohmic resistances you asked for:
- between the case of U2 and 0V - Black probe on the case of U2= 6,42M ohm
- Black probe on 0V = 20-30 M ohm
- between the case of U2 and V+ - Black probe on the case of U2 = 24,7 k ohm
- Black probe on V+ = 350-450 k ohm
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Do you have any idea what might be the probelm?
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Are you taking the 0V reference on the test pin at the back of the PCB?
There must be a short somewhere in the circuit. Look for a solder bridge somewhere. Is there an other component that gets hot (out transformer disconnected).
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We don't have a pin in the 0V reference. We are measuring it directly from the PAD.
The other components don't get hot. We already looked several times to the circuit to find any solder bridge but didn't find anything. Do you have any advice about what part of the PCB may have the short circuit? Is it possible that there is a problem with U1 or U2? What are your conclusions with the measurements we have provided?
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We are measuring it directly from the PAD.
Sorry, what is the PAD?
Remove U2. In order not to destroy any via or track, you must heat the 3 pins together with a large blob of solder.
Next mesure the DC voltage on the center pad of U2. If it is back to 14-15V then the issue is probably not in U1.
Replace U2 by a short (see attach pic) and briefly power up while monitoring TP2 voltage. Do not leave the power too long because the circuit will be overvoltaged.
If TP2 is still near 0V then the issue is probably not in U2.
(Or contact us for a workshop return)
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Hello, we finally found the problem! It is fixed and working smoothly now! Thank you very much for the patience and the hints that lead us to discover the issue. It was our fault! This is embarrassing but basically, we switched Q6 with U2 during the assembling :-X:-[:'(
Sorry, what is the PAD?
The PAD is essentially the same as a via. I think it is just a different name. Here is an article about that:
https://www.protoexpress.com/blog/what-is-pad-pcb-design-development/
Keep up the good work! :)