[n-tn]

When I connect my MP566 to power, first everything seems normal. I have 223V between 0V and TP2, -5V between 0V and TP8. However, when connecting a 1 VAC sine wave to the input the LED stays off when turning P1 to Max (it stays off as well when turning P2 to Max). I constantly have 0VAC between 0V and TP4, only when moving P1 the value shortly rises, but breaks down to 0VAC immediately.

I already double checked all the solder joints, nothing seems wrong there... any ideas how to find the error?

[JPK]

Is it the same with the DI input?

[n-tn]

It is the same when using the DI input. Only one difference: the LED turns green, when the input signal voltage is high enough.

[JPK]

Try to follow the signal
Send a good constant sine signal on input, like 0.5VAC measured between pins 2&3 of the input XLR. Check the level with the preamp connected because the input impedance of the preamp may pull the signal down.
Connect the voltmeter black probe to the 0V pin and trace the signal on TP1, TP3, TP4.
The reading should follow the input signal level.
The place where the signal drops will give you hints on where to look for.

[n-tn]

Unfortunately I dont't know the Pinout of my lunchbox, so I can't measure the input XLR with the preamp connected. (Do you maybe have any suggestion how to measure it?)
However with 0.5VAC between XLR-Pin 2 and 3 measured when not connected to the preamp, I have 0V on TP1, TP3 and TP4...

[JPK]

The pinout of 500 lunchboxes is standard. 
https://www.soundskulptor.com/forum/index.php?topic=41.msg98#msg98
You can also remove the XLR plug cover and connect directly on the plug.

I have 0V on TP1, TP3 and TP4...

Do you hear the Pad relay click when you switch to/from low gain?


Check the input transformer primary resistance between fingers 8 & 10 : 21 ohms.
Check the input transformer secondary resistance across R8 : 1.2 kohms.

[n-tn]

Thanks for the Pinout! So i have the 0.5VAC on the input pins with the preamp connected and turned on. (still 0VAC on TP1, TP3, TP4)

I don't hear any click sound when switching from/to low gain, only a click when switching the polarity.

Check the input transformer primary resistance between fingers 8 & 10 : 21 ohms.

What do you mean with fingers? (sorry, just confused there because the mic input transformer only has 9 pins)

Check the input transformer secondary resistance across R8 : 1.2 kohms.

Not shure if you meant it, when measuring the resistance across resistor 8 next to the input transformer, i have 1.18 kohms.

[JPK]

The golden fingers are the contacts on the PCB edge connector at the back of the unit. The are numbered top-down from 1 to 15.

I don't hear any click sound when switching from/to low gain

Check the solder joints on RLY1, R9 and the gain switch. Check the direction of D1.

[n-tn]

The golden fingers are the contacts on the PCB edge connector at the back of the unit. The are numbered top-down from 1 to 15.

Thanks! I have 20.8 Ohms between finger 8 & 10.


All solder joints on RLY1, R9 and the gain switch seem fine too me. D1 is in right direction

[JPK]

The input transformer looks OK and connected to the module input.
Check if you can measure the input signal on R7-R8 top pins.

[n-tn]

When connecting a 0.5VAC 1KHZ sine wave to the input, I can measure the 0.5 VAC over R7 and R8 (I not shure if it is because of my multimeter but every mesurement has different values... however definetally something between 0.3VAC and 0.5VAC)

[JPK]

With the gain switch on MID or HIGH, it should be stable at 3VAC, because amplified by the input TX.

[n-tn]

With 0.5VAC sine on the input and gain on MID/ HIGH, I have 2.6VAC over R8 and 1.4VAC over R7

[JPK]

If you have a signal on R8 but no signal on TP1 then the issue might be potentiometer P1.
Check the total ohmic value (between the 2 extreme pins) as well as the 0V to wiper (center pin) value when varying the pot position.
Do you confirm that the issue is also present on the DI input?

[n-tn]

I unsoldered P1 and it looks fine, it got 98k ohms total and the value from the centre pin grows up to 98k as well. When soldered in and measuring the resistance between 0V and the centre pin, the value rises and is stable until 28k but falls to 0 ohms when turning futher.

I definetly get no output no matter if using the normal IN or DI. The difference is that the LED blinks according to the transients of of the input signal. Futhermore I recognized that I can measure the voltage of the input signal between 0V and TP1, when using DI.

[TechWiz]

Is it the same with the DI input?

May I join the discussion? I have the same issue, except the DI Input works perfectly fine. Via XLR I don't get a signal already with TP 1. Could it be a bad realy or something like that?

Thanks for the support!

[JPK]

It could be a damaged Q1 (2N7000).
This device is sensitive to static electricity and must be handled with your body grounded. Generally removing your shoes is enough.

[n-tn]

It took me a while until i was able to continue this project...
I ordered a new 2N7000 and replaced the old one. I was extra carefull regarding static electricity and only handeled it grounded.
However everything remains the same. i noticed, when connecting a 1VAC sine wave to the input and measure between 0V and TP4 there is constant -5V DC voltage.... any further suggestions where the problem could be?

[JPK]

TP4 should stay at 0VDC when there is no input signal.
If not the case, you must check the components value, direction and solder joints of R27 to R32, U1, U2, D3, D4.

[n-tn]

Between TP4 and 0V there is a constant -4.6VDC voltage with no input signal. All the solder joints seem to be fine, U1 U2 D3 and D4 are in right direction.
I noticed that the Anodes of D3 and D4 are connected, which confuses me since according to the schematic only the kathodes should be connected...(i unsoldered both diodes and checked connection between the holes with a multimeter)

[JPK]

I noticed that the Anodes of D3 and D4 are connected
Both configurations anodes or cathodes connected work the same.

Between TP4 and 0V there is a constant -4.6VDC voltage with no input signal.
This is the issue.
If you remove U1, does TP4 return to 0V?

[n-tn]

When i remove U1, i have 13V on TP4 and the front LED turns red immediately when switching on.

[JPK]

Sorry:
If you remove U1, AND TEMPORARILY CONNECT U1-PIN6 TO 0V, does TP4 return to 0V?

[n-tn]

Yes in this configuration TP4 returns to 0V.

[JPK]

So U2 seems to be OK.
Maybe U1 is damaged.
Can you report the voltages on all the pins of U1?