1

MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: July 12, 2022, 03:30:36 PM »

So I reflowed all the solder joints on P1 and RLY2 and finaly it works, i was able to complete "Try to follow the signal" with all values aligning as they should. I can hear the relay click when inserting a jack in the DI. However, no click when switching the gain from MID to LOW.

Nevertheless, i finally solderd the output transformer and dont get an audio signal . Do you think i Should just reflow the joitns as well and hope it will work or is the problem somewhere else?

2

MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: July 09, 2022, 12:36:55 PM »

can't hear any relay click when switching gain...
independent from both pots, i have 0.3VAC on LOW and 3VAC on MID on R8 (I think with top pin you ment the one next to c1), with a 0.5VAC sine wave input.
What realy confuses the shit out of me though, is that there is some sort of passive amplification. With the lunchbox turned off and a 0.5VAC on the input i still get 3VAC on the top pin of R8.

3

MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: July 05, 2022, 01:38:02 PM »

So i did the measuremets and i cant follow the signal 

• with gain on mid, pot IN on 10 and OUT on 0:  0.03 VAC on TP1 and the LED flickers green...
• with gain on Low and both pots on 10: 0.03 VAC on TP3 and TP4

4

MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: July 04, 2022, 05:49:02 PM »

Thanks for your help! I replaced U1 and everything semms normal now, the -5DCV on TP4 disappeared.
However, I followed the Test Guide again and the gain seems lower than expected, when I connect a 1VAC sine to the input and turn the input to max, the LED stays off. Only with gain switch on mid or hight the LED turns green. Same with the output pot to max, the LED only turns red with gain on mid or high... I'm not shure if this is an issue, mybe i just have to wait longer for the tube to warm up...

5

MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: May 19, 2022, 01:39:51 PM »

Measured between the respective pin and 0V:

• -0.35 V
• -4.59 V
• 0 V
• -15.09 V
• 0 V
• -2.31 V
• 15.12 V
• 0 V

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MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: May 17, 2022, 06:18:47 PM »

Shure, should i measuere the voltages with U1 installed or removed?

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MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: May 16, 2022, 04:38:07 PM »

Yes in this configuration TP4 returns to 0V.

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MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: May 14, 2022, 04:53:30 PM »

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

9

MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: May 13, 2022, 12:27:19 AM »

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)

10

MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: May 09, 2022, 02:14:43 PM »

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?

11

MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: March 30, 2021, 12:39:47 AM »

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.

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MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: March 27, 2021, 06:45:13 PM »

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

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MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: March 26, 2021, 12:27:15 PM »

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)

14

MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: March 26, 2021, 11:49:39 AM »

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

15

MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: March 23, 2021, 09:48:47 PM »

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.

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MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: March 22, 2021, 07:16:06 PM »

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...

17

MP566 Tube microphone preamplifier / Re: Tube works, but no output voltage.

« on: March 22, 2021, 02:35:32 PM »

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

18

MP566 Tube microphone preamplifier / Tube works, but no output voltage.

« on: March 20, 2021, 04:29:50 PM »

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?