Sound Skulptor forums
Build help and troubleshooting => MP566 Tube microphone preamplifier => Topic started by: narekm97 on July 19, 2022, 09:17:32 PM
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Hello,
When I connected my MP566 to my lunchbox, I turned on the power and got a red light on the LED. After around 5 seconds the LED light went out and I couldn't get any signal through the preamp. Not sure where to look for the problem
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Check again the voltages then follow these tests: https://www.soundskulptor.com/forum/index.php?topic=209.msg981#msg981
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I have the very same problem as described above.
I also checked Topic: "In case your MP566 doesn't work as expected" - there are no "usual suspect" to be found on my build.
Neither the DI nor the Mic in let any signal through.
What should I do?
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Try to follow the signal as described in the same place here (https://www.soundskulptor.com/forum/index.php?topic=209.msg981#msg981).
It will tell you where to look.
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Thank you for replying.
I was not able to follow the instructions to "follow the signal" - I am new to this DIY stuff and I simply don't know what to do with the instructions.
Since this is my first build, I might have done something wrong, although I took special care to insert all the components in the right direction.
This is what happenes:
- I turn on the power of my R6, the LED flashes red for about a secont or two then it goes out
- the device emitts a faint smell, so I think some components are heating, the tube is not getting warm and the tube does not light up
Here is, what the test-restults are:
Test 1 (Short circuit test) -> 2,72 MOhms (If I got the test right, since there is no GND test pin on my board)
Test 2 (High Voltage Test) -> 0,03V and decreasing over time to 0
Test 3 (Low Voltage Test) -> -0,02 and rising to 0
Test 4 (Signal Check) -> I read 1,63V between OV and TP4, the LED stays off even if I max out the Output potentiometer
If you know what that means; I would be greatful to get any advice how to find and fix the problem.
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There is something wrong around U4, D14, D15, R47...R50.
Check value, direction and solder joints.
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Thank you. I checked the components you told me:
U4 - direction ok, solder joints ok
D14 - direction ok, solder joints ok
D15 - direction ok, solder joints ok
R47 - 330k - ORG-ORG- BLK- ORG-BRN measures at 105 kOhm
R48,49,50 - 47k - YEL-VIO-BLK-RED-BRN measure at 41kOhm
So there is one difference in R47... is that the problem?
(https://i.ibb.co/GWQVBd4/Mittel-IMG-9084.jpg)(https://i.ibb.co/Tg75vB0/Mittel-IMG-9085.jpg)
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From the picture I can see that you have the wrong IC in U4 (OPA134 instead of L272/TCA0372).
The OPA134 goes in U1.
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Thank you. I switched the ICs and now it tests as follows:
Test 2 (High Voltage Test) -> 223,7V
Test 3 (Low Voltage Test) -> 0,0 V
Test 4 (Signal Check) -> LED stays off all the time. I measure 0,003 VAC with in and out turned on to max.
Where else can i look for problems?
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You have probably destroyed the TCA0372 by putting it in the wrong spot. Maybe both IC's.
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Thanks for your help.
I replaced both ICs - but nothing changed. It still does not work.
Test 1 (Short circuit test) -> 2,72 MOhms
Test 2 (High Voltage Test) -> 223,8V
Test 3 (Low Voltage Test) -> -0,04 and rising to 0
Test 4 (Signal Check) -> LED stays off all the time. I measure 0,003 VAC with in and out turned on to max.
What could it be?
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The heater is not working.
Check the tube heater by measuring the resistance value between TP8 and V- (with power off): ~12 ohms.
If you don't get this value, check on the tube itself between pins 4 and 5 (bottom view).
(https://i.imgur.com/k2ItzgD.png)
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I measure:
between TP8 and V- => 12,3 ohm - Sometimes wildly jumping up to 15 to 20 Ohms
between pin 4 and 5 of the Tube (bottom view): 16 ohms and rising
between pin 5 & 6 (bottom view): 12,2 Ohms
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between pin 5 & 6 (bottom view): 12,2 Ohms
Do you confirm. This is totally uncorrect.
Can you check again the tube when disconnected?
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You are right. I was off by one pin.
I measured again with the tube disconnected: 127,5 Ohms.
Do I need a new tube?
(https://i.ibb.co/sRv5Dxk/Gro-IMG-9109.jpg)
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Is that between pins 4 and 6 (not sure from the photo).
If true, then you need to replace the tube. Contact us here (https://www.soundskulptor.com/en/content/6-contact).
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Thank you - I replaced the tube, now audio is coming through. But very very quiet.
The sound is not distorted or anything, but very quiet.
Here is what I did:
I set the Input Level on my apollo twin to 3/4 (that is 52dB)
I set the MP566 Input to 10 and output to 10
With the Gain Switch to low: I receive 46dB of level
With the Gain Switch to mid I receive 20dB of level
With the Gain Switch to hi: I receive 36dB of level (yes, acutally quieter than with mid)
Still seems something to be wrong. What else could I do?
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Try sending the signal via the DI jack and check if it is louder?
Try to follow the signal (https://www.soundskulptor.com/forum/index.php?topic=209.msg981#msg981)
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I re-checked:
Audio-Check:
POT in & POT out set to 10
Apollo Twin set to 65 dB input (fully turned up) - Noise is at 46dB constant
Line-in: | High-Z in:
Lo: -27 dB | Lo: -18 dB
Mid: -5 dB | Mid: -18 dB
Hi: -25 dB | Hi: -36 dB
Then I followed the signal. I sent the 1000Hz sine to the line-in, measuring 0,5V between 8 and 10 of the extender
| black probe | red probe
------------------------------
TP1 | 0,142 V | 0,207 V
TP3 | 0,137 V | 0,217 V
TP4 | 0,164 V | 0,227 V
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| black probe | red probe
------------------------------
TP1 | 0,142 V | 0,207 V
The value on TP1, TP3, TP4 are measured against 0V (black probe on the 0V test pin).
The value on TP1 should be around 3VAC with the gain switch on MID.
Do you get a better level when sending the signal on the DI input?
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Thank you for clarifying.
I remeasured: black probe to oV - red probe to TP1, TP3, TP4.
| Line-in | DI in
-----------------------
TP1 | 0,0V | 0,496V
TP3 | 0,0V | 0,0V
TP4 | 0,0V | 0,0V
Same results for Gain Switch lo, mid and hi
Potentiometers did not affect the measurement
I got 0,496V on pin 8 and 10 of the extender - my multimeter showed 1,044kHz - so the input should be right.
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You have 2 issues here.
- The input signal not reaching the first triode: Check the connections on RLY1, transformer T1, potentiometer P1.
- The tube amp gives no output: Check the connections on the tube, on potentiometer P2.
Check the usual suspects (https://www.soundskulptor.com/forum/index.php?topic=11.msg11#msg11).
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I checked all the connections. Solder joints look okay to me, only T1 looks a bit strange.
I remember having trouble soldering the 90° connectors of the tube PCB (A) to the Main PCB. The solder just didnt stick correctly.
Please find attached pictures of the joints.
Should I resolder?
RLY1
(https://i.ibb.co/RHkKY2p/Gro-RLY1.jpg) (https://ibb.co/RHkKY2p)
T1
(https://i.ibb.co/0XswSmX/Gro-T1.jpg) (https://ibb.co/0XswSmX)
P1
(https://i.ibb.co/0DVDYL0/Gro-P1.jpg) (https://ibb.co/0DVDYL0)
Tube socket
(https://i.ibb.co/M1wkM5p/Gro-Tubesocket.jpg) (https://ibb.co/M1wkM5p)
PCB Connector
(https://i.ibb.co/t2SdhMC/Gro-90degreepins.jpg) (https://ibb.co/t2SdhMC)
P2
(https://i.ibb.co/mTnnsf1/Gro-P2.jpg) (https://ibb.co/mTnnsf1)
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This solder joint on T1 looks bad:
(https://i.imgur.com/lS20Ray.png)
Should I resolder?
Do not hesitate to reflow any suspect joint. Your iron should be hot enough, around 320-350°C.
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I finally got the time, to resolder and do the tests again.
The signal is now coming through louder
- Pot IN and Pot OUT turned to max
- input level on my apollo is turned to max (65dB)
- Signal is louder but only in position mid gain (solid level up to 0dB)
- but Signal is very very noisy.
Test 4 (low voltage check) -> -15V
Test 5 (signal check)
| Line In |/| DI In |
| Lo | Mid | Hi |/| Lo | Mid | Hi |
----|--------|--------|--------|/|--------|--------|--------|
TP1 | 0,271V | 3,043V | 3,043V |/| 0,496V | 0,496V | 0,496V |
TP3 | 0,000V | 0,006V | 0,000V |/| 0,000V | 0,000V | 0,000V |
TP4 | 0,001V | 0,019V | 0,001V |/| 0,002V | 0,002V | 0,000V |
What else should I try?
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Test 4 (low voltage check) -> -15V
Your tube doesn't seem to be heated. Is it glowing in the dark?
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No the tube does not glow at all.
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No the tube does not glow at all.
This is the issue. What is the DC voltage on U4 pin 1 (against 0V)? Should be around +13VDC.
Check value, direction and solder joints of U6, U7, R51, C26, C28.
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black Probe on 0V + red probe on U4 pin 1 = -14,49V
red probe on 0V + black Probe on U4 pin 1 = 14,5 V
U6 - orientation ok, solder joint ok
U7 - orientation ok, solder joint ok
R51 = 7,6 Ohms, solder joint ok
C26 = 10uF - 63V, solder joint ok
C28 = 100uF - 25V, solder joint ok
I resoldered all the joints.
The tube is still not glowing.
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black Probe on 0V + red probe on U4 pin 1 = -14,49V
It should be positive.
Can you measure the DC voltages on the 8 pins of U4?
(https://www.soundskulptor.com/prodimages/debug/ic8pins.png)
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I confirm:
black Probe on 0V + red probe on U4 pin 1 = -14,49V (negative)
DC voltage measured against pin 8 of U4:
2,851V (slowly rising by +0,001V)
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Can you measure the DC voltages on the 8 pins of U4?
I mean all the 8 pins.
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1 -14,49 V
2 12,78 V
3 12,00 V (1 times sparks, when touching the pin )
4 -15,01 V
5 -5,677 V
6 12,53 V
7 12,55 V
8 2,850 V
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1 -14,49 V
7 12,55 V
8 2,850 V
With these values, the outpout pin 1 should be positive.
If the values are correct then U4 seems to be dead again.
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I bought a new 0372
The values have changed a bit, but pin 1 is still negative.
The tube is still not glowing:
1: -14,50 V
2: 12,80 V
3: 12,02 V
4: -15,01 V
5: -5,722 V
6: 12,40 V
7: 12,40 V
8: 2,922 V
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remove U7.
Check if U4 pin1 goes up to +12V now.
If not OK U6 is probably dead. Remove U6 and check again U4 pin1. If OK U6 is surely dead.
If OK look for a short circuit between TP8 and V- (tube removed).
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I removed U7 --> -14,51V on U4 Pin 1
I removed U6* --> 0,0V on U4 Pin 1
I removed the tube --> 5 MOhms (and falling) between 0V and TP8
*removing U6 was not possible without destroying it.
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removing U6 was not possible without destroying it.
To remove these 3 pins devices, simply heat the 3 pins together with a large blob of solder.
You can replace >U6 by this R-7812-0.5 or this PXO7812-500-S
I removed U6* --> 0,0V on U4 Pin 1
Not good. U4 might be dead again.
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I doubt it. I just replaced U4 two days ago.
It's the 3rd one now.
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I have bought a new U6 and a fourth (!!!) U4
(It is getting pretty expensive by now)
The values have changed significantly on Pin 4 and 8. Everything else remains the same.
The tube does not glow.
1 > -14,5V
2 > 12,15V
3 > 11,68V
4 > 2,86V
5 > 6,02V
6 > 12,22
7 > 12,21
8 > 5,66
Will it ever work?
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Can you measure the DC voltages on the 8 pins of U4, without U4 inserted?
Will it ever work?
Sure!
You can always decide to send it to us for a repair.
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I removed U4 and measured again:
1 > -14,9V
2 > 13,9V
3 > -15,04V
4 > -15,05V
5 > -5,393V
6 > 13,7V
7 > 13,7V
8 > 0V
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With U4 inserted, U4-pin4 = 2.86V
With U4 removed, U4-pin4 = -15V
Weird! U4-pin4 is connected to the V- rail.
Can you check the continuity between U4-pin4 and the V- test pin?
Do the voltage values on V- and V+ remain correct when U4 is in place (-15V, +15V)?
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New testing sessions, new results:
U4 Pin 4 with U4 inserted: -14,91V
U4 Pin 4 with U4 removed: -15,05V
Between U4 Pin 4 and V- > 0V
Voltage on V+ 14,99V
Voltage on V- 14,91V
But meanwhile C23 is not looking healthy anymore:
(https://i.ibb.co/jRT9dBf/Gro-IMG-9700.jpg) (https://ibb.co/YZ346YQ)
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New testing sessions, new results
I am puzzled. I will recommend you again to send it to us.
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Seems like a good idea.
Where can I send it?
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Contact me here:
https://www.soundskulptor.com/en/content/6-contact