1

LA502 Optical Compressor / Re: V- Rail returns low

« on: August 25, 2026, 06:53:28 PM »

2

MP512 Microphone preamplifier / Re: Clipping/distorting noise on High Z input

« on: August 25, 2026, 06:52:18 PM »

Your Q2 may be damaged, try replacing it.

3

LA502 Optical Compressor / Re: V- Rail returns low

« on: August 22, 2026, 04:52:17 PM »

Sorry !!! I mean't R35, not R39.
Double check R35. 
Check you didn't break the via which connect the bottom and top pads together. In particular on the bottom pin of R35 (near R17).

4

MP566-II Tube microphone preamplifier with HPF / Re: Signal gets heavy distorted and very quiet.

« on: August 22, 2026, 04:44:04 PM »

I can measure an AC Voltage on all TP's except TP2 and 3.
This is because there is a large DC offset which fools your meter. It is normal.

5

LA502 Optical Compressor / Re: V- Rail returns low

« on: August 21, 2026, 10:10:37 AM »

- Remove all the socketed IC's and and check it it is still the same.
- Check the value of R39 with the ohmmeter (1 ohm). It should almost look like a short.
- Did you need to remove a component during the build?

6

LA502 Optical Compressor / Re: 502 Compressor not working - test results.

« on: August 21, 2026, 10:05:55 AM »

7

MP566-II Tube microphone preamplifier with HPF / Re: Signal gets heavy distorted and very quiet.

« on: August 21, 2026, 10:04:29 AM »

Try tapping on components and connectors with a pen to try to locate the defective connection.
Resolder all solder joints that look suspicious.
Monitor a constant sine signal with your AC voltmeter at points TP1, TP2, TP3, TP4, TP5, TP6 to try locate the stage where the problem occurs.

8

MP512 Microphone preamplifier / Re: Clipping/distorting noise on High Z input

« on: August 21, 2026, 09:57:31 AM »

Check you didn't swap Q1 & Q2.
Check the voltage on R46 top pin (7.4V).
Check the solder joints on R45, R46, R47.
Check the direction of C23.

9

MP512 Microphone preamplifier / Re: Clipping/distorting noise on High Z input

« on: August 18, 2026, 11:04:36 AM »

Check that Q2 (2SK170) is in the correct position A.
Can you measure the DC voltage (against 0V) on both sides of R51: near connector (16V) and near C24 (15V) ?
Can you measure the DC voltage (against 0V) on both sides of R48: near C23 (8V) and near R41 (1V) ?

10

LA502 Optical Compressor / Re: Release time feels bit fast

« on: August 11, 2026, 09:27:39 AM »

Do you also find the release time too fast with the 10mS attack?
Check the value and solder joints of R32, C17, C18.
If still too fast try increasing R32 to 4.7M. You could also add an additional cap in parallel with R32. It will also slow down the attack time but you can compensate by decreasing R29.

11

MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« on: August 06, 2026, 04:47:16 PM »

Check that R46-L is connected to the 0V. Check on the resistor leads, not the PCB pad.
Reflow the solder joints of R45, R46, R47, R48.

12

MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« on: August 03, 2026, 05:43:17 PM »

No, that's very unlikely. The microphone is isolated from the preamp circuitry by the input transformer, so the preamp itself cannot damage it. The only parts directly connected to the microphone are the 48 V phantom power (through R1 and R2), the pad relay, and the input transformer. Under normal conditions, none of these should damage the microphone.

13

MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« on: August 03, 2026, 05:34:53 PM »

You have a broken connection.
Check that R47-Right is connected to
- R46-R
- R45-L
- U4-pin2
Check that R47-Left is connected to
- D11-L
- R48-R
- U4-pin6
If one link is missing, repair it with some wire.

14

MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« on: August 01, 2026, 07:48:11 PM »

Check the value of R47.
The value is 1Meg (brown, black, black, yellow, brown) but when measured in circuit, it should give you about 12k.

15

MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« on: August 01, 2026, 07:37:53 PM »

and than the signal fades away quite rapidly.
Try to follow the signal on TP1, TP4, TP5, TP6.
Check what happens between the initial working state and the final faded away state.
Also cjheck if something happens with DC.

16

MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« on: July 30, 2026, 06:57:36 PM »

What I see on Pin 6 of U4: with Gain at 0 and Out at 0: 3,26V.
The issue is here.
Please report the DC volts (against 0V) on U4 pins 1, 2, 3, 5, 6, 7, also 4 and 11.

17

MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« on: July 30, 2026, 06:51:56 PM »

Check the resistance value between:
- XLR pin2 and pin3, 
- XLR pin2 and 0V,
- XLR pin3 and 0V.
Also check your mic cable.

18

MP566-II Tube microphone preamplifier with HPF / Re: MP566-II Phantom Power problem - signal fades to zero

« on: July 30, 2026, 08:44:57 AM »

but the partslist with C1 on is crossed out
Make sure you are using the latest parts list. C1 must be populated.
Does the 48 V rail (on the finger connector, pin 15) remain stable when the voltage on the XLR drops?
What is the phantom voltage level on each side of R3?

19

MP566-II Tube microphone preamplifier with HPF / Re: Green LED (LD2) does not light

« on: July 30, 2026, 08:34:01 AM »

Check the bi-color LED with your DMM in diode test mode. It should light up in one color, then the other when you reverse the test leads.
Check that you receive the peak voltage of the input signal on U4 pin5, the same as pin3.
Check the DC voltage on U4 pin6. It should be a few millivolts above 0.

20

True-Grit Tube saturator / Re: TRUE GRIT Problème

« on: July 27, 2026, 06:23:56 PM »

21

True-Grit Tube saturator / Re: TRUE GRIT Problème

« on: July 27, 2026, 09:50:04 AM »

Il faut que vous restauriez la connexion entre P1-pin-haute et R7-pin-gauche en soudant un fil entre ces 2 points.

22

True-Grit Tube saturator / Re: TRUE GRIT Problème

« on: July 26, 2026, 08:32:54 AM »

Il est normal qu'une valeur de résistance mesurée dans le circuit soit plus petite que sa valeur nominale car elle est très souvent connectée à d'autes résistances qui diminue la valeur mesurée.
Vérifiez le bon fonctionnement de P1 (sur TG1) en mesurant la résistance entre la broche centrale et chacune des broches latérales : elle doit varier progressivement de 0 jusqu’à la valeur maximale.
Vérifiez que les broches latérales de P1 sont correctement connectées à R5 et R7, pin gauche, respectivement.
Vérifiez les valeurs de R5 et R7, à l'aide du code de couleur.

23

True-Grit Tube saturator / Re: TRUE GRIT Problème

« on: July 20, 2026, 08:34:54 AM »

Vérifiez le potentiomètre P1 : 20k entre les pins extrêmes, 0 à 20k entre la pin centrale et les pins extrêmes, en fonction de la rotation.
Vérifiez les soudures de P1, R5, R7, C9.

Vérifiez les connexion entre le switch SW2 et le connecteur J1 :
- pins 1-2 (à l'avant du switch) vers J1-pin2
- pins 3-4 (au milieu) vers J1-pin1 (pastille carrée).
Si une connection est coupée, il faut la réparer avec un petit fil.

24

LA502 Optical Compressor / Re: 502 Compressor not working - test results.

« on: July 18, 2026, 05:01:47 PM »

All seems to be good when U1 is removed.
So the issue must be around U1.
Check the value and solder joints of R6, R7, P1, C5.
Check with another OPA2134.

You said the device is not working, but you did not specify what the symptoms are?

25

EQ573 Equaliser / Re: EQ573-II ran in my lab

« on: July 17, 2026, 09:12:26 AM »

OK, you may have an issue somewhere.
Please wait until I am back at work (end of July) so I can help you.