1

CP5176 FET compressor / Re: SK25 output collapses and develops large negative DC offset at higher input

« on: September 12, 2026, 09:34:46 AM »

Does the LED lights up?
Check Q7 and Q9 with the DMM in diode mode. It is possible to test Q7 while in circuit and test half of Q9, the BC junction.
Test transistors
Generally DOA's are very difficult to troubleshoot because of the density and all the feedback paths. I would recommend to build a new one.

2

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

« on: September 12, 2026, 09:11:34 AM »

Actually, this is the normal frequency response of the original 1073. About -1dB at 20kHz.
I guess that the modern reissues are modified.
You can make the frequency response flat up to 20kHz by changing the value of 2 caps:
C47 at 100pF (instead of 470pF),
C60 at 47pF (instead of 150pF).

3

CP5176 FET compressor / Re: SK25 output collapses and develops large negative DC offset at higher input

« on: September 09, 2026, 04:07:17 PM »

at ~300 mVAC input it begins developing a large negative DC shift and its AC output collapses instead of continuing to increase.
It looks like the positive part of the output is not working. Check the solder joints of Q9, R12.
Can you explain your measurements:
R9 bottom: +0.12 VDC low -> -11.4 VDC high

4

MP573 Microphone preamplifier / Re: MP573 Build - No Output with XLR Connector

« on: September 04, 2026, 04:43:45 PM »

Too bad !

5

MP573 Microphone preamplifier / Re: MP573 Build - No Output with XLR Connector

« on: September 02, 2026, 05:35:48 PM »

The burnt relay is very probably what causes the DI/Mic issue.
You need to replace it.
Mouser/Digikey: TQ2-24V
TME : AZ850-24

Sorry for the missing cap. If you have an other one, you could also replace C2. C2 and C3 must be similar for the best CMRR.

6

LA502 Optical Compressor / Re: V- Rail returns low

« on: August 25, 2026, 06:53:28 PM »

7

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.

8

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).

9

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.

10

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?

11

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

« on: August 21, 2026, 10:05:55 AM »

12

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.

13

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.

14

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) ?

15

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.

16

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.

17

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.

18

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.

19

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.

20

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.

21

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.

22

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.

23

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?

24

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.

25

True-Grit Tube saturator / Re: TRUE GRIT Problème

« on: July 27, 2026, 06:23:56 PM »