[rolly]

Hi,

i've built two MP573's a few months ago, and they sound(ed) absolute amazing.

After initial tests and recordings i mainly keept using a single amp for vocals, leaving the second one untouched for a while.
Two weeks ago i wanted to do some stereo recording, but the second one now outputs an audible white noise. It's completely independent of any switch setting except the mute position of the polarity switch. Mute is working as intended and the amp itself does what it's supposed to do.

Voltages are looking good to me:

TP	ref.	measured
A	2.5 V	2.58 V
B	4.0 V	4.05 V
C	22.0 V	22.4 V
D	13.3 V	13.5 V
E	2.0 V	2.02 V
F	12,7 V	12.88 V
G	0.9 V	0.896 V
H	4.2 V	4.11 V
I	0.35 V	0.342 V
J	3.6 V	3.56 V
K	22.3 V	22.6 V
L	3.0 V	3.00V after 30 mins. of warm-up

Any advise on how to fix this?


Regards
Oli

EDIT: typo

[JPK]

Your measured voltages are good.
Can you measure the noise AC voltage level on the output?
Do you get the same noise after removing the jumper on CN1?
Do you confirm that the noise doesn't vary in level with the -/+10dB potentiometer?

[rolly]

With CN1 plugged in:

gain pot. set to -10dB

p-p mean	68,8 mV
RMS mean	4,98 mV

gain pot. set to +10dB

p-p mean	69,9 mV
RMS mean	6,9 mV

after CN1 has been removed:

gain pot. set to -10dB

p-p mean	68,8 mV
RMS mean	4,98 mV

gain pot. set to +10dB

p-p mean	71,1 mV
RMS mean	7,25 mV

Turning up the gain pot makes the noise a bit louder, and removing CN1 doesn't seem to make any noticeable difference.

[JPK]

Look for reversed electrolytics or tantalum between C24 and C29.
Is the V+ voltage correct?

Can you try the following:
Create a temporary short circuit between pins 2 and 3 of U2 (you can carefully use a small screw driver) and listen if it reduces the noise. If it does, then the noise source is U2 and it needs to be replaced.

[rolly]

I checked all electrolytics and tantalum caps between C24 and C29 as suggested, none of them is reversed.

TP2 -> V+ = 24,2 V
TP 0V -> V+ = 15,2 V
TP 0V -> TP2 = -9.0 V

Short-circuit between pins 2 and 3 of U2 does nothing except giving a short peak signal to the output.

[JPK]

Try removing C24.
Try replacing C8 and C27 (for testing you can use any larger value).
Try replacing Q4.

[rolly]

OK, then I have to order and build a 501 first to be able to dig deeper into the troubleshooting on my desk with all necessary tools in easy reach, instead of sitting on the floor in front of a rack.

[rolly]

What ESR value/range and tolerances for the caps do you recommend?

[JPK]

It doesn't matter.
We are just testing in case a defective cap generates the noise.
You can use any cap with a voltage similar or higher and a value similar or higher.