Noisy AT4033

[saint gillis]

  Dears, I'm repairing an AT4033, it works, but has a noise, a kind of highpass filtered little white noise.

  I've cleaned every part of the circuits carefully, replaced the electrolytic caps, replaced the capsule with a 100p capacitor, noise always there...

  I did my homeworks, here the schematic, any help appreciated, thanks :
http://www.sub-continental.com/diy/audiotechnica/AT4033.jpg

 

[Khron]

In some Shure KSM27 and KSM141, they seem to have used a faulty batch of 68pF / 120pF ceramic capacitors. Might be worth looking into? Or perhaps even the bigger values, if they're SMD ceramics.

 

[Gus]

A test/guess after the test and cleaning you did would be to check the regulator section with a scope for noise on the power supply

 

[abbey road d enfer]

  Dears, I'm repairing an AT4033, it works, but has a noise, a kind of highpass filtered little white noise.

  I've cleaned every part of the circuits carefully, replaced the electrolytic caps, replaced the capsule with a 100p capacitor, noise always there...

  I did my homeworks, here the schematic, any help appreciated, thanks :
http://www.sub-continental.com/diy/audiotechnica/AT4033.jpg

i would suggest you first identify the part of the circuit that's at fault. You can do it by shunting signal to ground, via  a RC (about 10uF in seried with 100r) in various parts of the circuit, particularly the bases and collectors of Q6 and Q8, and the source of Q1.

 

[saint gillis]

Thanks everyone.
So I've shunt to ground every transistor's base and collector with the rc network, always noise.
Then I removed yellow and black psu wires supplying the preamp, and put a 4K7 load between them, and the noise was present...

 

[abbey road d enfer]

Thanks everyone.
So I've shunt to ground every transistor's base and collector with the rc network, always noise.
Then I removed yellow and black psu wires supplying the preamp, and put a 4K7 load between them, and the noise was present...

It could be any of the components in the voltage regulator, but I would suspect first C14 and D5.

 

[saint gillis]

The electrolytics have been changed, and I've also tried to change the zener, didn't solve the issue..
I'll try to find the culprit , do you think I can put any low noise transistor in the regulator?

 

[abbey road d enfer]

do you think I can put any low noise transistor in the regulator?

The originals are rated at 120V, but I don't think it's necessary. They are not particularly low noise by modern standards, but they have a large hfe. I would not hesitate to replace them with the usual suspects BC550/560.

 

[Whoops]

  Dears, I'm repairing an AT4033, it works, but has a noise, a kind of highpass filtered little white noise.

Show us the noise, that can help understand what the problem is.

Record the noise and post a link here