Need help diagnosing buzz/hum in one channel of a two channel mic preamp

[aspenleaf]

Hello,

I'm trying to diagnose a buzz/hum in one channel of a Presonus MP20 mic preamp with Jensen transformers and LT1357 opamps.  The noise is only audible when the gain is over 75%, but channel one has the buzz/hum, while channel two doesn't.  I have gone over the circuit board looking for broken connections or cold solder joints and found a few, but it did not correct the problem.  I checked the ground points on the board for continuity and everything seems to be in order.  The buzz/hum happens whether a mic or instrument is connected or not.  It also appears at the channel outputs, the mixer output and the headphone output.  Engaging the 20db pad doesn't seem to affect the buzz/hum, only the level of hiss.

I couldn't find a schematic anywhere, but I'm hoping one of the more experienced readers here can offer some suggestions on solving this problem.

Thanks in advance.

 

[ruffrecords]

First thing to do is measure the hum/buzz frequency with a scope.. If it is mains frequency with harmonics then it is likely to be a magnetic interference problem from say a mains transformer to a mic transformer OR, if the unit has ac heaters, a problem with the heater supply OR a heater/cathode breakdown in one of the tubes.

If it issecond harmonic and above then it is most likely a HT supply problem - perhaps insufficient smoothing.

Cheers

Ian

 

[aspenleaf]

Thanks for the reply, Ian.

I'm not very schooled in electronics, so I'm not certain that I understand what you mean.  I don't have a scope, only a multimeter. 

It is a solid state, not tube, preamp.  There are harmonics above a 60HZ hum.  Channel two has a faint 60HZ hum that I can hear if I turn the gain up all the way, but channel one has that hum, plus a buzz.  There is a large doughnut shaped transformer next to the AC plug, and each channel has a Jensen transformer.  Could there be magnetic interference between the mains transformer and only one of the Jensen transformers, and how would I track that down?  It does it in any location throughout the house, using different circuits.

The preamp was built in 1999.  Is it possible that the electrolytic capacitors would cause this problem?  I didn't see any swelling or leaks.

 

[PRR]

> broken connections or cold solder joints and found a few

When you find a few bad joints, keep looking. They travel in packs, and there's always a few sneaky ones.

But I'm curious how a young OK-brand solidstate preamp could have any broken joints?

E-caps are worth replacing. Sorry I can't guess which ones. (And except the local power-supply caps, most would not raise buzz.)

 

[ruffrecords]

LOL, don't know why I thought it was a tube preamp. Must be going senile.

Anyway, it is unlikely that something has happened that would cause magnetic interference to occur - that should have been sorted at the design stage. The only way to check for it is to move the possible offending components further apart and that's not likely to be easy.

Probably better to turn your attention to the power supply circuits and check for more dry joints as PRR suggests. Do you have a schematic of it?

Cheers

Ian

 

[aspenleaf]

I haven't been able to find a schematic. 

I bought it used and the person I bought it from didn't pack it very well.  When it arrived I heard something rattling around inside and it turned out to be a piece of solder, so it must have been knocked around a bit.  I found 5 joints that weren't soldered at all.

I also found a few traces on the circuit board had been cut and sections of the circuit are bypassed with wire.  It appears to be the circuit associated with the inserts on each channel.  The inserts work, so I'm not sure what or why they were modifying in the circuit, but both channels appear to have the same modification, but only one has the buzz.