Mic Repair Puzzler

[Studio Parkside]

What would motivate spending a whole day repairing a $100 mic?
MXL v67 g
Intermittent loud hum sound cutting in and out. Checked for loose
connections/ cold solder joints. Resoldered most but all look fine to begin with.
wiggle test all wires and components no change. Phantom power steady at
capsule . This mic had a bad fall several years ago but continued to work well
until now.

I'm not a tech but i cant afford to pay someone and often manage to fix most things.
Have attached schematic. Because I have inspected everything visually i suspect internal component damage. Tested each side of the transformer and got a steady reading... BUT connecting directly from C8 neg
to R 11 junction bypassing the transformer sound is crappy but steady,leading me to suspect the transformer. Any thoughts ???
Scott Alger

[mjrippe]

So does the loud hum come in when the sound cuts out? If so check ground connections. Also re-solder every transformer connection if you suspect it. They are relatively heavy parts and can break solder joints more easily than caps, resistors, etc.

[Studio Parkside]

Hey and thanks for your reply

I have tested all connections etc and none seem to be the problem

The hum is constant. Bypassing the transformer and getting a steady signal
indicates to me that the amplifier circuit is working but the transformer is broken.
But as I said i am no expert , just hoping to avoid replacing to transformer if i am mistaken in my assumption .


Scott Alger

[LeeYoo]

Hi Scott.
IMHO, a mic can't hum by itself.
It can PICK UP hum, e.g. from a broken ground/body connection.
Or hum, introduced by the phantom power.
Is the hum instantly gone when you turn the phantom power off?
Did you measure the resistance between pin 2 and 3.
Should be no more than a few hundres ohms.
You're shure it is a steady 50/60 herz hum, or 100/120herz.
Not warbling/motorboating. That could indicate a faulty power cap in the mic.
Leo..

[Haz-Mat-Strat]

Check all of the pads carefully where all of the wires are connected. some times the pads lift and will have an intermittent connection. While the mic is on ,with a pencil eraser, chopstick, ETC, tap (lightly push) on all components, wires, and listen for static / crackles.

Re-solder all of the ground connections @ the board and XLR.

Check the contact of the body to the headbasket plate. Sand the top of the body edge to get better contact.

.