Repairing a Neumann KM84i

[ricothetroll]

Hi !
A friend of mine brang me that old KM84i that's not working anymore. The output level is VERY low and thus is mixed with the mic'/preamp's noise floor. I found the schematic here :
http://recordist.com/ampex/schematics/neumann/km8345sch.jpg
I'm wondering wether the problem is the capsule or the electronic part. I measured the voltages in the circuit and compared them to the schematic, everything looks OK. I can't really measure the polarisation voltages because my DMM's input impedance isn't high enough to not heavily disturb the measure, but I guess that there are no reasons why 10Meg and 1G resistors go wrong.
I have some Rode NT5 and I tried to connnect their capsule to the Neumann circuit with test wires but it didn't work any better than with the KM84's capsule. I also did the opposite test, trying to connect the KM84's capsule to the Rode electronics, without success either.
My questions are : what's the "standard" procedure to test a broken mic ? What else could I test ?
Best regards.
Eric

 

[radardoug]

You can feed an oscillator into the point where the capsule normally connects. Use a capacitor in series with your oscillator output. .47 uF will work, but not give you accurate frequency response. Use 22 - 47 pF to emulate the capsule capacitance. This will show up any impedance problems, whereas the .47uF will 'brute force' the input.
Common problems on the KM84 include the tantalum bypass caps in the power supply shorting, noisy fets, and sometimes breaks in the transformer connections. All pretty easy to suss out.

 

[ricothetroll]

Hi !
Thanx a lot for your answer !
I measured the DC resistance of the trafo and it appears that the lower secondary ("Geld-Blau", yellow-blue) is opened : I measure 4k4, that is actually R9+R10. On the upper secondary I measure a few ohms, so I guess this one is OK.
Can I short the broken secondary ? What are the risks to use one sec instead of two in series ?
Best regards.
Eric

 

[radardoug]

I think the way they wound those trannys they used all windings for maximum hum rejection, but there's no reason you shouldn't connect to the one secondary and see what happens. At the worst the mike will be a bit susceptible to hum pickup.

 

[radardoug]

Just thinking more, if you look very carefully where the wires go in you might find the break, and can then repair it. You will have to pull off some insulating layers.

 

[ricothetroll]

Hi,
I'll give it a try !
For the repair of the trafo itself, I must admit it's kinda scary, I once repaired a Fender Jazzbass pickup and that was already a VERY delicate job, so that tiny little transformer...
Thanx for your valueable advices !
Best regards.
Eric

 

[ricothetroll]

I shorted the bottom side of the trafo and the mic is now working perfectly ! I can't even hear the slightest hum
Thanx a lot for your help, Doug !