Broken Condenser Microphone MXL CR89

[Shepa]

Hi! I've got a broken mic. Do not know its history.
After connecting to phantom power and preamp it works for 30-60 seconds and then the autoexcitation starts. It begins to buzz lightly and then I can hear strong buzz only.
I could not find schematics for MXL 890, V89 or CR89 which supposed to be the same.
The circuit board looks fine except one 47 Ohm resistor, which looks burned.
Should I change this resistor only, or capacitors and op-amps could be damaged as well? Unfortunately, all op-amps names were scratched away by the manufacturer.
Thanks!

[R_J]

Re: Broken Condenser Microphone MXL CR89

Get some freeze spray and freeze the components to find which is causing the issue, I could not find the 47Ω you are refering to, What is its location number?
U3 might be a 4562 op amp, try wetting the top os the ic, maybe part of the number will show up.

[Khron]

Re: Broken Condenser Microphone MXL CR89

Looks like, as RecordingHacks mentions, the CR89 may well be a revision / new paint-job of the V89. Which would explain the "deleted" model number on the silkscreen - "MXL [something else] REV NC".

http://cdn.recordinghacks.com/images...89_circuit.jpg

Physically smaller ceramic capacitors this time around, but looks like the exact same circuit / board.

I'm 95% sure the two 14-pin SOICs are likely something like 40106 hex Schmitt trigger inverters - all the ceramics and the diodes underneith them point towards that.

https://i.pinimg.com/originals/cc/6b...0e8130c171.jpg

The circuit looks like a twist on the old Schoeps circuit - JFET input as a high-impedance buffer and phase-splitter, and instead of a pair of PNP's, they used that dual opamp to drive the output.

https://billthompson.us/bt/sch/cmc5-schematic.jpg

The two huge electrolytics (Nichicon Muse? eyeroll...) block the phantom-power DC from affecting the opamp's output. Tough to tell with those in the way, but at least one (if not both) of the 40106's are for supplying the opamp, i think. The other, likely to provide the bias voltage to the capsule.

[redwire]

Re: Broken Condenser Microphone MXL CR89

Post a pic of the other side of the PCB, with the burnt resistor.

It's a typical condenser microphone design.
U1, U2 are the voltage multiplier for the capsule bias voltage, usually +/-60VDC. Guessing CD40106B
U3 is just a buffer with number looking like LM4562 as others have mentioned.

[Shepa]

Re: Broken Condenser Microphone MXL CR89

Thank you!
http://cdn.recordinghacks.com/images...89_circuit.jpg
In this photo one of the blue-green resistors has a black ring as well. It means that it is not the burning mark. I thought they should look the same.
Yes, the two huge electrolytes are Nichicon KZ Muse 100uF 50V.
On the other side, two Nichicon KW(M) 10uF 50V around J-FET 2SK170-GR.
And on the exit Nichicon PW(M) 150uF 50V.
Maybe I will start with replacing capacitors.
Thank you for naming IC's !!!

[Khron]

Re: Broken Condenser Microphone MXL CR89

Not too sure the capacitors are the issue. "For audio" or not, Nichicon should be one of the very few really dependable brands, and since phantom-power is fed through a pair of 6.8k resistors, they couldn't have had a hard life at all.

And speaking of phantom power - even if you short both signal lines to ground, you'll get a max of around 14mA, all of which gets "burned up" in those two 6.8k resistors that are inside the preamp (that the mic is connected to). So there's technically and physically no way anything can "burn" in a phantom-powered mic like this. Unless something went VERY wrong with the preamp, but that's a different story.

Might i suggest starting with probing for some voltages first, before "shotgunning" with the cap replacement?

Those "blue green resistors" are actually a pair of 47uH inductors, in series with the outputs.

[redwire]

Re: Broken Condenser Microphone MXL CR89

...And speaking of phantom power - even if you short both signal lines to ground, you'll get a max of around 14mA, all of which gets "burned up" in those two 6.8k resistors that are inside the preamp (that the mic is connected to)...

If you short to GND a mic preamp input, there is a high current discharge.
For the repairs I have done, I find the signal coupling capacitors in the mic pre-amp/mixer (end) are charged to 48VDC and will hit a microphone's electronics hard if plugged in hot.
This can damage the mic's clamp diodes or other parts going to the XLR. But not burn up anything, as you say.