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AKG426 repairs
Old 7th August 2014
  #1
Lives for gear
 

AKG426 repairs

I have an akg422 and a 426 and I'm not 100% convinced that they are working properly, I have had issues with a level difference between capsules- a couple dB- and it seems to happen with multiple pattern boxes mics and cables. My normal troubleshooting isn't coming up with obvious issues.

I have called AKG and they generally aren't eager to talk about that mic much less fix it.

Any ideas, preferably in the USA where they can be looked at?

-Ben
Old 7th August 2014
  #2
Gear Head
I had a local AKG repair depot that used to be very helpful, but I believe those days, and that depot, are long gone. So I can't help you with repair advice, but if you decide to sell one of those mics 'as-is', please let me know.

Dave
Old 7th August 2014
  #3
Lives for gear
 

Sorry... Not selling any of them.

--Ben
Old 8th August 2014
  #4
Ben,

If you're comfortable poking around in the mics and have a DMM, or better yet an oscilloscope, here's a schematic. The two sections of a C-422 are "almost" identical, but there are slight differences. Both amplifiers are classic FET "source-follower" impedance converters followed by emitter followers driving the line transformers so are identical amplifier circuits. However, there is a difference in the way the electronics are powered: Phantom power for the lower mic provides the direct capsule bias for both the lower and upper mics (all 4 diaphragms), while phantom power for the upper mic provides the power for the impedance converters and the line drivers for both the upper and lower sections. For either mic section to function, phantom power must be provided on both channels. It's "not quite" two completely separate mics sharing one housing.

There are separate current sources (Tr3 & Tr13) for each channel, so you may want to check the operating voltage of each amplifier circuit (across C7 and across C17) to be sure that they are both approximately the same.

If you have a 'scope, you can also compare the audio signal voltage at the base's of Tr1 & Tr11 to compare the FET outputs. If they are equal for a given sound level it's possible that one of the coupling caps (C6-upper or C16-lower) may be going bad and reducing the level at the transformer. They are electrolytics, and are old enough that they may be due for replacement along with the C7, C17 power supply bypass caps. If the levels at Tr1 & Tr11 are not equal, then either the capsule's sensitivities have changed or the FETs may have changed over the years. If you're adventurous, you can switch the input connections from the capsules (input terminals: C1, R1 and C11, R11) between the upper and lower boards to determine if the imbalance is due to the capsules or the impedance converters.

Good Luck!

Schematic attached:
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Attached Thumbnails
AKG426 repairs-akg-c22-schematic.jpg  
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