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Interesting AKG 451 capsule Ebay purchase

I've been wanting an extra AKG 451 capsule for a while - been looking for an omni - I've got two cardioids and fancied an omni capsule. Been looking on Ebay for ages and one came up with the mic, but a really weird one - and from Denmark too!

The picture showed a 451, with the omni capsule, but an XLR female to male Tuschel connector, a small box with a Tuchel female, and on the other end . . a BNC socket. Complete with BNC to ¼" jack. I think this put people off, and I put a bid in for a realistic amount and got it.

When it came the mic was pristine, with original box. Inside the box was a 9v battery, so I plugged it in and turned the gain up and it worked fine, and level wise, it was was expected, mic level.

It seems it had been bought for connection to some test gear, hence the BNC for connection to a scope.

The seller told me the mod had been documented and was hidden under the foam - which revealed a before and after hand drawn circuit diagram. Took me a while to convert the Danish colours into English and even longer to work out how to dismantle it - it worked and involved removing a resistor, and reconnecting the transformer output back to a capacitor - the balanced transformer having been shorted out with a link and the input to it wired to the output. I've never seen the insides on the net - there are a few circuits, a service manual but there have been many revisions over the years including transformer fitted versions to adjust output impedance - so finding the right circuit to match the hand drawn one to double check took a bit of brainwork. I've attached the 'before' picture before I put it back to how it was meant to be - maybe at some point it may help somebody else. Fiddly, but not too bad. For those interested, the small red wire is actually shorting the transformer out, and on the component side, there is a short red wire that just needed reconnecting to the blue that is simple tucked down the side of the transformer - then removing the resistor at the XLR end. Of course, this also means anyone who wants to unbalance and power one of these (not sure why) could easily change the wiring TO this.
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