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pgraham
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Joined: Sun Dec 12, 2010 5:39 pm

Trouble understanding mic preamp connections in Meissner 9

Post: # 11779Unread post pgraham
Mon Dec 13, 2010 2:06 pm

I have recently acquired a Meissner 9 home suitcase cutter that I'm recapping, and I'd like to update the mic inputs to work with modern low impedance mics. The first step is understanding what the current circuit is doing, and that leaves me puzzled. The original has 3-pin connectors with a pin layout similar to the Amphenol 91-MC3M, but without the screw-ring connector. Pin 1 is ground, pin 2 goes to the preamp tube grid, but it appears that pin 3 is connected via an extra coupling cap to the _output_ of the preamp tube, off the plate after the coupling cap where the signal is heading for the next stage. I am trying to understand why that would be a connection on a mic jack and coming up blank. I can't find any reference info on what the pinout of the Ampenol connectors was supposed to be, either. If I replace these with ordinary XLR jacks and a mic transformer I guess I will just remove that connection (running pins 2 & 3 through the mic transformer to get the signal for grid in the usual way), but I'd love to have some idea of what it's for. I know a lot of Trolls have these cutters - any insights?
Philip
Portland, OR
Meissner 9-1065

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markrob
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Joined: Wed Nov 28, 2007 1:14 am
Location: Philadelphia Area

Post: # 11783Unread post markrob
Mon Dec 13, 2010 3:54 pm

Hi,

Is it possible it was used for a mute function? Perhaps there was a N.C. "push to talk button" on the mic that grounded this point to mute the audio.

Mark

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pgraham
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Joined: Sun Dec 12, 2010 5:39 pm

Trouble understanding mic preamp connections in Meissner 9

Post: # 11784Unread post pgraham
Mon Dec 13, 2010 4:54 pm

Thanks, Mark - that makes sense. The period mic that came with it doesn't have any switch, but I don't think it's original - doesn't look like the one on the manual cover. All the Amphenol cables I've seen are actually only 1-conductor + shield so I wonder if it was ever used.

I'm now thinking I'll try to make a pigtail connector that uses 2 pins of the existing connection and has a 1/4" female on the other end, so I can use old high-impedance mics with 1/4" jacks or use an inline XLR-1/4" impedance converter from RatShack with newer mics. Those are hardly HiFi, but for this cutter I'm sure it's fine. I have one of the odd 3-pin connectors from the existing mic I can use for one pigtail, with the benefit of converting that mic to a more-standard 1/4" plug; and I have some speakers for an old Bogen PA amp that use what I think is the same connectors that I've been wanting to convert to 1/4" anyway.

But first to test if it will actually cut, using the AM radio, which is really strong.
Philip
Portland, OR
Meissner 9-1065

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