Help on C12 Build...

[GeorgeToledo]

Thanks, I'll give it a shot!

 

[Gus]

Can you look at the part number on the three terminal regulator the part with the small heatsink?  Is it a 7806 or a 7906 or something else?
Have the two zener diodes been changed to two 60VDC ones?  The part number can be looked up.

Can you take a picture of the microphone board(s) I am curious if this is a cathode biased microphone.

A C12 can be thought of as cathode biased microphone mixed with a RC filter to make it kind of fixed biased

I think tracing the grounding circuit will be very helpful to find the hum issue.

 

[GeorgeToledo]

Gus, while this isn't my mic, these pics are identical to the mic I have:

http://www.m221b.com/AMI/C12CG/C12CGHI.html

I have examined everything, and it really does not look like the ground gets separated at any place.

I had a recommendation from Dennis at AMI/Tab, and he said that the power supply could possibly be modded to deal with the hum, at the expense of effecting the tone, and that I should use a grounded power outlet.

I looked at my 7 pin XLR cables, and ground does appear to make contact with the sleeve with the way it all fits together, with all of them. I didn't get into crimping the shielding as described… it wasn't immediately obvious how to do that without redoing the whole connector. I am going to dig into this more this weekend though, and hopefully resolve the problem somehow, probably by installing a grounded power receptacle.

 

[Banzai]

... and he said that the power supply could possibly be modded to deal with the hum, at the expense of effecting the tone, and that I should use a grounded power outlet.

Great way to convince a client not to fix his problem  . At the least I would ask them to clean it because all that dirt and dust can become conductive and short out components = dead power supply.

For the problem itself, can you post a picture of your (6-pin?) cable terminations? Unscrew the connector housings on both sides to show how it's wired.

 

[GeorgeToledo]

I'll post pics of those connectors in just a bit, thanks.

I was planning on blowing out the power supply with a can of air. What would be the procedure for anything that doesn't blow out with that - something like alcohol and a q-tip?

 

[GeorgeToledo]

Adding a grounded power receptacle to my old house wound up solving the problem. No extraneous noise at all, clear as a bell.

 

[Banzai]

Best is a lint free cloth, and wipe clean with isopropyl. Compressed air cans release chemicals.

Q-tips for stubborn parts.

 

[GeorgeToledo]

Thanks for the tip!

I'm glad I hadn't used the compressed air yet.

 

[Banzai]

Another great tool for this kind of job is a paint brush. Priceless for cleaning electronics in general.

Thanks for the tip!

I'm glad I hadn't used the compressed air yet.