G7 Mic Voltages

[Potato Cakes]

Hello, everyone,

I've dug out a G7 mic that I started quite some time ago, and after fixing numerous build errors and broken connections, I've managed to get stuck at the very end. PSU is outputting the proper voltages, so I went poking around inside the mic. I am getting correct voltages up to where it is circled in green, then the next component in line and I don't. I am a little perplexed, as I don't quite understand how you would expect to get same voltage on both sides of a 1G or a 33M resistor. The only thing I can think of is that the capsule is somehow causing this. I did check for shorts between the front and back diaphragm as well as the backplate with nothing to report. Any fresh perspective would be greatly appreciated as I want to minimize how many more times I need to resolder connections as the build is getting pretty mangled after I had to cut down to fit in a body that is slightly too small and jump  a number of broken contact points.

Thanks!

Paul

 

[volker]

You can't measure those voltages with a general purpose multimeter. The input impedance of the multimeter forms a voltage divider with the 33M/1G resistor. So from your measured 35V, your multimeter appears to have a 10M input impedance, which is exactly what would be expected, no fault there.

The reason you have the same voltage on both sides of the resistors is because there is no current flowing. Only a capacitor gets charged once, after that it is static. So with no current flow, there is no voltage drop across the resistor.

 

[Potato Cakes]

Thanks for the response, volker!

I guess I'm not sure at this point how to proceed. The tube seems to be getting it's voltages per the schematic and I can see it glowing. When I tap on the mic it is being transmitted out of the audio connection of the mic. So something is going in the vicinity of the capsule, but if my trusty Fluke can't tell me directly what is going on, then I'm scratching my head where to start looking. The capsule is brand new and I always take extra care when I install them. It could be bad from the manufacturer, but I don't know how to test that. It's made by a company here in Nashville, so I want to believe the chances of it being faulty are very slim.

Thanks!

Paul

 

[RuudNL]

The measured voltages are fine.
Maybe also check the anode voltage.
If this is somewhere in the middle between 0 and 160 V. I don't expect a problem with the circuit.
Easiest check is the 'finger' method.
Touch the grid connection of the tube. If you hear a loud hum, the circuit works.
(Don't by mistake touch the anode, this might cause an unpleasant surprise!  ;D)
If the circuit works, the capsule is the only suspect.

 

[Potato Cakes]

The anode is right at 75V, about have of what my total voltage. I meant to also include that in my findings. I'll try to make it hum and not shock me!

As the tube seems to pick up any vibrations induced on the mic and transmit audio.

I am still hoping it is not the capsule. Installing them is my least favorite part.

Thanks!

Paul

 

[Potato Cakes]

It is indeed the capsule. I had a cheaper c12 capsule laying around and I put it in there and it works. Now I have to see about getting the other one replaced.

As always, I appreciate the help.

Thanks!

Paul