Apex 460 Noise

[radardoug]

So how much ripple did you see on the scope?

 

[bernatvm]

I think this is normal ripple for a good PSU

I went to the lab earlier today and took at the voltages where the cable join the mic pcb with the oscilloscope. Didn't have time for any of the other tests. Here are some pics of the readings, I think there are no problems.

Heater

B+

I also attach an FFT of the noise. Snapshot is taken in a silence moment on a speech track. The gain is what I would usually use in that situation.

 

[radardoug]

You shouldn't be able to see any ripple.

 

[bernatvm]

Even at that scale? Those pics are taken at 10mV/division

 

[bernatvm]

Shorted pin 2 of the to ground and the noise is gone. That makes me think the problem is between the capsule and the tube. Do you agree? What could it be?

 

[JohnRoberts]

Scope measurements are relative to the ground lead, so check where that is connected?

JR

 

[bernatvm]

Scope measurements were taken referenced to the chasis of the microphone

 

[radardoug]

Shorted pin 2 of the to ground and the noise is gone. That makes me think the problem is between the capsule and the tube. Do you agree? What could it be?

Pin 2 of what? the microphone cable or the output cable?
If pin 2 is a ground, then your problem is fixed.

 

[bernatvm]

Pin 2 of the tube, sorry. It's signal input from the capsule to the circuit

 

[radardoug]

Yes, well that will certainly stop the noise. Try disconnecting the capsule and everything but the high value grid resistor. See if it is noisy. Then substitute a 1 meg resistor for the high value resistor. See if it is noisy.

 

[Mylithra]

You're sure youre not working with a microphonic tube?
Out of curiosity, why'd you pulled the 2 filter caps in the 12v PS?

 

[bernatvm]

You're sure youre not working with a microphonic tube?
Out of curiosity, why'd you pulled the 2 filter caps in the 12v PS?

I tried three different tubes all with same results, tube might not be the problem.

About the caps, which ones are you referring? I don't think I pulled any of them in the 6.3V (actually a typo in the psu schematic)

 

[davemascera]

I tried three different tubes all with same results, tube might not be the problem.

About the caps, which ones are you referring? I don't think I pulled any of them in the 6.3V (actually a typo in the psu schematic)

At the beginning of this thread doug said the ratio of bad tubes to good might be 100:1.

He also said earlier on to sub out the 200M with 1M and see if the noise improves and if it does, it may very well just be that the tube isn't up to the task.

 

[Mylithra]

About the caps, which ones are you referring? I don't think I pulled any of them in the 6.3V (actually a typo in the psu schematic)

In the first post I saw this:
I removed C9 and C10 which act as RF filters.

In the PSU ,these are the larger filter caps in the DC heater supply. Are these not the caps you're referring to?

 

[bernatvm]

In the PSU ,these are the larger filter caps in the DC heater supply. Are these not the caps you're referring to?

I was reffering to C9 and C10 on the microphone, filters on the transformer output.

 

[bernatvm]

At the beginning of this thread doug said the ratio of bad tubes to good might be 100:1.

He also said earlier on to sub out the 200M with 1M and see if the noise improves and if it does, it may very well just be that the tube isn't up to the task.

I tried the 1M resistor. What I got was the expected hpf on the signal, but the noise was still the same and went down to the sub area at a very similar level than before.

 

[davemascera]

Did you ever resolve the pin 7 and 4 grounding problem? As in make sure that they're at the low measurable limit of your ohmmeter with the new cable?

Have you measured the resistance of the head basket to other parts of the mic body itself? Could it be possible that the headbasket or mic body isn't quite as conductive as it should be? Try wrapping the whole thing in tin foil lol.

Try measuring a wire and making sure it reads that same as your probes shorted, attach it to PSU ground and touch the other side straight to the mic. You might get some sort of hum obviously but like it's worth it to see if the noise goes away.

Just some ideas.

EDIT: Also, why not try throwing those RF filter caps back on? Maybe something is heterodyning?