radardoug
Well-known member
So how much ripple did you see on the scope?
Apex 460 Noise
- Thread starter sethinspain
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Help Support GroupDIY Audio Forum:
bernatvm
Well-known member
I think this is normal ripple for a good PSU
bernatvm said: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
Well-known member
You shouldn't be able to see any ripple.
bernatvm
Well-known member
Even at that scale? Those pics are taken at 10mV/division
bernatvm
Well-known member
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?
Scope measurements are relative to the ground lead, so check where that is connected?
JR
JR
bernatvm
Well-known member
Scope measurements were taken referenced to the chasis of the microphone
radardoug
Well-known member
Pin 2 of what? the microphone cable or the output cable?bernatvm said:
If pin 2 is a ground, then your problem is fixed.
bernatvm
Well-known member
Pin 2 of the tube, sorry. It's signal input from the capsule to the circuit
radardoug
Well-known member
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.
bernatvm
Well-known member
Mylithra said:
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
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bernatvm said:
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.
bernatvm
Well-known member
I was reffering to C9 and C10 on the microphone, filters on the transformer output.
bernatvm
Well-known member
davemascera said:
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
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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?
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?
