Re: Microphone level adjustment question - Update

k9jri@att.net <k9jri@...>
 

I was surprised that nobody appears to be using a scope to set the drive level but that is not unlike the old days :) It appeared as though I might have both overdrive and when overdriven some stability issues in my kit.

So:

Using G8OWB's documentation for PE1BWI's stability mod I added the 220 ohm and the 1,000 ohm resistors. Most of the harshness that I was hearing on the local receiver was gone but there were still strong hints of overdrive.

At this time I set all of the bias pots and the drive pot to their CCW position and performed the necessary alignment adjustments. I was careful to not bring up the drive pot (R83) to far.

Although my Bird43P will not read individual voice peaks it was registering about 8 watts PEP on a steady Ahhhhhh. While all of the roughness in the audio now seems to be gone I will have a good 400 MHz. scope available tomorrow and will take a look to see if it is still flat topping. I suspect that it is.

I will follow up with the results of the scope tests and let you know what the real PEP is on normal voice peaks. Not a lot different than adjusting a Central ELectronics 10B with no test equipment.

I received one comment about RF getting into the electret microphone elements. That comment should have be in a LARGE BOLD font as it is certainly the case with the BitX20A that I have :)

73 - Mike - K9JRI

--- In BITX20@..., "k9jri@..." <k9jri@...> wrote:

When I listen to my Hendricks BitX20A on a local receiver I hear some really rough audio at times. If I use my electret mic it is worse than if I use an old 600 ohm dynamic mic with a DC blocking cap.

Back in the good old days, before ALC, I would use a scope to look at the RF envelope for flat topping but I no longer have a scope. Is there any simple way to detect flat topping other than to look at the output with a scope?

My concern is that if I am NOT flat topping the finals badly then it may be a sign of the instability problem pointed out by Martien (PE1BWI) and I should apply his stability mods.

I would appreciate any input you might have on the proper way to adjust the audio input levels without a scope AND and comments on the likelihood that the is the instability that PE1BWI was talking about.

Oh yes.... the local receiver was not being overloaded. Both the local receiver and the BitX20A were connected to 50 ohm dummy loads and I was listening with headphones to prevent any feedback possibilities.

73 - Mike - K9JRI

Join BITX20@groups.io to automatically receive all group messages.