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Replacing Electret Microphone With Audio Input?

D

Dave.H

Jan 1, 1970
0
You see the vertical resistor just on the input where the LR channels
come together? Put a 10K pot in place of it, with the junction of the
LR 10K's to the full clockwise position and ground on the CCW
position. Change the position of the 100 nf to the wiper of the pot
so it connects there and nowhere else.

--

OK thanks, I assume it doesn't matter if I use a linear taper or audio
taper? I want to use a linear taper.
 
D

Dave.H

Jan 1, 1970
0
OK thanks, I assume it doesn't matter if I use a linear taper or audio
taper? I want to use a linear taper.

I've got a 10K audio taper pot. Will that also work.
 
D

default

Jan 1, 1970
0
OK thanks, I assume it doesn't matter if I use a linear taper or audio
taper? I want to use a linear taper.

It doesn't matter to me . . . Seriously: If you anticipate using it
frequently to adjust volume you might be happier with an audio taper
pot. If you set it once and forget about it, it hardly matters.

The question is how constant is the volume from the 'pod? If you
think the control on your receiver is good enough to compensate use a
linear.

It doesn't matter one way or the other - it just "feels" right when
you use an audio pot for audio signals. I put a linear pot into a
stereo I built at my office/shop where I was working. It had 50 W a
channel and was too much for the little office but everyone who used
the control said it "didn't have enough volume." Never bothered me
though.
--
 
D

Dave.H

Jan 1, 1970
0
OK. I hope to get this transmitter up and running today. I will post
here with the results.

Built it and rebuilt it and it didn't work. I installed the 12 pF
instead of the trimmer cap, even took it apart and resoldered all
connections. Any suggestions why it doesn't work?
 
D

default

Jan 1, 1970
0
Built it and rebuilt it and it didn't work. I installed the 12 pF
instead of the trimmer cap, even took it apart and resoldered all
connections. Any suggestions why it doesn't work?


What method did you use to build it? It is a 100 MHZ circuit so
layout can be critical -

The next question would be what is the transistor doing? What is the
voltage from base to ground? What is the voltage from ground to the
emitter? (and if it were oscillating, taking a reading there would
stop it).

Can you / have you measured current consumption of the circuit?

What steps have you taken to verify it isn't working and what steps
have you taken to fix it other than checking your connections?
--
 
D

default

Jan 1, 1970
0
Built it and rebuilt it and it didn't work. I installed the 12 pF
instead of the trimmer cap, even took it apart and resoldered all
connections. Any suggestions why it doesn't work?

Two things come to mind assuming no mistakes in the construction.

I would disconnect everything west of the vertical 10K that joins with
the base of the transistor - it isn't necessary for oscillation.

Then I would replace the transistor with another type - I've always
had better results with the metal can version of the 2N2222 for FM
mics but change the transistor if you have another NPN good for high
frequency.

The other thing, assuming no oscillation, is the feedback cap. It
shouldn't be very critical but it can go a long way to getting a
transistor that won't oscillate to start. Too high and it won't work
and too low and it won't work . . .

Try a gimmick cap in there. If you have some wire wrap wire or cat 5
USTP those are probably ideal but any solid wire will work. Magnet
wire is another good choice. Vinyl insulation isn't ideal at that
freq so if you can get it to work with vinyl, change it for a lower
loss dielectric to get maximum range.

http://www.shol.com/kuggie/cttip/pix/gimmic.jpg

Two wires twisted together, but not connected, form a "gimmick" cap.

No magic formula for value of the cap - thinner insulation = greater
capacity, longer gimmick = greater capacity, dielectric constant of
the insulation determines capacity also, dissipation factor of the
insulation = Q (quality).

good insulations - Teflon, polyethylene, Tefzel, Delrin, enamel (low
dissipation factor)

Without modulation - it can be hard to hear the transmitter. It will
sound like a Thump when you tune through it on the receiver and it
should be able to kill the sound of a weak FM station when it is
tuned to the same freq.. On an analog TV with antenna, it will look
like wavy lines or a herringbone pattern.
--
 
D

Dave.H

Jan 1, 1970
0
Two things come to mind assuming no mistakes in the construction.

I would disconnect everything west of the vertical 10K that joins with
the base of the transistor - it isn't necessary for oscillation.

Then I would replace the transistor with another type - I've always
had better results with the metal can version of the 2N2222 for FM
mics but change the transistor if you have another NPN good for high
frequency.

The other thing, assuming no oscillation, is the feedback cap. It
shouldn't be very critical but it can go a long way to getting a
transistor that won't oscillate to start. Too high and it won't work
and too low and it won't work . . .

Try a gimmick cap in there. If you have some wire wrap wire or cat 5
USTP those are probably ideal but any solid wire will work. Magnet
wire is another good choice. Vinyl insulation isn't ideal at that
freq so if you can get it to work with vinyl, change it for a lower
loss dielectric to get maximum range.

http://www.shol.com/kuggie/cttip/pix/gimmic.jpg

Two wires twisted together, but not connected, form a "gimmick" cap.

No magic formula for value of the cap - thinner insulation = greater
capacity, longer gimmick = greater capacity, dielectric constant of
the insulation determines capacity also, dissipation factor of the
insulation = Q (quality).

good insulations - Teflon, polyethylene, Tefzel, Delrin, enamel (low
dissipation factor)

Without modulation - it can be hard to hear the transmitter. It will
sound like a Thump when you tune through it on the receiver and it
should be able to kill the sound of a weak FM station when it is
tuned to the same freq.. On an analog TV with antenna, it will look
like wavy lines or a herringbone pattern.
--

I have spaced the components too far apart, that could be the
problem. The only thing I've done other than rebuild it is check for
a signal on an FM radio, I will perform other tests later, I have
learnt to leave things like this for a few days then come back to
them.
 
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