Ramsey Electronics FM25A Assembly And Instruction Manual - page 12
FM25A
• 12
Audio Input Circuits
We encourage you to refer to the schematic diagram and learn the functions of
the following parts. If you wish, insert the parts groupings as one operation,
then solder and nip all connections.
Most of the resistors will be installed in a stand up fashion. That is, one lead will
go straight into the board while the other is bent around to go back down into
the other hole. The straight lead goes into the hole with the circle around it.
1a. Install R16, one of the small yellow trimmer potentiometers marked
"102". Don't confuse it with R6 which is marked "104".
2a. Install R11, the other yellow trimmer marked "102". These two trimmers
will allow you to adjust the input level.
3a. Install R30, a 4.7K ohm resistor (yellow-violet-red).
4a. Install R29, a 47K ohm resistor (yellow-violet-orange).
5a. Install R27, a 47K ohm resistor (yellow-violet-orange).
6a. Install R28, a 4.7K ohm resistor (yellow-violet-red).
For use in Europe, use 2.2K ohm resistors in steps 7a and 8a. These resistors
set the pre-emphasis characteristic for the FM25A. Pre-emphasis is a tech-
nique used in FM transmitters to increase the high frequency signal to noise
ratio.
7a. Install R9, 3.3K (orange-orange-red).
8a. Install R15, 3.3K (orange-orange-red).
9a. Install C14, .022 µF disc capacitor (marked .022 or 223).
10a. Install C22, .022 µF disc capacitor (marked .022 or 223).
Electrolytic capacitors have a right and wrong way to be installed. Usually, ca-
pacitors have a wide black stripe which indicates their negative lead and the PC
board or Parts Layout Diagram will show the positive side of the capacitor's in-
stallation hole. Be sure to place the ( + ) capacitor lead into the PC board ( + )
hole and the ( - ) lead into the ( - ) hole. Observe correct polarity when installing
the following three electrolytic capacitors:
11a. Install C17, 10 µF electrolytic capacitor. Don’t forget... the proper ori-
entation is noted on the PC board or Parts Layout Diagram.
12a. Install C21, 10 µF electrolytic capacitor. Observe correct polarity.
13a. Install C25, 10 µF electrolytic capacitor. Remember correct polarity.
14a. Install C42, a .01µF ceramic capacitor (marked .01, 103 or 10n).
15a. Install C43, a .01µF ceramic capacitor (marked .01, 103, or 10n).