Construction

Schematics

Here are all the schematic diagrams, starting with a block diagram that shows how the different schematics fit together. For a description of what each block does, refer back to the theory page.

Master Block Diagram
Audio Input Filter
Audio Input Buffer, Pre-Emphasis, and Matrix
10 KHz Lowpass Filter
3-Way Crossover
Bass limiter
Midrange limiter
Treble limiter
Limiter Voltage Reference
Audio Combiners/L+R Clipper
Oscillator, Divider, and I/Q Modulators
Limiter, Envelope Modulator, and Output Stages
Power Supply: Original Design or Supply with AC Line Filter
Pilot Tone Generator
Overmodulation alarm circuit

Circuit Board Layout and Parts Placement

Circuit Board, PostScript format.
Circuit Board, Adobe Acrobat (.pdf) format.
Parts Placement, Post Script format.
Parts Placement, Adobe Acrobat (.pdf) format.
Component Values, Post Script format.
Component Values, Adobe Acrobat (.pdf) format.

Many techniques are available for creating circuit board layouts. I transferred the layout to copper with techniks Press 'N Peel Blue film. The layout is laser-printed or photocopied onto the film. An ordinary clothes iron is used to transfer the design to a clean copper-clad circuit board. The hot iron causes the laser printer or copier toner to stick to the circuit board. The Press 'N Peel film is then carefully peeled off, leaving the toner patterns on the copper clad board. Blemishes can be touched up with a permanent marker at this point. The board is then etched in ferric chloride. When printing the designs, do a test print on paper first. Inspect this printout to make sure that the laser printer is not cropping the edges of the layout (this can happen with some models) and that the overall print quality is adequate. If the printer is not producing dark, well-defined traces (perhaps due to a nearly empty toner cartridge), do not proceed. Likewise, if the test printout has toner that shouldn't be there, do not proceed. If the printer will not print the entire layout on a letter-sized sheet, then the layout can be printed out on a larger sized sheet of paper and transferred to the Press 'N Peel film using a photocopier.

Three jumpers need to be tack-soldered to the circuit board. All of these are associated with the overmodulation alarm circuit. Connect jumper wires as follows:

From LM339 pin 10 to pin 2 of the Envelope Modulator NE602
From LM339 pin 11 to pin 1 of the Envelope Modulator NE602
From LM339 pin 8 to any point on the 5 Volt supply rail.

Also, note that the string of seven diodes in the clipper circuit is tack soldered between pins 1 and 2 of the TL072 operational amplifier IC in the combiner circuit. This is located on the lower left corner of the board (when looking at the component side of the board with the threshold potentiometers facing down).

Locating Parts

When putting together a shopping list, be sure to get the right values of capacitors and coils for the harmonic filters (both at the limiter input and at the envelope modulator output). These values depend on the operating frequency. For operation in the low portion of the band (530KHz to 800KHz), use the values marked "Low." In the middle of the band (800KHz to 1150KHz), use the "Mid" values, and at the high end of the band (1150KHz to 1710KHz), use the "High" values. With the circuit board layout provided, changing the components in these filters requires unsoldering. If you anticipate having to make frequent drastic changes in operating frequency, such as from 1610KHz down to 590KHz, you may wish to put the filters on separate boards and piggyback the filter boards to the main board. The appropriate filters could then be jumper-selected, on the filter board, or, with some re-design of the main board, the filter boards could be plug-in modules that can be quickly swapped when necessary.

A detailed parts list, complete with catalog numbers, is still under development.

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