The following instructions pertain to the new board marked with the number 051776. There have been several improvements made over the old board. Most noteable is the reduction of receiver birdies and a 12 mA reduction of the power drain. Only 2 receiver birdies remain; 146.000 and 147.100 MHz. The transmitter spurious outputs are at least 5 dB better and the broad band white noise is down much more, The white noise reduction also resulted in improved receiver sensitivity. I still don’t recommend using a high power amplifier with the synthesized HT220 but if you feel the need to do so here is the formula to compute the 2 major spurs so you will at least know where your QRM is,

Upper spur =	F + ((F/9) - 15.5555) for simplex
	F + ((F/9) - 15.4888) for -600 KHz
	F + ((F/9) - 15.6888) for +600 KHz
Lower spur =	F - ((F/9) - 15.5555) for simplex
	F - ((F/9) - 15.4888) for -600 KHz
	F - ((F/9) - 15.6888) for +600 KHz
	where F = operating frequency in MHz
example:	F = 146.520 MHz simplex
	lower spur = 145.796
	upper spur = 147.244

Another major improvement was the elimination of the two CD4011 ICs. Many people found it was hard to find CD4011s that would work right in the original circuit. That is no longer a problem since there are no CD4011s in the new circuit. Some people have ask why I used 9.10222 MHz for the reference frequency instead of 4.551111 MHz so the CD4020 reference divider would not be pushed to it’s limit. The answer is simple.... High accuracy crystals in the HC 18/U holder below 6 MHz are not available.

A word about IC sockets: You can use low profile sockets if you bond the leads that need to be soldered to the top side of the board out to the side so you can solder to them. Be careful that they don’t touch anything they should not.

Good Luck
Dale

1. Use a low wattage soldering iron with a small tip and #24 or smaller gauge solder.
2. Since the holes in the circuit board are not plated thru, you must solder the narts on both the top and bottom of the board where necessary.
3. The order in which the parts are installed is important because some parts overlap others. Install all the feed-thru wires first. Next install the dip-switch on the bottom of the board and solder it in place. Be sure it will be right side up when the board is installed in the radio. Put in all of the transistors next. Q1, 3, 4, 8, 9, 10 and 12 need one or more leads soldered to the top of the board. The remaining parts can be installed in any order but the three ICs should be put in last. IC sockets can NOT be used because some pins need to be soldered on top of the board.
4. The VCO coil is a 2.7 uH inductor. You can wind it yourself if you prefer.
   1. Select 1/2 watt carbon resistor greater than 1 meg-ohm with a straight smooth body.
   2. Cut a shallow notch in each end with a triangle file as illustrated.
      
      
   3. Cut a 19 inch piece of #36 magnet wire and remove 1 inch of insulation from each end.
   4. Wind 40 turns close spaced in a single layer on the resistor body and solder both ends of the coil to the resistor leads.
   5. Coat the finished coil with lacquer to secure the windings.
   6. Install the coil on the circuit board as illustrated below.
   
5. The transmit offset switch is a 3 position rotary switch that replaces the original "PL" switch. Some PL switches can be modified for 3 positions by removing a stop in the switch. Check the top of your switch for a pin which could be moved or removed.
6. The synthesizer can only be installed in the long omni housing with the privateline board and all other accessories removed. The extention sleeve must be modified by cutting out the antenna sheath and all other mounting tabs around the edges. The antenna cannot be fully collapsed when the synthesizer is installed. A rectangular hole must be out in the back for the frequency select switch. These modifications can be done with an Xacto knife and a mototool.
7. Wiring between the radio and the synthesizer should be done with #26 or #28 gauge stranded wire.
8. The parts near the VCO coil should be glued down to prevent vibration from modulating the VCO.

Test and Alignment

Test equipment needed:

• 20 MHz frequency counter
  
• 15 volt 100 mA DC power supply
  
• VTVM

Note: It is better to do preliminary test and alignment before you connect the synthesizer to the radio. It can prevent trouble later on if you locate any "bugs" now.

Reference oscillator adjustment

1. Connect 15 volts to the board. + to point I, - to the ground foil.
2. Connect the frequency counter lead thru a 20 pF capacitor to pin 10 on Z3, the CD4020.
3. Adjust C5 for 9.10222 MHz.

VCO and offset oscillator adjustments

1. Connect the frequency counter lead to point H (xmit drive).
2. Connect the VTVM to the + side of C14 (1 uF).
3. Set S1 (freq. select switch) to 147.000 MHz.
4. If the board is not in the radio, connect point F (ptt) to +15 volts. This puts the synthesizer in transmit mode. If the board is in the radio you must key the transmitter.
5. Connect a temporary jumder wire from point D to point B. This puts the synthesizer in simplex mode.
6. Turn on the power and adjust C7 for a 5 volt reading on the VTVM. If you get the 5 volt reading at this time all of the circuits are working ok.
7. Adjust C3 for 16.33333 MHz VCO frequency.
8. Remove the jumper from point b and connect it to point A. This puts the synthesizer in +600 KHz mode. Adjust C2 for a VCO frequency of 16.4000 MHz.
9. Remove the jumper from point A and connect it to point C. This puts the synthesizer in -600 KHz mode. Adjust C4 for a VCO frequency of 16.26666 MHz,
10. Remove the jumper from point P to + 15, Connect the jumper from point F to ground. This puts the synthesizer in receive mode.
11. Adjust C6 for a 5 volt reading on the VTVM. Adjust C1 for a VCO frequency of 14.4666 MHz.

Alignment is now complete and the synthesizer can now be installed. In the radio, some of the trimmer capacitors may need to be touched up after it is in the radio. Below is a list of the trimmers and their function,

C1   Receive Frequency adjust
C2   +600 KHz offset frequency adjust (transmit)
C3   Simplex frequency adjust (transmit)
C4   -600 KHz frequency adjust (transmit)
C5   Reference oscillator frequency adjust
C6   VCO range adjust (receive)
C7   VCO range adjust (transmit)

NOTE: If you have trouble with any of the test and alignment steps cheek for a 555.555 Hz square wave at pin 3 on Z3, CD4020. Some 4020s won’t work at 9.1022 MHz. If you have an input on pin 10 of this IC but no output on pin 3 you need to try another one.

How to use the binary frequency select switch

This switch consist of 10 spst switchs numbered 1 thru 10. In the following discussion an "ON" switch will be called 1 and an "OFF" switch will be called 0. The frequency is entered directly in BCD code.

Switch #1 is the MHz select switch: 0 = 146.XXX MHz, 1 = 147.XXX MHz

Switches #2, 3, 4, 5 are the 100 KHz select switches. They are BCD coded and can select any digit from 0 to 9.

Switches #6, 7, 8, 9 are the 10 KHz select switches. They are also BCD. coded and can select any digit from 0 to 9.

Switch #10 is the 5 KHz select switch. 0 = 14X.XX0, 1 = 14X.XX5

Note: The following is in courier font so the numbers should align in proper columns.

BCD code for switches 2 thru 9
BCD code, 0 = off, 1 = on

Switch number
2 3 4 5 or
6 7 8 9
-------
0 0 0 0 = 0
0 0 0 1 = 1
0 0 1 0 = 2
0 0 1 1 = 3
0 1 0 0 = 4
0 1 0 1 = 5
0 1 1 0 = 6
0 1 1 1 = 7
1 0 0 0 = 8
1 0 0 1 = 9
-------
8 4 2 1 = binary weight

Examples:

MHz 100 KHz   10 KHz    5 KHz
1   2 3 4 5   6 7 8 9   10  switch number

0   0 0 0 0   0 0 0 0   0 = 146.000
0   0 1 0 1   0 0 1 0   0 = 146.520
0   1 0 0 1   0 1 0 0   0 = 146.940
0   1 0 0 0   0 0 1 0   0 = 146.820
1   0 0 0 0   0 0 0 0   0 = 147.000
1   0 1 1 1   0 0 1 1   1 = 147.735

Caution, when operating above 147.400 MHz be sure the transmit offset switch is in simplex or -600 KHz. If not the transmitter will transmit at 148 MHz or above.

Photo of DIP switch in back of a HT220 (image from a eBay item)

Feedback is desired (suggestions, comments, errors, gripes, whatever) Michael Wright, KG6BFK, [email protected]

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