VHF is controlled by the signals
T8 high on Tx (IC11 pin 16)
R8 high on Rx (IC11 pin 17
B8 [144--148[ MHz (IC31 pin 14)
B8W [60--144[ or [148--200] MHz (IC31 pin 13)
L8 [60--200] MHz (IC32 pin 11)
Out of band Tx is impossible due to two reasons.
- Transistor Q38 realizes B8W and (not R8) to short circuit the VHF signal into the HF low pass filter.
- The IC38 TA4001F VHF pre-amp is only activated if B8 and T8 holds.
- Remove the main board from the unit. Keep the cutout hole away from you. This I consider the top of the board.
- Flip the board over its vertical axis and look at the underside. The cutout hole is now on the left. Locate IC32 on the right. There are two IC chips, and IC32 is a bit more to the bottom and to the right. The chip is at 3 cm from the right edge and centered vertically. Pin 1 is the top right pin.
16 15 14 13 12 11 10 9
| IC31 uPD4094BG |
1 2 3 4 |5 6 7 8 | o
o | +B8-/
| | L
| +---------(pin13 ic31)------B8W-----o 3
| o 0
| 8 7 6 5 4 3 2 1
| | |
| | IC32 uPD4094BG |
| | |
+------------------- 9 10 11 12 13 14 15 16
- Above pin 1 you will see two vias. The first via is connected to pin 1, and the second connects to a horizontal trace coming from the left. This trace is B8W and goes to the emitter of the PNP Q38 transistor. Cut the trace just before the via.
--- alternative not recommended ---
- Remove the shield from the top of the board. There are 30-40 solder points from the shield to the board.
- Under the board on the top of the board, remove R-353 and Q-38. They are located to the left of the IC-36 chip on the top of the board under the shield.
--- alternative ends ---
- On the underside of the board, Locate IC-32. There are two IC chips. IC-32 will be just down and to the right of the shield.
- Using a pointer, point at the left upper pin, and go toward the top of the board. You will find a trace that stops. It comes out from under IC-31, pin 14, and stops. It should be the sixth trace up from the top of the Chip. This signal activates the VHF preamp.
- Using a Xacto knife, (or something similar) Cut the trace in the bend halfway between IC-31 and the solder point (via).
- Make a jumper wire, and jump the connection, from Pin 11 of IC-32 to the newly isolated trace. Alternative: solder the wire from the via at pin 1 (bottom left pin) of IC31 and the newly isolated via.
- Reassemble the radio.
Transmit from 200 kHz to 200 MHz continuous (works from 1 MHz, Tx HPF)
ICOM does not warranty these mods.
Parts of this text are from: test at your own risk, have fun and 73 de OE1GQA Guenter - Vienna
Expanded VHF part 2
The VHF IC38 TA4001F 12.5dB preamp has a bandpass input filter from 140 to 150 MHz. If you try to transmit outside of this band you will either transmit nothing or spurious. (e.g. tx=70 MHz lo=140 MHz). See plottxold.pdf.
The four pin IC38 is located on the underside of the main board, just on the right of IC31. (cutout hole top left)
o C330 C329
o L99 L
IC31 o C333 C331 1
Note: L101 is ommitted in the schematic of the service manual. It connects at C334 and goes to ground.
New component values:
C334 82pF C333 1.2pF L99 180nH
L101 180nH L100 100nH C329 3.9pF
C332 10pF C331 2.7pF C330 10pF
Caps in 0603 size, Farnell or Johanson. Inductors in 1008CS size, Coilcraft. The result is given in plottxnew.pdf. The measured filter response was
even better; there was a resonance bump at 120 MHz and 150 MHz.
The power levelling did not work so good outside of the ham bands. This is due to the frequency dependent detection circuit on the PA board. The Tx power was +2dB at 120 MHz and +8.5dB at 165 MHz.
Spurious emission increases at the Tx band edges, 120 and 170 MHz, to about -30 dB. The VCO1 to VCO0 switch, below 120 MHz for my 706, increases the problem since the doubler output power is a bit low at the band edge.