Yaesu FT290R Some small mods

AUDIO DECOUPLING

It seems important to fit a 0.1uF cap into the receive audio line to the TNC. Fit it inside the five pin plug. The FT290R has no decoupling capacitor in the audio output (See wiring Diag. IC No.Q1027 (uPC575C2) off pin 7.

CAP CRUNCH

Many people have problems getting the Mk1 working on packet. This is due to the receiver filtering attenuating frequencies above 2kHz, thus the 2.2kHz tone is attenuated by 10-15dB. The standard capacitor crunch mod, originally designed to give more +ACI-sparkle+ACI- to the audio cures this. To do the mod, remove the case and turn the rig so the control knobs face away and the SO239 antenna socket is nearest you. At the back of the PCB is a relay (a box shaped component made of semi-clear plastic). Just above the relay are two green capacitors.
Using small pliers, grip the capacitor nearest the relay (C110) and crush it, removing all the debris.

DE-EMPHASIS - 1

There is a very +ACI-hard+ACI- de-emphasis in FT290R. This can cause serious problems copying packet signals with some demodulator chips, e.g. like the XR2211. This modification increases the intelligibility in noisy environments, and is also useful in FM voice communications.
Add a serial RC unit, R +AD0- 4.7k, C +AD0- 1uF tantalum capacitor between pin 9 of IC Q1019 and the collector of transistor Q1021. The positive leg of the capacitor goes to the transistor collector. These can be found as test points near to IC Q1019. Solder the components together, insulate them, then solder to the test points mentioned.
With this modification, FM reception will sound somewhat noisy, but louder.

DE-EMPHASIS - 2

Yaesu seem to have goofed on both the FM de-emphasis and the audio filter. A computer analysis of the audio circuit suggests:

1. Reduce C125 from 10n to 4.7n in filter
2. Reduce C124 from 2.2n to 1.5n (near Q1026)

This isn't perfect but it a lot better - the tones now seem nearly equal. There appears to be plain FM (cf phase) with no pre-emphasis on Tx+ACE-

RESET TO 145MHz

This mod resets the CPU to 145MHz and sounds a bleep to tell you it's been done. Remove top and bottom covers, locate the hole in the case near the SO239 socket covered by a stick-on aluminium label. Make a hole in the label and install a push to make, non-latching switch. Connect one side of the switch to ground and the other to pin 4 of J5003 which connects the keyboard and control units. A 1nF capacitor should also be connected from pin 4 to ground for decoupling. Replace the covers, dial up any frequency, press the button and the set should reset to 145MHz and bleep.

LISTEN ON INPUT

Place the set with controls towards you, remove the bottom cover and locate the black/white wire on SK1. It's the tenth one from the left.
Cut this wire at the socket and insulate the free end. Locate the green/white wire on SK1 and solder the anode of a 1N4148 diode to it.
Connect the cathode of the diode to the red/white wire on the PCB adjacent to the mic. socket. Insulate the diode leads to prevent shorts. When the call button is pressed, it allows listen on input on both +-shift and -shift. When you want the rig to transmit a tone burst you must close the PTT at the same time as pressing the call button.

CURING SYNTHESISER WHINE

Early 290s suffered from a high level of whine. Try moving the wiring harnesses around inside the rig whilst monitoring the whine on another rig. The real cure, if you have a delicate hand for soldering, is locate LO6 in the synthesiser and solder a 15pF capacitor in parallel with it. This can usually be soldered between the hot ends of C24 +ACY- C25 which are at the rear of the IC in the synthesiser unit.

MODIFIED SQUELCH

This mod reduces the difference in level between the opening and closing points and speeds up the operation of the squelch enabling the scanner to work better. Reduce C94 from 4.7uF to 1uF, increase R79 from 270K to 330K and add a 10uF tantalum or subminiature electrolytic between the leg of R78 (4.7K) and the body of the transistor T1007.

OPENING UP TO 148MHz

Remove the battery compartment and slide out the power board to allow access to the microprocessor board. Two tinned copper wire jumpers are visible on this board, next to the microprocessor. One is easy to see and the other is not because it is next to the main board.
Cut the jumper which is difficult to see. If the wrong jumper is cut then the frequency display will not be correct when the unit is powered up.



GW1NGL NA7KR Kevin Roberts Ham Radio
GW1NGL NA7KR Kevin Roberts Ham Radio

Page last updated on 09/10/2012 by Kevin Roberts NA7KR a colection of Ham Radio and Electronic Information