TS-430s Erratic Receiver Sensitivity


The problem is most evident when switching between certain band
segments. In some cases the received signal strength will jump between
good and poor at the moment, and shortly after, the band change is
done. The culprit is erratic continuity of the relay contacts on
receive in the bandpass filters. There are seven BP filters on the
Filter Unit with the following frequencies in MHz.

1.5 - 2.5, 2.5 - 4.0, 4.0 - 6.5, 6.5 - 10.5, 10.5 - 16.0, 16.0 - 23.0,
23.0 - 30.0

It is not generally well understood that relay contacts and mechanical
switches using metallic contacts are subject to poor contact when there
is little to no current flow through them. The primary cause is
oxidation of the metal. Gold plated contacts rarely have oxidation
problems, but are only found in more expensive equipment. Only a few
milliamperes of current through the contacts will promote self cleaning
and low contact resistance. This condition applies to the TS-430s and
probably to other radios. During receive there is no current flow
through the relay contacts in the signal path between the antenna
connector and the RF Unit PCB. A modification similar to this one may
apply to other radios with similar problems.

The seven bandpass filters are individually selected by relays on the
input and the output of each filter (i.e., two SPDT relays per filter).
One end of the selected filter connects to the ANT connector. The other
end of the filter connects to the T/R relay. During transmit the RF
current is sufficient to keep those relay contacts clean. During
receive there is no current through the other relay contacts to keep
them clean. This modification provides a few milliamperes of current
flow through all of the aforementioned relays during receive. It also
provides current through the relay in the attenuator when it is
selected and during transmit if the ANT connection has no DC path. This
modification is significantly cheaper and less laborious than replacing
the relays.

The modification consists of adding a 2200 Ohm, 0.25 watt metal film or
carbon film resistor. It may also be necessary to add a 4700 Ohm, 2
watt, metal film resistor across the ANT connector if your radio is an
older model. The TS-430s Service Manual lists this resistor under
General as R6. Where the 2200 Ohm resistor is installed depends upon
whether the model is older or later, but production changes I'm not
aware of could alter the location of parts in the instructions. The
instructions below cover known locations.

WARNING

This modification should only be performed by an experienced
technician. The author of this article assumes no responsibility for
any problems incurred due to performing this modification.

To begin the modification, remove the top cover and then the bottom
cover. Be careful to unplug the speaker cable before removing the top
cover fully.

Some radios have a 4700 Ohm, 2W resistor connected between the antenna
connector (SO-239) center pin and ground. You can determine if the
resistor is there with an Ohmmeter. If it is not installed, add it.

Some radios have a 100 uH inductor L69 installed next to connector # 1
on the RF Unit near the attenuator relay RL-1. The inductor is soldered
on the circuit side of the board. In other radios the inductor L41 is
installed on the Filter Unit next to connector # 1 instead of being
installed on the RF Unit. The inductor is mounted on the component side
of the board. The easiest way to determine which board the inductor is
on is to pull out the plug in connector #1 on the RF Unit. Connect an
Ohmmeter between the contacts on connector #1 on the PCB. If the
reading is near zero, the inductor is on that board. If the reading is
near infinite, the inductor is on the Filter Unit. To modify the PCB
with the inductor, follow the appropriate instructions below.


The inductor is on the Filter Unit
----------------------------------------

This modification consists of connecting the 2200 Ohm resistor between
L41 and the +13.8 volt circuit. The inductor and the resistor will be
installed under the circuit side of the Filter Unit PCB. The work area
is near the bottom left corner when oriented as described below.

1. Remove the six screws holding the metal cage that contains the Final Unit and the Filter Unit at the rear of the radio.
2. Orient the radio with the top up and you facing the rear.
3. Carefully pull the assembly away from the radio and separate the Final Unit from the Filter Unit.
4. Disconnect the two coaxial cables from the  Final Unit to the slip fit connectors on the Filter Unit to get better clearance between the two units.
5. Remove the six screws that hold the Filter Unit in its metal cage.
6. Near the edge of the board is connector #1 with two leads: GND & RA. Just behind the connector is D18. Behind D18 is a four pin connector #6. Note that pin 4 is furthest away from D18 and probably has a red lead in the plug side. The circuit side for the +13.8 volt connection has several parts connected to its pad.
7. Carefully pull the Filter Unit out to expose the circuit side of the PCB and the work area.
8. Remove L41 (100uH) mounted vertically just to the right of connector #1.
9. Bend the long lead on L41 to form a right angle with its body.
10. Cut a lead on one end of the resistor to the same length as the short lead on L41 and solder them together. Slip a piece of heatshrink tubing over the resistor to cover the soldered junction and most of the resistor's body.
11. Insert the long lead of L41 through the pad closest to connector #1 from the circuit side of the board. Solder it. The other pad will not be used.
12. Bend the long lead on the resistor to one of the soldered connections for the +13.8 volt line that also connects to connector #6, pin 4 and solder it. Cut off any excess lead.
13. Examine all areas where you have applied the soldering iron for solder bridges and remove as necessary.
14. Reassemble the radio in reverse order of disassembly. Be sure the wire leads going to the Filter Unit are free and clear before installing the six screws in the board. Also remember to plug the two coaxial cables back in their respective connectors.


The inductor is on the RF Unit
-------------------------------------

This modification consists of connecting the 2200 Ohm resistor between
L69 and the 13.8 volt circuit. The work area is the far left corner of
the PCB when oriented as described below.

1. Orient the radio with the bottom up and you facing the rear.
2. Remove all of the cable connectors from the PCB.
3. Remove the five screws holding the PCB. Note that the screw in the middle of the PCB is different than the others.
4. Carefully remove the PCB from the radio.
5. Unsolder the grounded end of L69.
6. Cut a lead on one end of the 2200 Ohm resistor to the same length as the free end of L69 and solder them together.
7. Cut the free end of the 2200 Ohm resistor to about 1/8" long.
8. Strip the end of an insulated hookup wire the same length and solder it to the free end of the resistor. 
9. Slip a piece of heatshrink tubing over the resistor and both solder joints. Shrink it.
10. Find connector # 4, which has five pins. It is to the right of and a bit below the center of the board. Locate pin 4 as the signal denoted as "14". The cable that plugs into this connector has a red wire in the desired location.
11. Dress the hookup wire to reach the proper pin on connector 4. Trim the wire and solder it. If the wire is routed on the circuit side, a few dabs of rubber glue will hold it in place. Alternatively, you can bring it around the edge of the board and route the wire under some existing wires on the component side of the board.
12. Reassemble the radio in reverse order of disassembly.


If anyone should question the voltage being applied to D18 on the Filter Unit, the schematic has an error. The schematic shows two reverse diodes in parallel. Because of this it was expected that a capacitor would be needed to isolate the diodes from DC, so that the diode with the anode connected to the voltage source would not be turned on. The two diodes are not in parallel; they are connected back-to-back in series. This device was tested out of the PCB board and there was no conduction of current up to the 30 Vdc level.

The nominal current through the relays is shown in the table below. It assumes that the 4700 Ohm resistor is installed. Note, however, the column labeled ZERO. This can occur in cases where the antenna, a voltage balun or an antenna tuner provides a DC path to ground. This condition will not damage the radio nor affect its operation. It should be noted that no current will flow through the ATTN relay in this circumstance. If that relay exhibits contact problems during receive, disconnect the coaxial cable at the ANT connector and cycle the ATTN switch several times to clean the contacts.



                                CURRENT PATHS               ANT. CONN.                    ATTN.  SW.
          -------------------------------------------------------------------------------------------------------
            MODE             I1              I2                   4.7K         ZERO                OFF           ON
          -------------------------------------------------------------------------------------------------------
               RX             2.00             0.0                    1                                           1
               RX             6.27             0.0                                       1                                          1
               RX             0.52            5.16                   1                                           1
               RX             6.27             0.0                                       1                                          1
               TX             0.52            5.12                   1                                           X               X
               TX             6.27             0.0                                       1                        X               X
          -------------------------------------------------------------------------------------------------------

NOTES:

1. The calculated currents using the 2.2K resistor are shown in mA.
2. Path I1 from the point where the inductor connects to the signal line and the ANT. CONN.
3. Path I2 from the point where the inductor connects to the signal line and the ATTN. relay on the RF Unit.
4. The 1's denote the active condition.
5. The X's denote a Don't Care condition.


Modification submitted by W0MAY