TS-850 Deaf and low RF output

Low receive sensitivity and low, or no RF power output as the frequency increases can often be attributed to a fault on the RF board within the Kenwood TS-850S. Quite often the problem is more noticeable at frequencies of 14MHz and above, usually with the RF output power dropping to only a watt at 30MHz.

Where’s the problem?

The problem usually lies on the RF unit (X44-3120-00) at IC1 which controls the switching for the band pass filters. Quite often an output pin goes low thus permanently switching on the incorrect band pass filter. When the filter is selected, received frequencies higher than the band pass filter are severely attenuated while the transmitter circuit also reduces the RF power to prevent damage to the output transistors.

To access the RF board you need to remove the bottom cover of the TS-850. With the radio upside down, the RF board is the one that has the silver screening plate at the rear of the radio. Remove the screening plate by undoing the four screws and carefully sliding it towards the front of the radio by a few mm so the copper earthing spring is away from the main chassis. Simply lift the shield away ensuring you don’t accidentally cut into any of the wires as the shield can have some quite sharp edges.

At this point you need to take note of where all the mini-coax connections are. It may be wise to take a few good quality close-up photos so you know where they go when you re-assemble the radio. Although the PCB does have markings on it to indicate what colour cable goes where, there are a couple positioned close to each other, so MAKE SURE you know exactly where they go! Likewise, you need to take note of the multi-connectors. Once you taken note, carefully unplug them. You will find that there are two mini-coax cables that form jumpers on the RF board and these need not be removed.

Remove the screws securing the RF unit to the chassis and lift the unit clear.

The RF unit

TS-850 RF BoardThe problem IC can be seen in this picture. If you look carefully, you will see that pin 2 has been cut and bent upwards. Pin 2 was disconnected to verify the fault in this case. It is possible that any of the outputs may be faulty, this is characterised by the output pin in question remaining at (or close to) zero volts when cycling through the bands.
IC1 removed from rf unitThis IC needs to be desoldered and removed, note that the PCB contains plated-through holes and you must ensure that the IC pins are absolutely free of solder before lifting it out the holes and free of the board.
Place the new IC in position and solder it to the PCB.

IC1 replacedI recommend that you use some extra flux paste when soldering the IC into place to allow a small drop of solder to run through the PCB holes, thus ensuring connectivity to both sides of the board.

Re-assembly is a reverse of removal taking note to not pinch or damage any wires. Once the RF unit is back in place, apply power to the radio and using a dummy load, test for full RF output power across all the bands. If this is successful, swap the dummy load for an antenna and check that the receiver is working correctly.
If everything is good, replace the cover, and return the radio to your working position and enjoy your Kenwood TS-850 to it’s full capacity once again.

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