Some TS870 owners report an annoying click in the headphones when going from RX to TX, while others complain of loud TX/RX relay noise. Both of these are only really apparent when using full break-in CW. Unfortunately my TS870 exhibited both of these problems, making QSK operation very unpleasant (as it is on most current rigs). In other respects the QSK is good; it's fast, the transmitted signal isn't clipped, and the sidetone is great. This article describes two modifications I have made which address the above problems, transforming the rigs QSK performance.
The modifications do not entail any track cuts or component changes, and the rig can be restored to its original condition at a later date if required. However, there are no undesirable side effects caused by these mods. You will need good eyesight and a steady hand. It goes without saying that I am not responsible for any damage you may cause while carrying out these mods.
To remove the audible click on RX-TX transition:
The TS870 uses voltage controlled amplifiers (VCA's) to control levels such as AF gain, sidetone etc. These are controlled by a microprocessor via digital to analogue converters. One half of IC29, which is a Mitsubishi M51131L dual VCA, is used to control the AF gain. A voltage of zero to just over five volts is applied to pin 8 of the IC from the DAC via a 100k resistor, corresponding to minimum and maximum volume. When switching from RX to TX the receiver is muted further back in the chain, but the audio is not muted, thus allowing through the switching clicks. By pulling the VCA control voltage close to ground with a FET and a resistor during TX the audio is effectively muted. This does not alter the sidetone or beep levels since these are set with their own VCA's. The TXC line is used to switch the FET, this line goes high on keydown, and stays high until 10ms after key up.
Parts needed:
1 x 10k low wattage resistor
1x 2N7000 or VN10 N channel FET.
20cm fine single core insulated wire (e.g. 'Verowire').
Fine tip low wattage soldering iron (circa 1mm tip).
Fine solder e.g. 0.46mm multicore
Fine solder braid in case of solder bridges.
Note the pin out for either of the above FETs is:
Pin view with flat side on left,
Drain - top pin
Gate - centre pin
Source - bottom (nearest) pin.
Remove power from radio.
Remove top cover then turn radio upside down and remove bottom cover.
With the rig front facing you, remove the large screening plate on the right hand side to reveal the 'TX-RX unit' (large four layer PCB). I found the seven screws in my rig were very tight - you will need the exact fit miniature crosshead screwdriver to avoid damaging the screw heads.
Locate IC29 near back centre of rig, in front of white 2-pin connector CN12.
There are three vias (holes/pads) in the ground plane just in front of IC29. Solder the FET source to the ground point nearest IC29.
Solder one end of the 10k resistor to pin 8. That's the centre pin of the left-hand row, near IC29 label on PCB. (Note the pin numbers run alternately, so pin 14 and pin 1 are at opposite ends of IC29, pin 1 being front right). Solder the other end of the 10k resistor to the FET drain.
Locate 26-pin connector CN7 at front of PCB. Carefully solder a piece of thin single core insulated wire to pin 15 (TXC). Note pin 1 is on right hand side. IMPORTANT: Be very careful not to bridge solder across adjacent pins. They are very small and close together. In case of a bridge, use fine solder wick to remove excess solder - use of a solder sucker may damage the PCB. An alternative connection point to TXC is pin 5 of CN11, left front of PCB. Connect the other end of the wire to the FET gate.
If desired, the effect of the mod. can be heard by disconnecting the 10k resistor from the drain, powering up the rig, then dabbing the 10k resistor lead onto the drain using an insulated probe, e.g. a plastic pen, while keying a string of dots in full QSK with some background noise. Disconnect power and resolder the resistor/drain. Refit screening cover.
To stop the relay noise:
First off make sure menu item 51 is switched to OFF. Positions 1 and 2 in menu 51 engage the linear amplifier switching relay, which makes an awful racket. If using a linear some other means of switching it should be employed, either derived from the keying line or the rigs '12V on TX output'.
The antenna changeover relay K302 is a Matsushita NR-HD-12V reed relay, and is actually extremely quiet in operation, but its switching noise is mechanically amplified and transmitted through the rigs metal casing. By removing the relay from the PCB it can be sonically isolated and is then almost completely silent in operation.
Parts needed:
5cm of 'Extra-Flex' wire, e.g. 128 strand x 0.05mm with silicone rubber insulation, RS 359-807, (or alternative thin flexible insulated wire).
Cotton wool.
Medium wattage soldering iron (circa 3mm tip).
Solder e.g. 0.46mm multicore
Fine solder braid.
Remove the large screening cover, top centre rear, to reveal the PA LPF board and the small antenna relay board at rear. Unsolder both antenna/ground connections. Unplug the internal connectors; one white 3-pin, and three coaxial, noting which is which. Remove the three fixing screws and remove PCB.
Very carefully desolder and remove K302, which is the larger relay on the left of the PCB. This is best done using fine solder wick since the PCB holes are plated through. The other relay, K301, switches the internal ATU in/out of circuit and does not operate when in full break-in mode.
To sonically isolate the relay it is stood off the PCB using five 1cm lengths of thin extra-flex cable. Only the five outer pins on the relay are used, the inner two that were soldered to the groundplane can be left unconnected. The wires are carefully soldered so as to prevent the solder wicking along their lengths, thus preserving the flexibility of the wire. The relay then stands just above the PCB and is not rigidly mounted. Refit the PCB and all connections.
Next, for good measure, place a layer of cotton wool around and on top of the relay. Refit the screening cover.
Replace the outer casing and enjoy! Full break-in is now as it should be - silent and smooth.
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