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TS711/TS811 CW PTT Modification

When using the TS711/811 in CW mode the break-in behavior can’t be switched off, i.e. as soon as the KEY-line/input is active (ground) the radio will go into transmit and RF power is generated. Normally when using an external poweramplifier or preamplifier some sort of sequencer/timing device is used which will control the sequence when going from transmit to receive and vice versa. An example of such a constellation is given in the figure below:


Fig 1: Sequencer controlling radio, power amplifier and RF switches

With an unmodified TS711/811 this is not possible. Even worse is the behavior when no key plug is attached to the key input  jack at the back, radio is set to CW mode and PTT is engaged: Immediately a full power CW carrier is generated!. 


For both issues (CW carrier when no key attached and CW break-in not able to deactivated) a modification is available.

CW carrier when no key attached modification

When no KEY is inserted in the key input jack, signal “STS” is floating and the KEY line will be pulled low via Q12 (IF unit) and a carrier is generated when PTT is engaged.

Control Unit

Fig 2: IF Unit X48-1400

However, when a key is inserted in the key input jack  “STS” is connected to “CWB”. CWB is held at 8V when CW mode is selected. This will prevent Q12 from pulling the “key” signal low and no carrier is generated when PTT is engaged.

Display Unit

Fig 3: Display Unit X54-1820

Modification is to remove two wires attached to the key input jack assembly and connect them together directly, so “STS” will always be held at 8V when CW mode is selected.

Detailed steps:

Display Unit

Fig 4: Connecting wires from display unit

De-activate break-in

As soon as CW key line”KEY” is activated a CW carrier will be generated because transistor Q16 will be pulling line “SS” low. Signal “SS” is the main PTT line which will put the transceiver in transmit. Among others “SS” is  directly connected to the microphone PTT and pin 13 of ACC2 (PTT).

AF unit

Fig 5: AF unit X49-1180

 Modification is to force the base of Q16 to ground. This will hold the radio in receive mode even when the CW key is activated, only when the PTT is activated (via the handheld microphone, or via the external PTT input on accessory connector ACC2 pin 13).  Point to note is that the sidetone is generated, irrespective the state of the PTT line input.

Detailed steps:

Short Circuit R

Fig 6: Short circuit resistor R88

Use after modification

After the modifications a sequencer can be utilized to control the radio in both SSB and CW modes.  With reference to figure 1 how to connect things up: