Kenwood TL-911 Linear Amplifier Service Manual

Features

The Kenwood TL-911 is a linear amplifier offering ample 2KW PEP input for high power operation in SSB. It features a class AB grounded-grid configuration for stable, distortion-free SSB transmission, with a linear switch for CW (A1) operation. An overcurrent detection circuit prevents heat trouble from abnormal operations. Individual plate currents of the power amplifier tubes can be monitored. A dynamic balance circuit ensures uniform power sharing among the tubes for full efficiency and safety. The ALC circuit is dual-type for improved linearity. An SWR meter is included for quick antenna matching. The unit is designed as a desk-top type, compatible with the TS-515 transceiver. Switching between exciter and linear amplifier operations is simple via an antenna-through relay.

External View

This section describes the external controls and connections of the TL-911. The front panel features a meter, various knobs (e.g., for tuning, band selection), toggle switches (e.g., for power, linear operation), and indicators. The rear panel includes connectors such as a receptacle type M, a US socket, a cord bushing, and a fuse holder.

Top Chassis View

This view illustrates the components located on the top chassis. Key components identified include vacuum tubes (V1-V5, identified as 6LQ6, and V6 as 6CW5), variable resistors (VR1-VR8), capacitors (C27-C44), transformer T1, relay RL1, switch S1, variable capacitor VC1, and a cooling fan.

Bottom Chassis View

This view details the components on the bottom chassis. Visible components include variable resistors (VR1-VR5), a cooling fan, relay RL2, capacitors (C27-C29, C43-C44), a US socket, a fuse holder, and the rectifier unit (X43-1010-00). Switches S2-S5 are also indicated.

Circuit Description

Outline

The TL-911 linear amplifier circuit is a hybrid type, incorporating 6 vacuum tubes, 3 transistors, 27 diodes, and 1 thyristor. The unit is designed with distinct sections for different functions, allowing for shielded final amplifier sections and independently mounted rectifier and control units.

Final Power Amplifier Section

This section houses five 6LQ6 tubes within a shielded box, cooled by a powerful fan. The tubes operate in Class AB for SSB and Class C for CW in a grounded-grid configuration. The LINEAR ON switch manages bias switching. A regulated power supply ensures stable screen grid voltage, minimizing distortion. The section also includes a dynamic balance circuit for uniform power tube characteristics, a dual ALC circuit for improved linearity, and an SWR meter circuit.

Power Supply Section

The power supply utilizes a large transformer with ample current capacity. Rectification is performed by a rectifier unit (X43-1010-00), featuring a bridge rectifier composed of 8 diodes. Voltages for tube plates, screen grids, bias circuits, and relays are supplied by specific diode configurations.

Operation of Protection Circuit

The protection circuit monitors cathode currents via resistors (R1, R3, R5, R7, R9). It is designed to activate if a cathode current of 240 mA persists for 20 seconds or more, or if it exceeds 400 mA instantaneously. Upon activation, a thyristor triggers a relay (RL1), which in turn de-energizes another relay (RL2). RL2 then applies a high negative voltage to the screen and control grids of the power tubes, cutting them off, and lights a protection lamp. The antenna output is automatically switched to the exciter. The circuit can be reset by shifting the exciter stand-by switch to receive mode or turning the LINEAR ON switch off.

Figure 1 illustrates the Block Diagram of the amplifier. Figure 2 details the Operation of the Protection Circuit, showing the sequence of events. Figure 3 presents the Protector Characteristics, plotting cathode voltage against power tube current.

Troubleshooting

Connecting Remote Cable

This section provides diagrams illustrating how to connect the remote cable between the TL-911 and various transceivers, such as the TS-515 and TS-510, and also for the TX-599. It details pin connections for different functions like ALC, VFO REMOTE, and AF output.

Adjustment

Required Measuring Instruments

• Voltmeter (1MΩ input resistance, 1.5-1000V range)
• Stabilized supply (2V and above, capable of accurate voltages including 2.4V, 4.0V, 7.5V; 1.5A or greater)
• Power meter (50 or 75Ω impedance, 500W or greater)
• 100 or 150Ω dummy resistor (non-inductive, 50W) for SWR calibration
• Precision type voltmeter (Class 0.5 with 2.4V and 4.0V graticules)
• Exciter (3.5-28 MHz, SSB and CW)

Adjustment Procedures

1. Screen Grid Voltage Esg (V): Adjust VR5 (rear panel) to obtain DC 70V at pin 1 or 7 of tubes V1-V5, with exciter in transmit mode and linear-on switch set to SSB.
2. Base Current Ip (static balance): Adjust VR1-VR5 (10kΩ) on the control unit (X53-1000-00) to achieve 15 mA per tube for each individual switch position (V1 through V5), with exciter in SSB transmit mode.
3. Plate Current with Input Applied (dynamic balance): Adjust TC1-TC5 (rear chassis), VC1 (PLATE), and VC2 (LOAD) on the panel. With exciter in CW transmit mode, adjust VC1 and VC2 for maximum output at 7.15 MHz. Then, shift the Ip switch (V1-V5) and adjust TC1-TC5 so that each tube draws approximately 240 mA. Repeat adjustments as necessary.
4. Protection Circuit (I.P.L. instantaneous protection level adjustment): Adjust VR6 (I.P.L. SET) on the control unit. Disconnect remote cable and shunt pins 4 and 5 of the remote socket. With the linear amp power switch on and linear-on switch off, adjust VR6 until the circuit trips instantaneously when a 4V voltage is applied across the cathode resistor. The protector should trip within 0.5 seconds for 4.0V and 5 seconds for 3.8V.
5. Protection Time (PROTECT TIME): Adjust VR4 (100kΩ) on the rear. Apply 2.4V across the cathode resistor and adjust VR4 so the protector trips 20 seconds after voltage application. (2.4V corresponds to 240mA cathode current).
6. Ip Meter Calibration:
   • V1 through V5: Adjust VR7 (10kΩ) on the control unit until the Ip meter reads 240mA when 2.4V is applied across the cathode resistor.
   • IPA: Apply 7.5V to terminal 16 of the rectifier unit (X43-1010-00) and adjust VR8 (10kΩ) until the meter reads full scale (7.5V corresponds to 1.5A plate current).
7. SWR Meter Calibration: This involves multiple steps using 50Ω and 100Ω dummy loads, adjusting VR1, VR2, and the SWR SEN knob to ensure accurate readings in FWD and REF modes.

Voltage Measurements of Tubes and Transistors

Vacuum Tubes

Transistors

Parts List

Parts List of TL-911

Parts List of Control Unit X53-1000-00

Parts List of Rectifier Unit X43-1010-00

PC Board Diagrams

This section includes diagrams illustrating the physical layout of components on the PC boards for the Control Unit (X53-1000-00) and the Rectifier Unit (X43-1010-00). These diagrams show the placement and connections of various resistors, capacitors, transistors, and diodes.

Specifications

Color Code

Resistor Color Code

Capacitor (Mica) Color Code

Unit = pF. Capacitance being less than 10pF is ±0.5pF on tolerance.

Schematic Diagram

This section contains the main schematic diagram of the Kenwood TL-911 Linear Amplifier, detailing the interconnections of all components, including tubes, transistors, resistors, capacitors, inductors, switches, relays, and power supply elements. It provides a comprehensive overview of the circuit's design.