Electronic T/R Switching and the Ameritron QSK-5
by Greg Latta AA8V

Circuit Board and Parts Identification

Click On A Section of the Photo Below for Information on That Part of the QSK-5 Circuit Board:

Ameritron QSK-5 Circuit Board

Safety Note: Working on or testing the QSK-5 and associated circuitry may involve operating the amplifier containing the QSK-5 with the cover off. This is extremely dangerous since very high voltages are present when the amplifier is turned on, sometimes in close proximity to the QSK-5. If at all possible, do all work with the amplifier off and unplugged. The operator assumes all risk and liability in such matters! Never operate the amplifier with the cover off unless you are experienced with working around very high voltages!

Circuit Board and Parts Identification

QSK-5 Circuit Board Overall View: Electronic high power T/R switching is a complicated business, as evidenced by the complexity of the QSK-5 circuit board. The notated photo above points out the important sections of the circuit board. You can click on the photo at right or the link below it for enlarged views of the circuit board.

Click on the image for a larger view. Click here for a super detailed view.

Fuse Lamps: The fuse lamps are located on the top of the circuit board to the left. These light if the current through the receive diodes is excessive. When the bulbs light up, their resistance increases, protecting the expensive diodes. If the current becomes too much, the lamps burn out, just like a fuse. If this happens, you cannot receive while using the QSK-5. Do not bypass the bulbs or try to use a fuse in their place. Unlike a fuse, the resistance of the bulb increases as the bulb lights up, further protecting the receive diodes. The lamps are replaced by unsoldering them and installing new ones. Any 50 mA 12V or 15V bulb can be used. You can purchase replacement lamps from Ameritron or find them at Radio Shack, if you are lucky. It pays to keep a couple of extra bulbs on hand.

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Receive Diodes: The receive PIN diodes used in the QSK-5 are M/A-COM type MA4PH301. These are the small blue units in the close up photograph at right. As you can see in the photo, the folks at Ameritron decided to double the number of diodes used in the PIN diode receive switch by replacing each original diode with two diodes in series, with a 1 Mohm equalizing resistor across each one. This gives a total of eight diodes is series for the PIN diode receive switch For more information on how the receive switch functions, you can go to the How PIN Diodes and PIN Diode Switches Work page. For all the information you ever wanted to know about the MA4PH301 diode, click here for the MA4PH301 data sheet from M/A-COM. This data sheet is in .PDF format, so you must have Adobe Reader installed to read it.

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Transmit Diode - Amplifier Input Switch: A single M/A-COM type MA4P4001B PIN diode is used in the QSK-5 amplifier input switch. This is the same type of diode used in the amplifier output switch below. The diode is mounted underneath the large RF choke on the top left side of the circuit board shown at right. (The diode is not visible in the photo and is easy to miss.) A 330 kohm equalizing resistor is connected across the diode, and, for DC, this diode is connected in series with the amplifier output diodes, and is controlled by the transmit switch driver. For more information on how the amplifier input switch functions, you can go to the How PIN Diodes and PIN Diode Switches Work page. For all the information you ever wanted to know about the MA4P4001B diode, click here for the MA4P4001B data sheet from M/A-COM. This data sheet is in .PDF format, so you must have Adobe Reader installed to read it.

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Transmit Diodes - Amplifier Output Switch: The PIN diodes used in the QSK-5 amplifier output switch are M/A-COM type MA4P4001B. These are the large blue units in the close up photograph at right. A hole in the circuit board below each diode allows cooling air to reach the diode. Four diodes are used for the amplifier output switch. The diodes are wired in parallel for RF and in series for DC. A 1 Mohm equalizing resistor is placed across each diode. These are visible through the cooling holes in the circuit board. For more information on how the amplifier output switch functions, you can go to the How PIN Diodes and PIN Diode Switches Work page. For more information on the MA4P4001B diode, click here for the MA4P4001B data sheet from M/A-COM. This data sheet is in .PDF format, so you must have Adobe Reader installed to read it.

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Switching Transistors: The PIN diode switch driver is located on the circuit board where the high voltage lead connects to the circuit board at point E. The driver utilizes four 2N6740 high voltage power transistors to turn the PIN diode switches off and on. To operate properly, the switch driver needs about 600V DC in addition to the usual 12V DC that powers the rest of the QSK-5. The 600V connects to the QSK-5 at point E on the circuit board, in the middle of the four 2N6740 transistors. For more information on the operation of the switching transistors, go to the PIN Switch Driver page. For more information on the 2N6740 transistors, click here for the 2N6740 data sheet. This data sheet is in .PDF format, so you must have Adobe Reader installed to read it.

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T/R Input: The T/R input circuit is located at the bottom of the board by pin D. The 1N4007 diode connected to pin D is to protect the QSK-5 in case a signal of the wrong polarity is connected to the T/R input of the amplifier. If connected to the wrong polarity, the diode simply prevents any current from flowing through it into the QSK-5. Q5 is the input switching transistor and is barely visible in the photo under the red high voltage wire. To learn more about how the T/R input circuit functions, you can go to the T/R Input and Timing Circuits page..

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Timing and Handshake Circuits: The timing and handshake circuits are located at the bottom right hand corner of the QSK-5 circuit board. (On my circuit board I have mounted a QSK indicator circuit on a small piece of vectorboard on top of the LM-324 op-amp, so the op-amp is not visible). Q6, the QSK loop return driver, can be seen at the bottom of the photo. The power tab transistor is Q7, an FET transistor used to control the bias on some amplifiers. For more information on how the handshake circuit operates, go to the Handshake Circuit page. You can learn more about how the timing circuits work by going to the T/R Input and Timing Circuits page. The QSK indicator circuit on the small vectorboard is my design and lights an LED on the amplifier front panel whenever the QSK-5 is placed in transmit mode. You can read about the indicator circuit by clicking here.

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If you have any questions or comments, you can send E-Mail to Dr. Greg Latta at glatta@frostburg.edu