The 1Watter QRPp Transceiver
Service Manual

Last Updated Tue, 13 Jul 2021 17:10:46 -0400

 Japanese Translation of Service Manual by Takayama Shigekazu "Shig" JA1XRQ
 Spanish Translation of Service Manual by Rudy Lopez CX7RL
Block Diagram
Voltage Controlled Crystal Oscillator (VCXO)
Receiver Section
    Low Pass Filter
    First Mixer
    Crystal Filter
    Frequency Response (by K3ASW)
    Audio Detector Mixer (BFO) with AGC
    Audio Amplifier with Mute
Transmitter Section
    Transmit Carrier Oscillator Mixer
    Transmit Band Pass Filter and Preamps
    Transmit Driver
    Transmit Final Amplifier
Keyer and QSK Operation
Printed Circuit Board


The 1Watter is a CW Transceiver designed for one watt RF output on any band from 160 to 10 meters.
It was designed to be inexpensive yet perform as well as most commercial transceivers but on limited frequencies.
The printed circuit board size is small, only 2.5" (63,5 mm) by 3.8" (96,5 mm)
It employs full QSK switching and drives a 4 or 8 ohm external speaker.
All 1Watters with a serial number greater than 400 use the same PCB.

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Block Diagram


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Voltage Controlled Crystal Oscillator (VCXO)

The VCXO, a series-tuned Colpitts oscillator is the heart of all 1Watters. It controls
both the receive and transmit frequencies. A MPSH10 transistor was chosen for the oscillator
because of it's high Current Gain - Bandwidth Product, a minimum of 650 mHz (FT).
This will ensure reliable operation at frequencies higher than 20 mHz
The VCXO is the source of the oscillator signal at pin-6 of U4 (~700 mV P-P)
and the carrier oscillator mixer input pin-1 of U2 (~200 mV P-P)
Circuit from 160 Meter 1Watter

The position of the tune control potentiometer (at J1) has a significant influence
on the oscillator signal waveform shape and amplitude.

Below is the waveform at U4 pin-6 with zero (0V) volts applied to the MV209 varactor diode.
VCXO waveform with 0 volts

Below is the waveform at U4 pin-6 with five (5V) volts applied to the MV209 varactor diode.
VCXO waveform with 5 volts

Below is the waveform at U4 pin-6 with ten (10V) volts applied to the MV209 varactor diode.
VCXO waveform with 10 volts

Voltage measurements for Q4 (MPSH10) are E=3V6, B=4V1, C=8V0
These measurements were from a scope using a 10x probe.
My Fluke 23 Multimeter caused loading issues with measurents.

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Receiver Section - Each section of the receiver chain is described below.

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Low Pass Filter including a series resonant Transmit/Receive Switch

With a 50 ohm antenna source connected to the antenna & ground connections, an rf signal passes through two
quarter wave low pass filters, connected back to back. This filter is also known as a 5 pole half wave
low pass filter (LPF) composed of C1, L1, (C2 & C3), L2 and C4. The other side of the LPF connects
to a series resonant Transmit/Receive switch consisting of (C5 & C51), (D1 & D2) and L3. During transmit,
the 2 schottky diodes conduct and limit the amount of rf to the next circuit which is U4, a NE602 class HF mixer.
Circuit from 160 Meter 1Watter

The LPF above is for 160 meters, more specifically 1,810 kHz. The formulae for calculating C1,2,3,4 and L1,2 are:

C(pF) = 1,000,000 /(2π*f*Xc) or 1,000,000 / (6.28 * 1.81 * 50) = 1,760 pF - nearest standard value = 1,800 pF

L(uH) = XL / (2*π*f) or 50 / (6.28 * 1.81) = 4.4 uH = 23 turns on a T37-1 toroid

You can use the site at http://toroids.info to easily calculate
the values of L & C with a 50 ohm reactance to design a half wave filter.

To check the half wave filter for bad solder joints, use an ohm meter and measure the resistance between the connection
marked Antenna and the square pad of the trimmer cap C5 (left side). You should measure near zero (0) ohms.
Measure from Antenna to Ground; it should be an open circuit.
Measure from the right side of the trimmer cap C5 to Ground; it should be near zero (0) ohms.

The right side of the LPF connects to the series resonant Transmit/Receive Switch. This L/C circuit is designed to be
series resonant at the operating frequency with the capacitor to be of a low practical value. We want this circuit to
have negligable influence on the LPF while in transmit mode but also to pass an rf signal while in receive mode.

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First Mixer

U4 mixes the incoming rf signal with the VCXO frequency to produce an intermediate frequency (IF).
The IF is dependent upon the band of operation. The example shown below mixes the 1,810 kHz antenna signal
with the 8,192 kHz VCXO oscillator source and produces a 10 mHz IF and 6,382 kHz IF.
The 6,382 kHz IF is removed by the 10 mHz band pass crystal filter connected to the output of this circuit.
Tuned circuit/transformer T4 transforms the 50 ohm signal source to closely match the 3000 ohms balanced input impedance of U4

Circuit from 160 Meter 1Watter

Voltage measurement for U4 pins 1 & 2 is 1v43 and pin 5 is 6v8
Q6 is a Emitter follower amplifier; it helps match the 1500 ohm output of U4 pin-5 to the xtal filter.
Voltage measurement at the Emitter of Q6 is about 6v1

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Crystal Filter

The crystal filter was designed using Advanced Empirical Methods Technology (AEMT)
Another definition of AEMT is guessing what should be suitable component values and testing the circuit.
Circuit from 160 Meter 1Watter

The Filter uses high frequency 10 mHz crystals, we had to use larger than normal values
for loading capacitors (680pF) in the filter to help narrow the filter response for CW use.
All crystals cases should be grounded to prevent pickup of stray RF signals.

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Frequency Response

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Audio Detector Mixer (BFO) with AGC

The output of the crystal filter connects to a BFO / Audio Detector Mixer via a balun transformer.
The balun transforms the 150-200 ohm xtal filter output to a balanced 3,000 ohm input to the BFO.
Also connected to the input is the Collector of the AGC amplifier Q7. As the AGC source signal
from the speaker increases, the Diode D7 and cap C52 will rectify and filter the signal.
As the signal approches 0V6, Q7 will start to conduct which will lower the voltage (and gain) at U5 pins 1 & 2.
Voltage at U5 pins 1 & 2 with low audio is about 1v4.
Voltage at U5 pins 1 & 2 with very loud/strong audio is about 0v65.
Circuit from 160 Meter 1Watter

The values of capacitors C34 & C35 are chosen to make crystal X6 oscillate about 600 Hz higher than the
center frequency of the Crystal Bandpass Filter and X2, the transmit mixer carrier oscillator frequency.
You may change the values of C34 & C35 to accomodate your listening style. Lowering the values
of C34 & C35 will increase the peak audio response to something higher than 600 Hz.

You may be able to increase the input gain of U5 by increasing the bias voltage on pins 1 & 2 by installing
a resistor Rx on U5 between pins 1 & 8. The resistor value should be near 27K; you will need to experiment
with different values. Depending upon the NE602 manufacting lot, the resistor may increase gain or may not.
I have not checked to see if there are any negative effects to receiver signal quality by making this mod.
Voltage measurements for pins 4 & 5 of U5 should be near 6V2 with antenna disconnected
and near 8V0 with antenna connected and receiving a loud/strong signal.

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Audio Amplifier with Mute

The differential output of the BFO/Mixer connects to a differential mute control circuit
which connects to the differential inputs to U6, a common LM386 audio amp
The mute circuit is controlled by the Keyer circuit. In receive mode, the Mute control voltage is 12 volts
which is a reverse bias to diode D6, hence no current flow in the diode allowing the J-Fets Q8 & Q9 to conduct.
In transmit mode the Mute control voltage is 0 volts, turning off both J-Fets and muting the audio signals to U6.
The RC Time Constant of R13 and C36 determine the QSK delay from transmit to reveive mode.
The value of C39 determines the high audio frequency roll off for U6.
C41 and R14 compose a "Boucherot cell" which prevents audio oscillations at very high volume.

Voltages at U6 pins 1 & 2 are near zero (0) and pin-5 near 5v8

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Transmitter Section - Each section of the transmitter chain is described below.

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Transmit Carrier Oscillator Mixer

This mixer only receives power when the transceiver is in transmit mode.
The U2 mixer generates the transmit frequency my mixing the VCXO frequency of 8,192 kHz with the carrier
oscillator frequency of 10.000 kHz. The mixer generates 2 main frequencies, VCXO+Carrier and VCXO-Carrier.
The frequency of interest is 1,810 and is passed by the BPF while the 18,192 kHz is rejected by the BPF.
The frequency of the carrier oscillator should be the same as the center of the crystal filter bandpass
and is partially set by capacitors C21 & C22 and adjusted by the number of turns on L6.
The differential output of the carrier generator mixer connects to the transmit band pass filter.
Circuit from 160 Meter 1Watter

During transmit, the voltages on pins 1 & 2 should be about 1v4 and pins 5 & 6 should be about 6v3.

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Transmit Band Pass Filter and Preamps

The output of the Transmit Carrier Oscillator Mixer connects to the transmit BPF.
T2 converts the 3,000 ohm differential output down to about 35 ohms. The main filtering is done by T3,
which is a very high Q circuit and is a bit of a chore to adjust for best performance.
Some BPFs are touchier than others; all depends on your 1Watter band and transmit frequency.
Circuit from 160 Meter 1Watter

The high Q of T3 is partially achieved by using a J-Fet (Q5) to amplify the signal,
and further amplified by a transistor emitter follower stage (Q3)
which provides a low impedance drive signal to the driver stage.

Voltage at the base of Q3 is measured at 3v4 and the emitter is 4v2. I know this makes no sense but
it is confirmed by both my multimeter and scope. I'm measuring or doing something wrong.
Anyone reading this...please confirm or set me straight because what I read and see is nuts.

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Transmit Driver

The transmit driver is a simple 50 ohm input and 50 mW output amplifier.
Bypass capacitor across the emitter resistor is not used because this prevents instability issues with the amp.

Voltage on the emitter of Q2 is about 1v3 in transmit mode.

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Transmit Final Amplifier

The final amplifier is a Class-B amplifier. The diode D4 conducts on the negative cycle of the RF input which
results with a positive 0v6 volt DC voltage across the diode. This means that the final transistor amplifies
the entire positive half cycle of the RF input because the 0V6 bias charge is stored by capacitor C8.

D3 limits the collector voltage to 30V to protect the final rf amp. Yes, I know the specs say 20 volts max but you
can get away with more...at last I have not seen any blown amps using the 14 1Watters that I have built and tested.

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Keyer and QSK Operation

The operations of the keyer is descibed at http://kitsandparts.com/keyer.php


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Complete Schematic of a 1W30

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Printed Ciruit Board Version 2.0

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