7MHz QSK tranceiver, using MIZUHO QP-7 and other unit
JG1EAD, Haruo SEMBA
This is 7MHz CW QRP tranceiver completed by fully utilized MIZUHO QP-7 ,VFO-7 and other small units. This combination is very popular in Japan. Full break-in (QSK) , side tone, and AF filter function are installed to add more originality,
1.Construction of the QP-7
QP-7 was assembled based on manual. Instead of fixed crystal, VFO signal is injected from terminal. +12V is supplied to final transistor from common12V supply line. The other part was supplied from transmission +12V supply line. Keying is done by the transmission supply. Thus, the driver TR emitter keying terminal is grounded.
2. VFO-7D modification and alignment
The original VFO-7D unit supplied from MIZUHO has 200KHz range. This unit equip varneer reducer for fine tuning, still the tuning is severe for 30KHz JA's CW allocation. To reduce the range, two 20pF dipped-Micas are added in a parallel with two rotator of variable capacitor in the unit. Then adjust core-A and B on the board. This will give the range 6.995-7.045, 50KHz range. If you make the dial-plate, you can read 1KHz resolution. In this case you can not use original dial-plate, I cut a circular from plastic plate and instant lettering was done. VFO stability and linearity in the frequency is excellent after the mod.
3.Construction of the receiver part
A simple DC receiver becomes the simple QP-7. I picked up a 2SK241 and a MC1496 for my receiver, becasue they are in my stock. The 2SK241 is for the RF amplifier and MC1496 for the mixer. MC1496 is replaceable with other DBM ICs' such like SN16913, 76541. If you want use diode ring circuit for this purpose, you should add an additional Amplifier to compensate the loss.
4. T-R switching and other
To achieve complete full-break-in(QSK) switching and reduce pop-noise during T-R transient, following points are important.
+ Do not use huge de-coupling capacitor in transmitter and receiver circuit. The value around 0.1uF is recommended.
+ Coupling capacitor for AF input should be around 0.1uF.
+ AF-mute of 10m-sec delay during transmission to receive.
When you use DC-receiver of the counterpart of transmitter, Its receiving frequency must be shifted to obtain beat. Practical method often used by DC-receiver users is zero-in to the target station and RIT tune to favorite tone. Audio-peak filter obstruct to find the zero-in position. This transceiver has one-push bypass switch also release the RIT function.Diode-SW is better for full-break-in transmitter. For 1 watt power Mitsubishi MI301 is used. Pay your attention to enough forward current and low-pass filter to antenna terminal, otherwise this diode will be comb-generator! Bias resister of 220-680ohm is appropriate this case.
5. Chassis assembly
For my practical transceiver, chassis assembly is important. Sub-flame is installed in the case to improve mechanical strength. The VFO is settled to the frame. VFO-frequency is readable from front window. Case is painted olive. This olive color is my favorite.
Full-break-in convenience is remarkable. Good QSK transceiver has no audio fluctuation even in small period during CW code. Without power meter, you will feel anxious if the TRX send actually. Peak-filter also works very fine. Response of the filter almost compete similar crystal IF filters. Higher harmonics are reduced to -63dB(2nd order), -73dB(3rd order) by adding the LPF. This comply with regulation. Perahaps,the original QP-7 with maximum power will not satisfy -40dB harmonics. Local leakage to the antenna terminal is S5 with direct connection of FT900. This is equivalent to -95dBm. Several disadvantage in DC-receiver are strong broadcasting station reception during night and non-AGC audio attack by strong signal. I get excited at QSO with the home-brew simple transceiver and straight key.