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H.F transceiver

I have bcen a reader of your admira-. ble magazine sińce the Wireless Weekly days, and I have madę up many of the projccts without serious problems. But recently I tackled your HF transceiver (October 1985) and found one that stumped me.

I wonder if there might be some errata still to come. Take 018, for exam-ple, the voltage table indicates 9 Volts on the collector, but it derives its poten-tiai from an 8 Volt raił! Also the bias should be 0.8V, but the bias resistors arc 180k and lOk. (I have changed the 180k for a 150k, did I do wrong?)

Also 032: the table shows 5.0V on the emitter; my measurement is 0.4V (Battery 12.78V).

But this is the one the Serviceman might appreciate. The 05 and 6 drivers and the 07 and 8 power amplifiers have 12.78V on the collectors, and 0.82V on the bases, but the quicscent current for the drWers is only 23mA (it should be 50mA), and only 34mA for the PA (it should be lOOmA). A thorough check has been madę of the Circuit and the transistors have bcen checked with an ohmmeter. No decrease in the Vcc can be noticed when the transmitter is ener-gised. Ali seems to be operating nor-mally in the receiver and in the transmitter up to and jncluding the RF pre-drivers. (Fr. AT, Vanimo, PGN).

• This design originated from Dick Smith Electronics, as you probably realise, and we can only offer limited help. You noted that Q18 derives its collector voltage from a 8V raił, although the voltage table indicates 9V. You are right, of course; this is not possible. However, on page 11 of the kit manuał, you will see that the supply voltage is specified as 8.00 — 9.00V. The voltage table is based on a 9.00V supply, and not on 8.00Y.

With regard to the values of the bias resistors for Q18, ii might help to know that the 10k and 180k are not the original values as published in EA. These were 10k and 82k. From your findings, we expect that 180k is too high, and your.150k gets closer to what it should be. You could even try 120k or 100k.

The voltage table shows for Q32 an emitter voltage of 5.00V and a base voltage of 1.0V. This must be wrong: if the emitter voltage is higher than the base voltage, then the transistor would not conduct. Your measurement of 0.4V makes a lot. morę sense, so perhaps the original should have read 0.5V.

As far as the difference in the quiescent current of the drivers and the power amplifier is concerned, we don’t think the Circuit diagram is wrong. The differences In current are not likely to influence the performance of the transmitter slgnificantly.

Electronic Wattmeter

I am having trouble with the Electronic Wattmeter, of September 1983. Are there any alterations or errata to the Circuit?

With 1.5V on TP3 to TP2, VR3 has no effect on zeroing the meter but VR1 does zero the meter. There is a + or — drift from meter zero, With 1.5V on TP3 to TP1, VR1 has no effect on zeroing the meter but VR3 does zero the meter, with ereater zero stabilitv than from TP2.

W ith the connections as Fig.2, VR4 has no effect on the adjustment of the meter to 3kW. There is a -2.15V on the positive terminal of the meter and ad-justing VR4 has no effect on the -2.I5V. My positive supply is 12.16V, while the negative supply is ll.52V. (A.W., Elwood, Vic.)

•    There has indeed been some errata on this project. The text should say “with 1.5V between TP3 and TP2, adjust .VR1 for zero. With 1.5V between TP1 and TP3, adjust VR3 for zero." Notę also that point 9 on the PCB overlay goes to the negative meter terminal.

“Screecher” car alarm

In the Screecher Car Alarm, of August 1986, where do the wires from the terminal błock of this project connect to, in the car? I’m having difficulty working this out. (C.Mc.G., Avondale Heights, Vic.)

•    To wire the Screecher Burglar Alarm to your car, first find the writing on the printed Circuit board next to the ' terminal błock. The

• •

connections are as follows:

. V+- connects to the +12Volts (fuse box or battery positive); .    •    •

gr connects to the body óf the car or the negative battery terminal;

S- connects to one of the speaker wires and S+ to the other. The 100fiF capacitor is also connected here (see article);

. i2 connects to a normally Iow (no voltage) sensor switch which goes high (12 volts) to trigger the alarm;

H connects to a normally high (12 volts) sensor switch which goes Iow (0 volts) to trigger the alarm (e.g. door switch side of courtesy lamp).

Low distortion oscillator

I have recently built (from an Altrori-ics kit) the Ultra Low Distortion Oscillator as describcd in EA, December 1986 — January 1987. Although it was a simple matter to adjust all frequencies for equal output lcvel, I found the job of adjusting each frequency rangę so that the frequency scalę calibrations are correct to be impossible.

On feeding the output of the oscillator into the EA 7-digit 5(X)MHz Fre-quency Meter and adjusting the trim-pots VR1 to VR4 so that the frequen-cies for each rangę were correct at the “100” position on the scalę, I found that all other frequencies were crowded from both ends of the scalę towards a position centred between the 20 and 25 positions. After adding 4700 resistors in series with the 150k resistors across both sections of VR5, I was able to get correct scalę alignment at the centre of the pot’s travel — that is at the 18 position. At the 10 position, with the rangę switch set to the xl position, the fre-quency meter read 11.4Hz and re-mained at this frequency until the pot was moved abovc the 11 position from where the frequency began to incrcase to the 12 position where it was correct. From here up to the 18 position there was a gradual crowding of frequencies.

At the high end of the scalę the crowding was morę severe, so that at the 50 position the frequency meter read 97.8Hz. From there down to the 100 position the frequency only alters by 2.2Hz. From the 50 position to the

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ELECTRONICS Australia. September 1987