24 April 2015

Pixie kit - so cheap because ...

While buiding the cheap Pixie kit I came to a dead spot when placing few capacitors. So I did few investigations.

First of all, the 10uF felt too small for the marked 25V rating. I searched my junk box for an older 10 uF 16V cap and they are physically the same. See the picture:

Something else puzzled me: the value of C2, C4, C8, C11. It is specified 100 nF but the marking to look for is 103, 10*10^3, 10'000 pF = 10 nF. Which is the correct one? I will search online for other 40m Pixie diagrams and place the correct value.

Then I moved my study to resistors. I had felt something wrong when bending leads: too thin. The comparison with "old" 1/4 W and 1/8 W volunteer samples from the junk box confirms the anomaly. Even the 1/8 W resistor has thicker leads! (click the picture to get a larger version).

Same conclusion for the supplied "dummy load" (blueish resistor): much thinner leads than an old 0.5 W resistor (I have no idea when I bought it in the last 20 years).

Summing it up. I think the Pixie kit is so incredibly cheap most probably because the supplied components are not top quality. Electrolitic caps with wrong voltage rating. Resistors with wrong leads, at least from 3 dB ("one stop" in photography terms, or half) lower power rating. The print error on parts list would be even easier to correct. I wonder what's wrong with the supplied BNC socket (lossy? wrong impedance?) and transistors.

Nevermind. I have already soldered a socket for transistors so that I can try different, more trusted, parts. At 7 MHz the extra stray capacitance should not matter. The price paid is good for the PCB, if it is OK as well...


M1KTA said...

Hi, got one sent to me to comment on ... I'd agree, components seem terribly cheap versions and not quite right so if you got a junk box should be able to substitute the right value. That said it costs peanuts.

Shame did not come with a 7.030 MHz xtal but I guess if enough in EU build one and keep it on 7.023 (ish) there will be a group of them running so there will be a new qrp centre of activity on the bands hihi

I'd be interested if you or anyone managed to use a club 7030 crystal. Perhaps use some sockets and swap xtals?



Paolo said...

@M1KTA: I plugged a 7030 kcs in HC49/U case and it oscillates just fine. A couple of kHz low actually, while the supplied 7023 kHz oscillated at 7022 kHz. It is not from the GQRP club, but shouldn't be much different.

I will try to rebuild my Pixie kit with old parts from the junk box, at least for the RIT/XIT circuitry.