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Author Topic: Pixie II for newbies  (Read 1220 times)
KC0ODY
Member

Posts: 78




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« on: February 02, 2004, 08:19:24 PM »

Hi all,

I'm very new to QRP and also new to kit building. Well, I have built about 4 receivers, but I still consider myself very green in electronics. Anyone who can read and solder can put together a kit, but just a couple of days ago, I realized that I really need to expand my knowledge base concerning electronics.

I picked up a Pixie II kit recently from a QRPer. It came with some cursory instructions... i.e., a schematic and a list of parts in the kit as well as an Altoids tin to put it all in. I also got some jacks for headphones and an antenna, and a switch which I presume is an on/off switch.

Now, for you experienced types, I'm sure this would be all you would need to get started; however, I've never put together a kit without detailed instructions, so I'm a bit confused as to what goes where on that tiny circuit board. Please don't be too critical of me at this juncture; t's just where I am in the process. I've never gone to school for electronics and never learned how to read a schematic, so I'm not exactly sure where to start.  However, I am bright and eager to learn.

There are a couple of people in my local radio club who are kit builders, but neither of them are available right now to answer my questions, and I haven't found anything in my web searches that would teach me how to correlate a circuit schematic with what part goes where on the board.

Can anyone point me to a site that could teach me how to interpret a simple schematic? I know how to read the parts on a schematic, but I don't know how the schematic itself translates to exactly where those parts go on a circuit board. I hope I'm making sense here...? Thanks for ANY help you can give me... I am really new to all this.

Jackie
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WB2WIK
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Posts: 20595




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« Reply #1 on: February 03, 2004, 04:36:14 PM »

I'm not familiar with the "Pixie II," but it sounds like it came with a printed circuit board and a bunch of parts, and a schematic, without any way to easily correlate those things.  

Most PCB layout folks add a silk screen to the top side of the circuit board with component designations, "R1," "C2," "Q3," IC4," etc. that correlate directly to the schematic diagram.  That silk screen can, and really should, even show which pin is Pin #1 on every IC, which are the base-emitter-collector or gate-source-drain for every transistor, and so forth.  It takes only minutes to do this, and costs pennies to add the silk screen to a commercially fabricated board, so it's unusual for a board not to have this data.

However, it sounds like the board you have doesn't!

If you had a multi-layer board there (a circuit board containing connections routed between multiple board layers which you cannot see -- such as a computer mother board), without a component screen on the top surface, it would be literally impossible to know where anything goes.  R1 might be right next to R2 on the schematic, but could be on opposite sides of a circuit board, and 15" apart, with the connection between them made by an internal layer you cannot see -- it would be impossible for anyone to know this.

But usually, simple circuits and projects for hams use only double-sided boards (all connections are made on the top side, or the bottom side, of the board and there aren't any layers in between).  At least, then you have a fighting chance of figuring out how to lay the components per the schematic.

Other than taking your time and gaining some experience in this (or going to a technical school and taking classroom courses and labs in the subject), there's not much you can do.  Of course, if you can find a local Elmer who's an experienced technician, engineer or homebrewer, he or she might help a great deal!  But there isn't necessarily any rhyme or reason for the way printed circuit boards are laid out, other than a good layout expert with a strong engineering background will usually "optimize" the layout for best performance, based on the actual circuit.  Most DC circuits aren't path or layout-critical, but AC and RF circuits can be (so can high voltage circuits)!

WB2WIK/6
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WB6BYU
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Posts: 13234




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« Reply #2 on: February 04, 2004, 05:50:22 PM »

the Pixie is a crystal-controlled direct-conversion
transceiver, in a rather rudimentary but quite functional
form.  The interesting quirk is that it uses the final
output transistor as the receive mixer.

Clearly this may take a lot more coaching than is practical
on a forum such as this, but I'll give it a go and try
to keep it short...

First I would go through the parts list and identify
each part, and which symbol it corresponds to on the
schematic.  Hopefully you have a book that shows the
different symbols (resistor, capacitor, transistor, etc.)
Make a copy of the schematic and lay each piece on it
over the corresponding symbol.  Fortunately there are
not a lot of parts, but you will have to match the part
values and types.

Next, look for some obvious clues for parts placement.
I expect the audio amp is an 8-pin DIP IC, and there
should be a set of 8 holes on the board that it fits.
If the board isn't marked, you will have to figure out
which way it goes in the holes.  Look at the schematic
and see which pins are grounded, then check the board
wiring - the ground trace should be quite obvious.
Stick the IC in the holes and bend a couple leads to
hold it in place (DON'T solder it yet!)

The transistor holes may also be obvious, and if the
final is in a TO-39 case there shouldn't be any ambiguity
about which way it goes in. If the oscillator uses a
transistor in a TO-92 case, however, the leads will be
in a straight line, and (just like the IC) you will
have to figure how how they go.  There may be some
clues for other parts, such as hole size, lead spacing,
etc.

Once you get the obvious parts in, pick a pin that you
can identify and see what parts are connected to it on
the schematic, then look at the copper traces on the
board that go to that pin.  Some parts will be easy:
a capacitor to ground, for example, will mean looking
for a grounded hole that is spaced to fit the part.  A
second lead on the same pin, then must belong to the
second part.  Commonly parts are layed out neatly
parallel to the board edges - if that seems to be the
case with your board, it will reduce the number of possible
ways a part can be installed once you find where one
end of it belongs.

From there it is like working a jigsaw puzzle:  sometimes
you have to play "what if" games.  (Here it helps to keep
track of which parts you are sure of, and which you are
just trying in place to see if that solution works.)
Remember there will probably be holes in the board for
the output and power connections.

Fortunatly the Pixie is not a big kit, and you shouldn't
have much trouble figuring out the parts.  

Or, if you don't feel up to QUITE that much of a
challange, here is a web site that has a lot of Pixie
information, including a photos and tutorials.

http://www.qrpp-i.com/pixie2.htm

But it might be fun to try your hand at figuring out
the parts placement first!
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KC0ODY
Member

Posts: 78




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« Reply #3 on: February 04, 2004, 09:13:15 PM »

Dale-- this information has been VERY helpful-- can you tell this is my first time doing this sort of thing? :-) Thanks to Steve, too, for your input.

I think I'll try my hand at the "jigsaw puzzle" method first. I won't solder anything in place until I'm pretty sure of what goes where. This will be good practice for me in reading schematics and learning about the circuit itself.

I'll also check out the qrpp-i site as well.

Thanks again for your help, gents!

73 de Jackie, KC0ODY
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AG4RQ
Member

Posts: 300


WWW

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« Reply #4 on: February 10, 2004, 11:02:09 AM »

Jackie, do a Google search for "Pixie", "Pixie2", "Pixie 2", "Pixie II" and "PixieII". You will come up with a lot of the same stuff I did regarding the Pixie.
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NJ0E
Member

Posts: 48




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« Reply #5 on: February 13, 2004, 05:17:28 PM »

hello jackie,

i have completed a pixie 2 kit, the one sold by halted
electronics. in the instructions for mine, the parts
layout on the pc board is on the third page near the
bottom.

if you have any questions about the parts or how they
are oriented, pls let me know. my email address is
available on qrz.com.

the pixie works nicely. a couple tips; when you're
deciding where to locate the connectors, try to make
them pretty close to the pins they connect to on the
board. i made the mistake originally of running the
lines going to the headphone connector under the
one going to the key jack, and got some popping and
squirrely noises in the headphones when i was keying.
i changed the location of the headphone connector
and that fixed the problem.

also, the switch is probably to switch in and out a
capacitor to "pull" or slightly change the frequency
of the crystal between receive and transmit. if you
don't provide for a slight frequency shift between
receive and transmit, you may not hear a station
calling you back at all, or if you hear anything, it
will just sound like an unmodulated carrier thumping.

the pixie is alot of fun; i outfitted mine for 80
meters, and have worked stations with it as far
away as florida from my qth in texas.

best of luck to you.
73,
scott nj0e
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