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Friday, January 31, 2014

G0MGX's Minima



Wow, Mark didn't waste any time in building his Minima. FB OM.   His web site has a lot of good info on this project:  http://g0mgx.blogspot.co.uk/


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Thursday, January 30, 2014

BITX 20/40 Update #7: Cabinetry and Socketry


With the 20 Meter Oscillation Exorcism behind me, I have decided to take a break from electronics and do bit of woodwork.   I took the walnut (I think) box that I bought via Amazon and cut out a big piece of the wood on the front.  That's where the rectangular piece of copper clad board will go -- it will be the front panel, supporting the AF gain control, the bandswitch, the main tuning control, and the mic jack.   A similar copper clad board will be on the back, this one supporting the antenna jack,  speaker jack and 12V input jack (with space for a linear amplifier T/R control jack).  (George Dobbs, G3RJV, calls this the "socketry.")

That beautiful copper sheeting will line the inside of the box.

I found the soft wood on this box to be very easy to work with.  The little saw pictured above made the woodwork easy.

I have boiled linseed oil and clear polyurethane for the finish. 

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

Monday, January 27, 2014

BITX20/40 Update #6: 20 Meter Exorcism


The transmitter was working fine on 40, but was horribly unstable on 20. In the past, this kind of thing would really drive me nuts, but experience has made me more patient. I know that "taming the beast" is part of the homebrew process. 

I knew that layout was part of the problem:  I had significantly less room on the board with this rig than I'd had with the BITX17:  the additional bandpass filter and low pass filter, and the associated relays, used up a lot of copper clad real estate. So by the time I built the PA chain, the inputs were too close to the outputs.

The fact that the rig was stable on 40 but not on 20 led me to believe that this was not a problem of insufficient decoupling.  Instead, I thought that I was getting additional inductive feedback at the higher frequency.

I noticed that the instability disappeared when I put the 1X scope probe on the input to the first amp in the RF chain (Q14). That was an important clue.  Looking closely at the circuit, I realized that the base of Q14 had a long lead (several inches) up to the low pass filter.  I had experienced problems with this lead on the BITX 17 project and had cured it with a relay at the low pass filter -- this relay took one end of the lead out of the circuit on transmit, preventing it from becoming a little radiator.  I used that mod in this rig, and figured that that cured the trouble.  Wrong.   The other end of that lead was still connected to the input to the RF power chain.  It was picking up enough RF to send the PA chain into oscillation.

I put a SECOND relay at the other end of the line.  That took it completely out of the circuit.  And the instability disappeared.  I fired up the rig and worked California on 20.  Very satisfying.   

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

Thursday, January 23, 2014

BITX 20/40 Build Update #5: It is ALIVE!

I got the BITX 20/40 on the air this morning.  The receiver has been working for a week or so, but as usual it was a bit of a struggle to tame the transmitter.  I got up early this morning and started poking and probing.  I played with the driver and final coils a bit.  I had used the same trifilar toroids that I'd used in the BITX 17, but this rig didn't seem to like them. So I went with FT-50-37 bifilars -- that seemed to work better.  That IRF-510 seems to put out about seven watts on 40.  (I still need to tame the transmitter on 20).  The 40 meter signal looked great on the 'scopes (RIGOL and Tek!).  Shortly before 8 am I worked AD4SY who reported that I was filling his shack with booming audio.  Life is good.  

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

Wednesday, January 22, 2014

Digi 'scope, Analog 'scope


Carl (M0ICR ex 2E0TEC) saved the day with my Tek 465 -- he sent the replacement HV multiplier that put the 'scope back in action.  (Ace Tek repairman Tad WA1FQO made the repair.  Tad sells Tek scopes on E-bay -- check out his products: Search for fqo63 on ebay.)

Carl notes a number of similarities between our workbenches and recent projects.  All he needs now is a Drake 2-B!

Alan Wolke said he'd be interested in my reactions to having both a digi scope and an analog scope on the bench.  I can already see areas where one might be a bit better than the other.  The Tek is better at watching a fast moving audio or SSB waveform.  But the Rigol doea all kinds of math tricks.  Check out the FFT function shown on Carl's page:  http://iqrp.net/


Bill,
I am very pleased that you finally got it working with the help of brothers from the fraternity.  Having listened to your latest Podcast I think that must be your alter ego!  I have A Tek 465, Rigol 100MHz DS1102E and have built 2 Bitx, one for 20m and the other for right bands- using a DDS VFO  (3 if you include the 80m version by Steve Drury G6ALU - look up MKARS80) ...I also have 2 copies of SSD!
My website is here:  www.iqrp.net  where you can see both scopes in my shack and images of my Bitx and my Hans Summers WSPR beacon!
I have just returned from my local radio club social meeting (the Radio Society of Harrow) and took great pleasure in listening to Soldersmoke 158 whilst driving there ... half on the way there and the other half on the way back.
Very best wishes from London (where it is cold, wet and miserable !)
72/73 de Carl M0ICR (ex 2E0TEC from Nov 13)
Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

Monday, January 20, 2014

SolderSmoke Podcast #158: BITX 20/40, Farhan's "Minima", BITX Bandsweep


SolderSmoke Podcast #158 is available:

http://soldersmoke.com/soldersmoke158.mp3

January 20, 2014
MLK Day. Tough winter.  Shack heated by Heathkit.
BITX 20/40 Almost done:
Bandswitching arrangement.
VFO construction using spreadsheets.
2N2222s wired in backwards! (CBE not EBC! Who knew?)
Other amusing building errors.
Crystal filter evaluation -- the G3UUR method.
Sweeping a filter with an Arduino DDS.
Building a BITX in LTSpice.
New Rigol 'scope (now 100 MHz).
Tek 465 REBORN!
QRP HATERS -- They are out there!
MAILBAG.
BITX 20/40 dual band bandsweep

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20
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