http://soldersmoke.com/soldersmoke173.mp3
7 March 2015
Bench Report: Pete's Progress on the Let's Build Something Rig:
http://www.jessystems.com/LBS_Detail.html
Ben's cool case for his LBS rig
Bill's Tale of Woe: QRO troubles with the BITX 40
-- QRO amplifier taking off on 40 (but not 17)
-- Criticism and public humiliation on 40
-- Troubleshooting
-- Suggestions from Allison
-- A sad realization about my VFO frequency selection
-- Exorcism needed
-- Pete suggests a digital solution
-- Wow, my 'scope has an FFT! Almost a spectrum analyzer!
-- Some thoughts on trouble shooting
-- On the meaning of "BASTA!"
The Spring 2015 Issue of Hot Iron http://www.walfords.net
More on Pete's KX3
Encouraging other hams to build
Posts
This blogger tends to lose my comments 2 out of three tries so here I try again...
ReplyDeleteBITx, Had I known before hand I'd have sent you back to check it out. Its a cool package but often the experiments with it lead to odd behaviours that and request for help. Often they are hard to sort out as details aka data is missing.
The VFO in the 75M range is a non starter. Why? harmonics fall in the desired band and no amount of filtering can remove them. Even if the VFO were prefectly clean the mixer has its own non-linear function will remake the missing harmonics and drive one mad. ;)
The poor EB63 was not rated for signals below about 2mhz
so it was trying and in pain over that.
The other BITX bite is the IRF510. Its a good device but people often discover (or plain diss it) it can have very high gain and at 10mhz and down its a hot device. The bitx power amp design while fundamentally good is prone to layout induced issues and its made a few people scream basta till they understood the problem. One thing the Amp
(IRF510) was prone to is stable on 50 ohm load but no so on a reactive load (untuned antenna or amplifier input).
Scopes and radios... Rarely use one. My favoured tools are a RF voltmeter and receiver or spec analyser. The usual problems are RF level either not enough or too much
and spurious products. A scope can do waveform but a
clean sine will still have up to a 20dBC harmonic content
and that's high. Your Rigol FFT function will see much more use as you appreciate what it can tell you. Definitely a
cool toool.
The 40M (and other bands) ESSB, frequency police, and callus quip bunch are generally not worth listening to. One jumped on me for be 20 off, my comment was it made him sound better to me, end of exchange. Most in reality hear your running HB and often want to know more. They are worth talking to.
Amplifiers,while EB63A was making you mad I'd gotten a
classic CB amp a Siltronix SLM-100 of circa early 80s. Sufficient to say its not legal for any mode or band. Low pass filters are missing and biased class C. But the one I have had a good pair of SRF2017 (aka MRF460). So the solution was to add bias and rip out the so called RX amp and add a 10M LPF internally. The result was pretty close to the EB63 (120W) and I run it at the 6-70W level as the MRF460s are a lower power device. One add was a input pad of 3DB to insure the FT817 didn't over drive it on any band then feedback to flatten the gain for the same reason. External switched LPF for 80 through 10 completes it.
Oh I took off the stupid buttons as well. Spec analyzer says good enough for prime time and 40M gang had the usual comments on the FT817 hand mic audio (ok but not HIFI...) but now louder.
NOte always run SSB radio near their best output into the amp. If the radio puts out too much add a resistive pad
at the amp input (for TX only). This is because relying on
ALC, mic gain or other means poof power reduction is unreliable and can easily result in distortion or worse killing the amp. That pad assures that the radio always seens a good SWR at its output as the amp may not have a good input SWR on all bands. Also it tends to decrease the signal to carrier ratio and that's not good. This is something the bitx is prone to (unless tweaked it tends to leak carrier).
LBS, tell me more. Please? Always looking at other ideas and radios.
Allison
Thanks Allison. And thanks again for all the good ideas and advice. Yea, that VFO freq on 40 is a formula for madness. BASTA! Back to the drawing board. LBS is Pete and Ben's project. It started as a Direct Conversion receiver using an Arduino. DDS. When completed it is a full SSB transceiver using an Si5351 to generate both the VFO and BFO/carrier sigs. Pretty neat: http://www.jessystems.com/LBS_Detail.html
Delete73 Bill
Bill, I listened to your "tales of woe" on 40 recently. If my memory serves me correctly, I started listening to SolderSmoke back in 2006. Since that time I can tell a difference in your electronic's knowledge growth. You have nothing to be ashamed of in building your own gear. I liked your comment alluding to the fact that the biggest and loudest critics on 40 meters are people who are appliance operators and have never built their own gear. The high tech, pan adapter equiped noobie can now proclaim technical superiority (in their own mind) to those of us who chose to build our own gear. Keep the solder flowing and keep building your own gear!
ReplyDeleteAl, N8WQ