[AMRadio] FW: Homebrew receiver

[David Knepper]

Brett and others, I wonder if we could somewhat emulate the same results as
you had by eliminating the RF stage from let us say a NC-300 receiver and go
directly into the mixer stage.

I am sure that this has been tried before for operation on 160 and 80
meters.

Just a thought that is not so original.

Thank you.

Dave, W3ST
Secretary to the Collins Radio Association
Publisher of the Collins Journal
www.collinsra.com
----- Original Message -----
From: Brett Gazdzinski <brett.gazdzinski at mci.com>
To: <amradio at mailman.qth.net>
Sent: Wednesday, October 15, 2003 8:46 PM
Subject: [AMRadio] FW: Homebrew receiver


> This is what I plan on sending to Electric Radio, along with
> pictures.
> What do you guys think?
>
> Brett
> N2DTS
>
>
>
>  I wanted a complete home brew station, and since I have
>  a homebrew pair of 813,s, modulated by one of two modulator
>  decks, push pull parallel 100TH,s, or a pair of 4sc250b,s,
>  and a classic push pull rig with link coupling, using 812,s
>  modulated by
>  a pair of 811,s, only a receiver was needed.
>  At first, I thought I would build something simple that worked
>  just well enough to be able to copy AM under good conditions, just
>  so I could say I had a home brew station.
>  But I wanted something a little better than the regen receiver
>  type of radio, maybe a simple superhetrodyne.
>  I did loads of research, looked in Bill Orr, and all my old
>  ARRL handbooks, looking for simple receivers.
>  All the circuits had some sort of problem, complex tapped coils,
>  hard to get parts, poor designs, etc.
>  I also looked at the diagrams for things like my Gonset G76, the Scott
>  model SLRM I have, the Hallicrafters sx17, and the R390.
>
>  I decided to base the receiver on the Scott SLRM, since it works
>  very well, has good fidelity, uses 8 pin tubes and a 455Khz IF.
>  I ran into problems though, as the Scott was built to reduce emissions
>  out the antenna, with loads of shielding and an rf amp with
>  tuned circuits.
>
>  I accumulated parts, and started construction with the basic layout
>  of two tuned circuits on the antenna input, an RF amp, a separate
>  local oscillator and mixer, two stages of IF amplification, hifi
>  detector, s meter circuit, agc circuit, and power supply.
>
>  Since it was to be experimental, I used octal sockets for everything,
>  the antenna coils, the local oscillator coils, and the IF
>  transformers.
>  The receiver started out with plug in coils to change bands.
>
>  I laid out all the parts, leaving room between things to allow
>  room for experimentation, and mounted the basic parts.
>
>  I tried various circuits for the local oscillator, using coils
>  wound on ceramic forms, B+W coil stock, and slug tuned ceramic
>  coil forms.
>  This step would have been very difficult without the aid of a
>  very nice  spectrum analyzer I have through work. It allowed me
>  to look at the  frequency output, harmonics, hash, drift,
>  frequency range, amplitude, all at the same time.
>
>  At first, I went with plug in coils in the local oscillator,
>  used the rf amp,  using the spectrum analyzer to peak things
>  and check gain.  The mixer was easy, then to a filter.
>  I planed on using a mechanical filter, but they are
>  expensive, and a little tricky to put in the circuit.
>  I found a company on the web, kiwi, who makes various filters, and
>  went with one that has an op amp input, three filters of slightly
>  different center frequencies (sets bandwidth) and an op amp output,
>  and runs off 10 to 30 volts dc.
>  There is no loss through the filter, and its quite similar in results
>  to a mechanical filter. I used a 5.5kc model.
>  It mounts on Velcro, and has pig tail shielded wires to hook up
>  to the IF system.
>  This filter is easy to add to any receiver using 455 KHz as an IF,
>  and really works fantastic.
>
>  I copied the IF system out of the Scott, and used a hifi detector
>  on one of the AM web pages.
>  It took some experimentation to get the agc takeoff and IF gain
>  control systems working well, then I added the S meter circuit I stole
>  out of the Bill Orr handbook using a 6SN7.
>
>  Taking the receiver for a test drive revealed problems.
>  Startup drift was excessive, muting the receiver seemed impossible,
>  the RF amp caused all sorts of problems, and the if amps were
>  unstable.
>
>  As a test, I hooked the antenna up to the mixer input, and bypassed
>  the rf amp, and had very good results, so I removed the rf amp
>  completely, and went with two tuned circuits then into the mixer.
>  Some experimentation with the antenna link on the input coil boosted
>  gain quite a bit.
>  I ordered a selection of NPO caps, and did weeks of experimentation
>  on the local oscillator stability, changing components,
>  design, putting the coil in a metal plug in can to shield it, and
>  got the stability much better, but still have startup drift for
>  the first 5 minutes.
>
>  Careful shielding and reducing the gain of the IF eliminated the odd
>  oscillations I got at times, and the receiver was working quite well.
>
>  I did not like the tuning dials I had, marking the frequency was hard
>  with the drift, and I have a real problem marking the frequency
>  so it looks nice on the dial.
>  I needed something better, and found the almost all digital
>  electronics digital frequency readouts, basically a frequency counter
>  with a selectable frequency offset.
>  You program the thing to offset the IF frequency, in my case, 455Khz
>  lower, and all you need to do is get the pickup close to the
>  local oscillator tube, and the display reads the exact
>  receive frequency down to 1000 Hz.
>  I used their backlit display, which looks nice, and a real accurate
>  frequency readout is very nice to have.
>
>  The performance of the receiver was astounding!
>  With the transmitting antenna used, sensitivity was very
>  good, fidelity was great, I use a marantz amp on all the receivers
>  in the shack, to a big three way speaker, and the homebrew sounds
>  the best, because of the low distortion fi fi detector I guess.
>  The biggest surprise is the noise level.
>  Since the tube count is low, and the mixer design is a quiet one,
>  the receiver is incredibly quiet.
>  Its MUCH quieter than anything else I have, or have ever had.
>  Forget the modern rigs, the IC chips just can not run quiet, and
>  there are so many of them in modern rigs that the noise and distortion
>  in any modern rig I ever used is way high.
>  Comparison to my very well working r390a was dramatic, I could CLEARLY
>  hear signals that were well under the hash level of the r390a,
>  the signals were unreadable on the R390a, but very good
>  comfortable copy on the homebrew receiver.
>
>  After the results I got out of the homebrew, the plan changed from
>  something I could use sometimes, just to have a complete home
>  brew station, to the receiver of choice.
>  This caused problems.
>  I had the receiver mounted in a rack cabinet, and had to run around
>  back to change the plug in coils, a real pain in the butt over time.
>  So out came the receiver, and a new front panel and band switching
>  was added, along with 160 meters.
>  Tuning was changed to a system using TWO back to back vernier
>  drives, the tuning range was changed to cover only part of the
>  ham bands, giving very nice slow tuning range.
>  A bfo was needed for zero beating AM signals, so I found and built
>  a 455Khz crystal oscillator circuit, with a variable output
>  level by way of a pot in the screen voltage.
>  The level control is on the front panel.
>  The bfo also allows me to copy cw and ssb quite well, without
>  a product detector, so I can listen to the ssb guys complain about
>  AM.
>  The receiver moved into a cabinet on the operating desk, and
>  was integrated in the shack with muting and so on, and is
>  the main receiver now, the others are almost never used...
>  The only problem the receiver has, and it does not bother me, is
>  the startup drift. From a cold start, it drifts about 1kc
>  over about 5 minutes, then is rock stable.
>  This might be due to the choice of octal tubes, the actual tube
>  used effects the drift quite a bit.
>  Experimentation with npo caps can reduce the drift, but it starts
>  drifting the other way over longer periods of time, and I think its
>  better to have 5 minutes of drift and stop, rather than drift less
>  but over longer periods of time.
>
>
>
>  I was quite surprised about how easy it was to build, and how much
>  raw fun it was to design the thing, and do all the testing
>  and development.
>  You sure do learn a lot when you build something step by step, without
>  any overall design to start with.
>  Every system must be analyzed, built, tested, changed, other
>  things tried, etc.
>
>  The end result looks a little rough inside, as it was
>  changed quite a bit, deleting the RF amp, adding band switching,
>  etc, but it still looks ok.
>  Its been totally reliable and stable for about a year now, with
>  quite a lot of use.
>
>  My next project is a superhet receiver using 7 and 9 pin tubes, using
>  things I learned from the first one:
>
>  Start off with band switching,
>  Forget the RF amp, its not needed on the low bands at all,
>  Do NOT leave a lot of space between things, but put the tube sockets
>  and IF cans close together as possible, along with the local
>  oscillator parts and band switch.
>
>  It will also include two filters, 4.5Kc, and 5.5Kc.
>  The 5.5 was a great overall choice, but a 4.5 will help
>  when things get crowded on the bands.
>
>  Building a good receiver for AM reception is not as hard as most
>  people think, and I encourage people to give it a try.
>
>  I have no formal electronics background, all I know I got out
>  of books and by playing around, so if I can do it, almost
>  anyone can.
>
>  Parts are not a limitation, although it may take some time to assemble
>  all you need at a reasonable cost.
>  Things like IF cans can be got out of old tube radios, old table
>  top AM radios are a good source of parts, as well as mouser
>  electronics, Antique electronic supply, ham fests, even radio shack.
>
>
>  You may find you can build something better than anything you can
>  buy for almost any price, as YOU pick what is important,
>  I only wanted part of 80 and 40 meter coverage, low noise, and hi
>  fidelity, along with reasonable frequency resolution.
>
>  Old tube receivers like the Scott SLRM, SX17, SX28 can be quiet and
>  hi fidelity, but lack frequency resolution, good filters, and cover
>  more bands than I need.
>  Newer tube receivers like the R390 series, the Collins 75a series,
>  the National nc300/303, Drake and others have some good points,
>  but lack fidelity, bandwidth choices, look ugly as stink, or have some
>  other drawback.
>  All new ham equipment seems to be very high in noise and distortion,
>  and you may THINK some of that stuff sounds good, until you
>  compare it to a good AM signal through an old tube hi fidelity
>  receiver like the Scott SLRM or the SX17.
>  Even with output from the detector into a good hi fidelity
>  amp and speaker, there is no comparison between the new and
>  old stuff on AM.
>  I integrate all the receivers into the Marantz amp, and I can
>  jump between various receivers quickly, all tuned to the same
>  signal, and the difference is dramatic.
>  I have tested many, Kenwood ts440, icom 735, Kennwood r1000,
>  IC 756pro, and others, and they are all poor receivers for AM
>  if you want fidelity.
>
>  Brett
>  N2DTS
>
>
>
>
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