[AMRadio] FW: Homebrew receiver

[David Knepper]

Very fine, Brett.  In all the receivers that I have at the Collins Radio
Center, I like the NC-303, the best for AM


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: Thursday, October 16, 2003 8:50 AM
Subject: RE: [AMRadio] FW: Homebrew receiver


> Dave,
> Many receivers did just that.
> I don't remember if the NC300/303 used the rf amp on the low bands
> or not.
> The NC300/303 was one of the best AM receivers I ever had.
> I think it needed better audio stages, but I always used a detector
> output.
> Frequency resolution and stability were very good, bandwidth choices
> were good, although I don't remember how sharp the filters were.
>
> The looks of the thing were quite Art Deco, and did not turn
> everyone on, but they are very good receivers in my book.
> Hard to mount one in a rack though, with the rounded corners.
>
> I am not sure how much the RF amp adds to the noise level.
> A well designed rf amp section can actually reduce noise levels.
> Noise mostly comes from mixers, and overall tube counts.
> All mixers add some noise, some designs are much better than others,
> and the more there are, the more noise you get.
>
> I used single conversion, with a quiet mixer setup, and
> used two tuned circuits of very high Q in the input,
> along with resonant dipole antennas for 80 and 40 meters,
> so I don't get any images or other problems, as signals
> out of band are attenuated very much before making it to
> the mixer. This is not always the case in the general receiver
> setup, as many bands are covered, low Q broad band coils are used,
> and who knows what antenna will be used.
>
> As far as the NC303/300 goes, if you can get past the style,
> the only big improvement to be done would be the audio output
> (if used), and maybe the addition of a KIWI filter module.
> These are quite like mechanical filters, very sharp.
> One of those would be very easy to add, without hacking
> up the receiver.
>
> Brett
> N2DTS
>
>
> > -----Original Message-----
> > From: amradio-admin at mailman.qth.net
> > [mailto:amradio-admin at mailman.qth.net]On Behalf Of David Knepper
> > Sent: Thursday, October 16, 2003 4:31 AM
> > To: amradio at mailman.qth.net
> > Subject: Re: [AMRadio] FW: Homebrew receiver
> >
> >
> > 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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