Switching to a Mechanical Filter from 1967 for my HRO-ish Receiver (with video)

From RSGB Handbook 1982

Having overcome the difficulties with the National NPW Dial and Gearbox, I turned my attention to the 455 kHz filter.  I had been using this old Toyo CM - 455 kc filter (Date stamped August 1969).  CM stands for "Crystal-Mechanical."  These filters are hybrid with some of the features of a crystal filter and some of the features of a mechanical filter. For more details go here: 
https://soldersmoke.blogspot.com/2016/04/the-meaning-of-cm-in-toyo-cm-455-filter.html

I was disappointed by the CM filter.  It seemed very lossey, and it just didn't seem to be of sufficiently high Q -- it seemed very broad. I could hear the other side of zero beat.  It was barely a "single signal" receiver,  and being "single signal" is the whole point of a superhet. 

I remembered that Pete Juliano had sent me a Japanese-made 455 kc mechanical filter. Maybe this would do better.  Last night I did a quick comparison test and -- wow -- Pete's filter was much better.  The Fifth Edition of the RSGB Handbook seems to agree with my assessment, noting that mechanical resonator filters were superior to the Crystal Mechanical hybrids (see pages 4.17 and 4.18) 

Pete's filter is from the Kokusai Electric Company.   Part# MF 455 ZL.  (Date stamped May 1967). "ZL"indicates lower sideband.   I checked and indeed the passband goes from just above 452 kc up to about 454.5 kc.  This is a 40 meter receiver and SSB on 40 is LSB, so this filter would work perfectly right?  Not so fast!  Sideband inversion had to be considered.  

I was running my VFO from about 7455 to 7755 kHz.  This means that the modulated incoming signal would be SUBTRACTED FROM the VFO signal to get to  the 455 kHZ IF.  And when that knd of subtraction happens, we have sideband inversion.  The LSB signal will look like a USB signal when it reaches the filter. 

My BFO was running right at 455 kHz, using a ceramic resonator at that frequency. I briefly considered just shifting it down to 452 kHz, but this proved to be difficult.  Then I got a better idea. 

I could just shift the VFO down to 6545 to 6845 kHz.  This would mean that the VFO frequency would be subtracted from the incoming modulated frequency.  There would be no sideband inversion.  I had been thinking about doing this frequency shift anyway, thinking that VFO stability gets better as you go lower in frequency. 

REMEMBER THE RULE:  If you are subtracting the modulated (signal) frequency from the frequency of the local oscillator or VFO, only then will you have sideband inversion.  See: 
https://soldersmoke.blogspot.com/2015/05/sideband-inversion.html

Moving the VFO was easy. I am using a variable capacitor with several variable caps on the same rotor.  I just moved from the smallest variable cap to the middle variable cap -- this added capacitance to the system and lowered the frequency.   I also added three additional turns on the coil.  This put me very close to where I needed the frequency to be.  I added one additional 9 pf cap and this put the VFO freq right where I wanted it. 

I was really  glad to include Pete's filter in this receiver.   The mechanical resonator technology fits very well with the very mechanical old-tech theme of this project (it already had a gearbox -- a mechanical filter seemed to fit right in).   It is a fascinating device -- it is almost like having a set of tuning forks all tuned to 455 kc (see above for the RSGB description of how it works).    And having it from from Pete adds a TREMENDOUS amount of mojo, juju, and soul to the new machine. 

Icing on the cake: As I type this, I am listening to Fred K3ZO converse in Spanish with hams all through South America.  Fred preceded me by three decades at the U.S. Embassy in Santo Domingo, but when I got there the local hams were still talking about him  -- he was much loved and admired by the Dominican hams.  TRGHS. See Fred's story here (scroll down a bit): http://www.gadgeteer.us/DRDISP.HTM 

Solar Cycle 25 -- The High Frequency Oracle Has Spoken (THFOHS!)

Not quite as authoritative as a spoken message from the The Radio Gods themselves (as in TRGHS), but the HF Oracle seems to be well connected.   Time to start planning for Moxons and Hex Beams!  

Read the full poetic report from the Oracle here: 

https://forums.qrz.com/index.php?threads/the-hf-oracle-prognostication-for-solar-cycle-25.626433/

National Dial and Gearbox Problem Resolved

Thanks to all who offered advice and assistance.  With help from you guys -- and  especially from Pete Juliano N6QW -- I think I have this fixed.

Pete was right -- the problem was really with the variable capacitor.  The one I was using was kind of stiff and irregular in its motion.  I found another one in the junk box that was easier to turn (it has one set of ball bearings).  This fixed most of the problem.

I also spent more time making sure the shaft of the cap lined up perfectly with the shaft of the gear box.  This also helped a lot.

I realize now that some of the "stickiness" that I occasionally feel while tuning may be coming from the dial -- not from the gear box.  It looks like my dial took a hit that slightly bent one portion of it.  It seems that the numbers have a bit of trouble clicking over on that portion of the dial. Some lubricant may help there.  But I can live with it.

The receiver now tunes very smoothly and I can go right back to a frequency and find the signal exactly where I left it.  There does seem to be a very slight difference depending on whether I "approach from above" or "approach from below" -- but this is not a big deal.

Check out the pictures of the receiver and the VFO.  Note the "cardboard from a coat hanger" coil form.  The winding is held in place with nail polish varnish.  I had planned on having the variable cap, the coil , and the associated fixed caps all in a box for better thermal stability, but the VFO is very stable even without the box.  I have the VFO running 455 kc ABOVE the signal frequency.  It runs from about 7455 kc to about 7800 kc.  I could have set it up to run 455 kc BELOW the signal freq.   That would have made it a bit more stable (it is easier to attain VFO stability at lower frequencies) but VFO is so stable that I probably won't mess with it. I followed DeMaw's rules:  Physical stability, NP0 caps.  For the NP0 caps, put several of them in parallel to get the desired capacitance value. Keep heat-producing active components away from the coils and caps.   

Thanks again to Armand WA1UQO for giving me this amazing piece of radio history. And thanks to Tim Sutton for the big box that holds this receiver. 

James Millen knew what he was doing.  See: http://www.isquare.com/millen/millen-page.htm

M0KOV's Real Life Dilbert Moment -- His Mom Took Him to the Doctor

M0KOV's BITX

Dilbert's mother took him to the doctor because of ham radio.  Jean Shepherd's date said that Shep's mother should "take him to a doctor" (he was obsessing over his Heising modulator.)  And now we learn that Steve M0KOV was hauled into the doctor's office because of his "obsession with electronics."  You are in good company Steve.  The Knack diagnosis is confirmed. 

From Steve M0KOV: 

Just watched young Dilbert at the doctor's surgery.

Although I was late getting my ham licence, I'm sure that I fit the standard knack victim mould. I built my first radio at the age of 10 and even before then I preferred to be bought batteries, switches and lamps rather than sweets. Within a couple of years my small bedroom comprised of a bed, somewhere to throw my clothes and an electronics work bench. The bench and  floor were completely covered with half built electronic projects, ex military radios, tools, my beloved old Heathkit oscilloscope and the rest. 

Now, my true Dilbert moment. I remember being in the family doctors surgery and my mother was discussing my inability to get to sleep (a perfectly normal ailment for a 13 year old male). She was voicing her concern that it might be my obsession with electronics,  and it was all going round in my head and keeping me awake. Funny, later in my life she never seemed to be bothered if the worry of studying or exams kept me awake.

Thanks for another great podcast.

73 de Steve M0KOV

SolderSmoke Podcast #209 SKN, Old Rigs, Movie Review, Safety Tips, Mail Bag

SolderSmoke Podcast #209 is Available: 

13 January 2019

http://soldersmoke.com/soldersmoke209.mp3

An HT-37 "With Presence!"  Even on CW!  A Straight Key Night Story
Saving a much-loved HT-37 
Drake 2-B Alignment
Tweaking the Mate for the Mighty Midget
Fixing a Deaf DC Receiver
A Wobbly National HRO-ish Gear Box 
Minimum Discernible Signal Tests
Pete's Festive Holiday Transceiver in a Bottle

Movie Review:  "First Man"  Neil Armstrong goes to the moon. 

Safety Tips for 2019:  Fuses, Hard Drives, Flu Shots. 

MAILBAG: 
Dave G6AJW builds Pete's Sudden Transceiver
Rogier KJ6ETL builds a new shack
Steve N8NM Helps save the HT-37
Jim AL7RV W8NSA builds Parasets
Dave AA7EE puts a beacon on the air -- Please Listen! 
Jan OM2ATC builds and documents an Si5351 VFO
Bruce KC1FSZ Homebrew's a BITX
KB1GMX's Tip on IRF-510 Oscillation Prevention
Ryan W7RLF Homebrews a DC Receiver

Listening to August K5HCT from the East and West Coasts on 40
Tim WA1HLR rebuilding his 1968 transmitter.  

Old Station On-The-Air -- A Transmitter Half as Old as Radio Itself

I'm biased, but I think it is "a thing of beauty."   With some unexpected free time available, today I followed up on my HT-37 work by cleaning and aligning the Drake 2B.  I was assisted in this by one of the many great YouTube videos by Alan Wolke W2AEW.   

The Drake 2B's 50 kc and 405 kc oscillators were right were they were supposed to be -- no adjustment required.  And I don't think they have been peaked since 1973.  The 455 kc IF can was also very close to perfectly tuned. 

Since most stations from the mid-1960s had some QSL cards on the wall behind the rig, I pulled out some old ones and put them up.  Many of these contacts were made by the rigs in the picture. 

If we say that 1897 marked the first use of radio, and if we assume that this HT-37 was manufactured in 1959,  that means that my old transmitter has been in operation for almost HALF of the history of radio.  

The AL7RV (W8NSA) Paraset, Regens, Test Gear, and Schematics Varnished into Enclosures

Jim W8NSA ex AL7RV is an old friend and a really talented radio builder.  Over the years he has sent me many boxes of very FB radio parts.  I'm sure that almost all of my homebrew rigs have parts from Jim somewhere in the circuitry.  Jim recently sent in an update on his radio building efforts. 

Bill,

I managed to cobble up some regen's over the summer.  Most fun was my version of Dave Richards, AA7EE  Sproutie, it turned out so nice I have it on my bedside table. Yes, the Sproutie has displaced the trusty old Transoceanic, brings back memories of late night SWL'ing with my Space Spanner as a teen.  

The Sproutie worked well enough I'm considering a build of the Dave's Sproutie MK II, maybe not in such a grand cabinet or with such fine metal work as Dave's but a functional copy.

I also built the regen receiver from the SOE Paraset design, other then a coil winding mistake the receiver worked well at power up. 

I built the Paraset receiver to test a number of mods others have suggested to improve the receiver's performance.  Using the mods involves modern solid-state components and additional band-spread functionally. Many consider the mods be in poor taste but they go well making a much better receiver.

I am ready to build my copy/version of the Paraset, now that I've missed SKN I have almost a year to get it built.
  

I've been gathering Paraset parts for years, was lucky to get in on the group purchase for a set of 3 Paraset knobs made from the same molds by the same company SOE used when building the transceivers in the 1940's, having a real part of that history is exciting, to me at least. 

Paraset Parts (see picture above)  (some parts shown may not be in the proper location)

At last I have all the major components.  I am at the point of gathering up the various resistors and small fixed capacitors needed. Some builders have gone to the efforts of embedding newer, smaller, parts in the carcass of vintage 1930s & 40s parts hollowed out and repainted.  Too much effort for me, I want a functional radio, not a museum grade replica, but I really do want to keep the look of the SOE issued set.

Case and front panel...

Many of the U.S. Paraset builders are using a wooden craft box from Hobby Lobby, It is just about the correct size and shape.  You can see at the right side of the photo a detailed panel layout.  Building the Paraset is not quite Heath Kit but with so many talented builders having made so much information available I feel so lucky to follow the effort. 

Years ago, during our RV travel days, I picked up a box of Bliley AX2 40 meter crystals.  Good price and just about every one of the crystals were found to be working.  Have a total of eighteen 40m crystals from 7.0024Mhz to 7.0986Mhz, have two 80m crystals in the CW portion but none of the AX2 crystals are for the traditional QRP frequencies, close but...   AF4K and others are selling HC49 crystals for the QRP frequencies, problem is the 6L6/6V6's used in the Paraset are known to be real crystal crackers.. (as was my 1964 novice rig!). 

I'm considering the use of a transistor oscillator and driver stage to feed the 6L6 as a PA in effort to save the crystals and maybe allow the use of an outboard DDS VFO, yes I can hear the moans of dismay from other Paraset replica builders

The box included a number of sockets and crystal adapters too.  Was great find. Having so may 40m crystals I'm starting to have thoughts of optimizing the Paraset build for just 40 meters. I can improve the sets performance but of course lose 80m and possibly 60m.

Today's project while catching up on SolderSmoke Podcasts is a test device to aid in checking the resonant frequency of L/C networks. It's always nice to prove your math is correct! 

See: http://www.agder.net/la8ak/m11.htm

Building it Manhattan style into an old rat shack plastic box.

I have always loved the way WW II equipment had the schematics varnished into the covers or cases.  I try to do the same with most of the things I build.

Schematic reduced to fit, ink-jet printed. Using an old credit card I spread a very thin layer of E6000 clear adhesive on the case, apply the paper image, then using a clean credit card or the edge of piece of circuit board feather the schematic into the adhesive. 

Let dry for about 10 mins, wipe off the excess glue around the edges.  Let dry for 24  to 48 hours and cover with a few thin coats of Testors Clear Lacquer Overcoat.. several thin coats, follow the recommendations for re-coat time on the spay your using.

Please read the cautions for using E6000.  Pete may not be able to purchase it in his state!

The Paraset antenna output connections use a porcelain crystal socket, the AX2 crystal purchase included a few dead or empty crystal cases, I modified two of them to use as adapters.  

OH! and yes, I'll be using outboard low pass filters with the Paraset! 

Thanks again to you and Pete for the blog and smoke efforts!

73,  Jim W8NSA aka AL7RV