Solid-Stating an HT-37 VFO -- Advice Needed

 

Original HT-37 VFO Circuit

A couple of things before I start:  

First, this is not my fault.  The Radio Gods are to blame.  I innocently tried to by an HT-37 tuning capacitor on e-bay, but the seller sent me the entire VFO unit.  The only thing missing was THE TUBE. Clearly, that was a sign, right?  

Second, this is a work in progress.  That is why my diagram (below) is a bit ugly.  I am looking for your input and advice on how I might do this better.  I will understand if religious principles prevent some of you from participating in this endeavor. 

I am trying to solid-state this device WITHOUT major surgery, and without adding any reactive components that would change the resonance or tuning range of the original.  The original circuit tunes from 5 to 5.5 MHz and that is fine with me.  

I started out by just sticking a J-310 FET into pins 1, 2, and 5 of the tube socket.  I put 12 V on the drain and the thing oscillated right where it is supposed to.  That was a good sign.  

Here is what I have done so far: 

Bill's initial solid state conversion of HT-37 VFO

Mechanically, my effort has been very simple.  At first I tried to fashion a more serious male socket for the FET using two broken 7 pin tubes.  This didn't work well.   

So then I just ran three short wires up through the center hold of the tube socket to the connections for pins 1,2, and 5.   I superglued the J-301 to the chassis and made some non-reactive connections: I put a 47 ohm resistor on the source,  and a 220 ohm resistor on the drain. I grounded the drain for RF with a .01 uF cap to ground.    I added a 100k resistor and a diode on the gate.   Oh yea, I put a couple of ferrite bead on the FET gate lead.  (See pictures below.)

Three lead up through the center hole

A rare look inside an HT-37 VFO

The original thermatron circuit has an output bandpass transformer, a 3900 ohm resistor and a coupling cap.  I left them in the circuit, but they are not doing anything. 

The output from the source of the FET looks pretty good.  I can see some VHF on the trace, but I suspect this is from my FM broadcast nemesis at 100.3 FM (one mile away).  On a receiver, I can hear some AF noise on the signal, but this may be the result of the RFI from THE BIG 100 -- WASHINGTON'S CLASSIC ROCK. 

So what do you folks think?  What else could I do, or should I do?  

SolderSmoke Podcast #156 -- November 4, 2013 -- Interview with Peter Parker VK3YE of Melbourne, Australia

November 4, 2013
Special hour-long interview with Peter Parker, VK3YE
-- Early experiences with radio
-- CW
-- DSB Gear
-- Simple gear, and gear that is TOO simple
-- VXOs, Super VXOs and Ceramic Resonators
-- Building receivers
-- Chips vs. Discrete
-- Making the leap to SSB
-- The Knob-less wonder and the BITX
-- No need for a sophisticated workshop
-- Advice for new phone QRPers

Peter's Blog: https://vk3ye.com/

Peter's YouTube Channel:  https://www.youtube.com/user/vk3ye/featured

Mike Caughran KL7R's Last Podcast

 

Mike KL7R (SK) during a visit to the AL7FS shack.

This was Mike Caughran KL7R's last podcast. He died in a car accident shortly after we made this program. January 13, 2007. Mike's oscillator work. Michigan Mighty Mite. Lasers, diodes, and Einstein. Laser communication experiment. W7ZOI-KL7R QSO on SKN.M0HBR's feedback amps.The new comet. Saturn, Jupiter and calculation of c. 17 meter QSOs. New SPRAT CD. MAILBAG: China enigma, VE4KEH, M0DAD, GU0SUP, M1CNK, K4AHU, KD4EDM, KG9DK, AA6KI, VA7AT ON5EX

---------------------

Mike's Obituary from the February 2007 ARRL Letter:

Mike Caughran, KL7R, SK: Well-known low-power (QRP) and homebrewing
enthusiast Michael S. "Mike" Caughran, KL7R, of Juneau, Alaska, died January
22 of injuries suffered in an automobile accident in Hawaii. He was 51.
Caughran may be best known as one-half of the team -- with Bill Meara,
N2CQR/M0HBR -- that created and produced the weekly SolderSmoke podcast
<http://www.soldersmoke.com/>. "I think people were drawn in by Mike's
friendly voice and manner," Meara commented on a memorial page for KL7R
<https://kiwi.state.ak.us/display/mc/Home>. A member of ARRL and the Juneau
Amateur Radio Club, Caughran also wrote articles for the Michigan QRP Club's
T5W newsletter and he was an active ham radio contester. "Mike was one of
those people who you instantly like because of his honest, straightforward
and humble way of talking and expressing ideas," said Mike Hall, WB8ICN, who
edits T5W. "His co-hosting of SolderSmoke provided me hours and hours of
enjoyment." Caughran was an IT professional with the State of Alaska.
Survivors include his wife and son.

On our 17th Anniversary: SolderSmoke Podcast #179 -- TENTH ANNIVERSARY SHOW -- A Walk Down Memory Lane

On August 21, 2005 Podcast #1 was uploaded to our old GeoCities host. Just prior to that Mike KL7R set up a Yahoo Mail account.  I still use it. Yahoo sent me an e-birthday card! 

I put our 10th Anniversary Podcast on the YouTube Channel today.  Click above. Show notes below 

---------------------------------

22 August 2015

YESTERDAY MARKED 10 YEARS OF THE SOLDERSMOKE PODCAST
-- A clip: The first minutes of SolderSmoke #1
-- A trip down SolderSmoke memory lane.
-- The SolderSmoke lexicon -- words and phrases we use (a lot).

BENCH REPORT

-- Pete's antenna project.
-- Pete's new Blog: http://n6qw.blogspot.com
-- Bill's big amplifier problem fixed thanks to Allison KB1GMX.
-- Six digit freq readout with an Altoids case.

THE Si5351 PHASE NOISE CONTROVERSY

-- ALL oscillators make noise.
-- Keeping things in perspective: It is 100 db down!
-- Observations and tests from LA3PNA, NT7S, and K0WFS:
http://k0wfs.com/2015/08/21/si5351-phase-noise-and-thd-tests-using-an-agilent-e4402b-spectrum-analyzer/

http://nt7s.com/2014/11/si5351a-investigations-part-7/

-- Try it, you'll like it! The benefits trying things on real rigs.

NEWS
Interviews on "QSO TODAY" with Eric 4Z1UG.
Horrible band conditions.
Looking at Saturn with telescope.

MAILBAG

Another recruit for the CBLA: Paul KA5WPL.
Ron G4GXO on Bell-Thorn and Eden9 SSB rigs.
Rupert G6HVY on Kon Tiki radio and Mr. Spock.
Mikele's Croation BITX rigs.
Dean AC9JQ's TIA.
Bryan KV4ZS will build an LBS receiver.
Dave Anderson give Pete good antenna advice.
Steve Smith moves in from the garage.
Pete has built 12 SSB transceivers. Intervention time?

------------------

TRIGGER WARNING: Solid-Stating Old Tube (Thermatron) Gear (Including -- GASP -- R-390As)

Look at that.  Well, maybe some of you shouldn't. (I'm thinking of you Grayson.)  I found the Charles Smith YouTube channel while innocently looking for ideas on how to solid-state the HT-37 VFO assembly I recently bought on e-bay.  Charles Smith has some really great ideas in this area.  He solid-stated a Heath VF-1.  But he took it all to an extreme when he solid stated an R-390A.   Take a look at how he built the replacements for the thermatrons:  He used those plastic wall sockets that you screw into sheet-rock when you need to hang a picture.  This is real genius. He used the tube filament lines to carry DC to these new sockets. 

R390A Solid State Conversion Video #1:  https://www.youtube.com/watch?v=xhWzX874wYo

Charles Smith's YouTube Channel 

https://www.youtube.com/channel/UCpfh8gxL64qd22RaNPR-QFw/videos

He has videos on the HQ-170 (DEAN:  Just say NO!) and the SP-600.  He also covers the HQ-110, which is uncomfortably close to my HQ-100.  

Who is Charles Smith?   What is his callsign?    Charles Smith is KV4JT.  Here is his QRZ page: 

https://www.qrz.com/db/KV4JT

He has some great humor and wisdom in his videos: Procedures that are difficult or more trouble than they are worth are called "bugger-bears."  He advises that if your IF cans are stuck, you should "find a way to unstick them!"  Indeed you should!  He builds a cool jig to hold the IF section of the R-390A while you are working on it.  He provides similar protection (with two long screws) to the VF-1.  

Even though some of you will have to go to therapy after seeing all this, I say THREE CHEERS FOR CHARLES SMITH! 

SolderSmoke's 2006 Interview with Farhan (and pictures from his 2019 visit)

Farhan in N2CQR shack with ET-2 on bench

October 2019

Here is the YouTube version of the SolderSmoke Podcast #34

 https://youtu.be/q-L4IE9f1aA

Farhan with Bob Bruninga WB4APR (SK) 

https://www.qrz.com/db/WB4APR

With Bill and Abe Lincoln

With Einstein

Correcting an Einstein equation

With Elisa

Tuning Bill's BITX 20

VK3IO's Wonderful Antenna Tuning System

 

I have been ridiculed because my manual antenna tuner (that is screwed into the wall of our car port) requires me to step outside into the cold or heat to change bands. 

On August 18, 2022  -- with my Hex Beam, not with the tuner -- I worked Ron VK3IO on 20 SSB.  Check out Ron's antenna tuning system.  He used the same balanced tuner that I use, but he has them set up for FIVE bands.  FB.  The wire antenna was working very well -- with 100 watts to the wire Ron was booming into Northern Virginia. 

Ron's QRZ.com page: 

https://www.qrz.com/db/VK3IO

Watch out for those spiders Ron!  

Pete N6QW's First SolderSmoke Podcast

May 26, 2014. Pete Juliano's first SolderSmoke podcast. I think we both thought that this would be a one-time appearance. But one show led to another and soon we were in a permanent podcast collaboration. This was a very fortunate development, for me, for the podcast, for all our listeners, and for ham radio. Thanks Pete!

A Blast from the Past: The First SolderSmoke Podcast

I am gradually -- maybe one every few days -- going to put older SolderSmoke podcasts up on the YouTube channel.  Here is SolderSmoke Podcast #1.  This podcast was first uploaded on August 21, 2005. 

Be sure to check out the new Playlists on the SolderSmoke YouTube channel: 

(22) SolderSmoke - YouTube 

Thanks to Ciprian YO6DXE for the wallpaper!  And thanks to Dave K8WPE for the idea of reposting "Old Smoke." 

Daylight Again on the Sunrise Net! Walter KA4KXX Builds a PTO

Dear Bill: 

I had never built a PTO, but after reading Farhan's Daylight Again Transceiver article I cobbled one together with parts and pieces I had on hand. My observations so far are as follows. 

1. The frequency-determining capacitors (shown on the schematic as three 470 pf) are very critical, so I feel the usual experimental cut-and-try technique is a must, even using all NP0 and C0G types. 

2. After I built the 2 MHz version like the article (see first photo), the stability was terrific, but when I tried building the companion Daylight Again crystal filter, I was only able to get a bandwidth of 1.6 kHz, which is too narrow for my taste in an SSB radio, so I decided to build the same filter design but with 11 MHz crystals, where I could easily achieve a 2.8 KHz BW. 

3. Therefore, now I needed a higher frequency VFO, so I merely reduced the capacitance (from about 1200 to 370 pf) without changing the coil and I am very impressed with the performance of my 4 MHz PTO (see second photo). The bandspread easily covers the entire 40M band, CW and Phone. 

4. However, whenever I transmit on the 40M Phone band, I like to first set my VFO within 10 Hz of the operating frequency. That way, if I talk for five minutes or so and get up to 15 Hz of drift (which is quite common with many radios when I operate portable outdoors in the sun and wind), it will not be noticable and I avoid receiving any "you are off-frequency" chastising. But the shortcoming I have with this PTO inductor is that the 1/4-20 bolt has a coarse thread, so it is very difficult for an old fellow like me to get within even 20 Hz of a particular frequency just using this common bolt. Therefore I believe a better choice would be the fine thread 1/4-28 two-inch brass threaded bolt which is available from industrial supply houses like McMaster-Carr. However, for CW use or those with a very steady hand, the 1/4-20 works well enough. 

5. I solved my fine tuning problem by adding a varactor circuit using a common 1N914 diode in series with a 100 pf capacitor, operating from 0 to 6 volts. Another advantage to adding this feature is that since I have not so far enclosed my PTO, I can mount the varactor potentiometer several inches from the PTO so my hand capacitance does not affect the frequency like when tuning with the bolt. 

6. An easy way to "do the math" in my case with the common Sanjian counters is to simply create a small lookup table listing half a dozen common frequencies and stick it on the radio. For example, 90% of the time in the morning I am tuned to my favorite SouthCars Net frequency of 7251, so using a BFO setting of 10,999.900, I simply set the PTO to 3,748.90 on the 6-digit 10 Hz resolution counter I normally use (see third photo). 

7. I am currently using my Daylight Again PTO on a daily basis with an NE602 receiver, and I am thinking of adding a locknut to the bolt so it does not wiggle when I jostle or move the radio, essentially giving me a crystal replacement oscillator that I can use for any single 40M frequency. To date I have been able to listen for hours at a time indoors without even any touch-up of the varactor fine tuning. 

8. Also, if continuous frequency readout is desired without building a noise filter circuit board, a separate power supply for the counter is a solution. For portable operation I use Lithium Polymer radio control model airplane batteries which are light, small, and cheap, so one 12V 2000 mAH battery for the transceiver (allows a half hour of transmitting at 15 watts) and a much smaller 12V 350 mAH battery with a series resistor to reduce the current and brightness of the counter has worked well for me. 

73, Walter KA4KXX 

Orlando, FL

Help SolderSmoke! Playlists for SolderSmoke YouTube Videos

In response to popular demand, I am putting up YouTube Playlists for SolderSmoke videos.  

Most of these lists are rig or project specific.  For example, my Hammarlund HQ-100 has its own playlist. 

But there is also one MASSIVE playlist with about 214 YouTube videos.   This one is especially good if you just want to keep SolderSmoke videos playing in the background as you work on rigs in your hamshack.  This video will also cause a big increase in the "SolderSmoke hours watched" metric of YouTube. 

Here are the Playlists (more will be added over the next few weeks): 

(20) SolderSmoke - YouTube

Here is the big Playlist with 214 SolderSmoke videos: 

(20) Polyakov Direct Conversion Receiver on 80 meters - YouTube

What Coil for the Polyakov Input Circuit? How to calculate a coil value for resonance.

So,  what is the value for L1 and L2?   What coil should I use?  

Michael AG5VG had that question.  And so did I when I built this receiver.  See below for the process I used in answering this question. 

On Tuesday, August 9, 2022 at 10:53:32 PM EDT, Michael S  wrote:

Good Evening Bill,

My name is Michael and I really enjoy your podcast with Pete. I have also spoken with him in regards to the design of a 20M bandpass filter I made for a homebrew rig.  I am currently in the process of making a 20 meter DSB - SC type. Thank you for all the information that you speak and teach about during your podcast. I also enjoy the humor. It's great.

The Polyakov is a simple DC receiver and it amazes me and how the sound quality is. My question is, what is the turns on the toroid for the antenna primary side and the radio secondary and how did you figure out the turns because looking at the schematic it doesn't give that information that I can see. Also how you resonated it with the variable capacitor that looks like a 365pf air variable. 

Thank you for your time and keep up the great work on the podcast and the content on YouTube.

73s,

Michael

AG5VG

My response: 

Good questions Michael.   When I saw the SPRAT article I too was struck by the fact that it didn't give a value for the coil.  But DK2RS did have a large value variable capacitor... And he was billing this as a dual-band (80-40) rig.  So I figured he wanted that LC circuit to resonate as low as 3.5 MHZ and as high as 7.3 MHZ.  So, with a variable cap that goes up to 350 pf, what value L should I use?   I started by calculating the resonant frequency of the frequency mid-way point: 5.1 MHz.   I figured the variable cap should be around 162 pf at the mid-way point.  At this point I went to the on-line resonant frequency calculator: https://www.1728.org/resfreq.htm (a REALLY useful site!).   This site revealed I needed a coil of about 6 uH.  This put me in the ballpark.   But then -- with the site -- I tested it with the values of the variable cap I had on hand.  Mine was 23pf to 372 pf.  (you really need an LC meter to do this kind of thing). 

Again at the resonant freq calculation site:  23 pf and 6uH = 13.5 MHz           372 pf and 6 uH = 3.3688 MHz

This would have been OK, but I wanted to move the frequency range down a bit, so I tried. 6.5 uH 

23 pf and 6.5 uH = 13 MHz                 372 and 6.5uH = 3.23 MHz

Now, how many turns?  First look at the overall coil -- don't worry about taps at this point.    I use the Toroid Turns Calculator: http://toroids.info/

 Start by asking yourself "What core do I have on-hand?    Let's say you have a T-50-2 (red/clear).   The calculator shows you need about 36 turns.  Do-able, but physically kind of tight.  

I found a big core in my junk box.  A T-106-2.  The calculator showed I'd need about 22 turns on this core. It was much easier to get these turns on the larger core.  

You have to measure the core after you wind it to make sure you are at the desired inductance.   One side of the main coil went to ground, the other side to the top of the variable cap. 

 Now for the taps and secondaries:  The schematic shows a tap.  This is usually about 1/4 of the number of turns up from ground.   I picked about 5 turns, and wound a little tap in there at that point -- that tap went to the antenna.  You also have a secondary coil --no value is given, but based on experience I guessed around 5 turns -- I wound these turns on top of the primary one lead went to ground, the other went to the diodes and the switch. 

 The last thing to do is to see if the circuit resonates on both bands that you want to receive.  You can do this with a signal generator, or with the band noise:  Hook up an 80 meter antenna.  Put the cap closer to its max value and tune the cap -- can you hear band noise?  Or can you hear (or see on a 'scope) a signal at 3.5 MHz?   You should be able to peak it with the main cap.  Try to do the same thing on 40 meters -- here the variable cap should be closer to minimum capacity. 

 That's it.  That's how I did it.   You can do it too!   Good luck with the Polyakov.  

 One hint:  Building the VFO is the hard part.  You can get started by using a signal generator in place of the VFO.  Just make sure you have the level right -- around 620 mV input.  

 Good luck -- Let us know if you have trouble. And please let us know how the project goes.   

 73    Bill    N2CQR  

20 meter rig built by AG5VG