The Scrounger A Ham Radio Transmitter Project

 

“The Scrounger” Ham Radio QRP Transmitter Time Span 1964-2012 By WB4CKM

When I was 12 I built this CW ham radio transmitter that was described in the September 1964 issue of Electronics Illustrated magazine. The article is available here, here, from KH6SKY of RadioSky I still have several cards from contacts madePublishing. with this set on 40-meter band, QSL at 7195 kHz. Back then, I the Novice band was 7150-7200 kHz. June 23, 2011- Yesterday and today I started restoration of my Scrounger transmitter; 1. I discovered by measurement that my grid leak resistor was actually marked as an 820 resistor instead of the required 82k . It measured today as 1300. The brown color band looked more orange than brown, and I never measured it in my youth. Amazingly, the set worked anyway. I found and installed an unused and good 82k resistor from my parts bin. It measured 82.5 k.

 

2. The dual section power supply filter capacitor unit had signs of electrolyte leakage and neither section measured correctly (supposed to be 20uF @ 450 volts per section). I removed the bad unit from its cardboard sleeve and replaced it with two good 22uF units that I salvaged from an abandoned TV restoration that I did several years ago. 3. I removed the RF output coil assembly and its supporting terminal strips, and performed a light cleaning of the outside of the aluminum chassis, using only damp and dry paper towels. July 22, 2011 4. A couple of weeks ago I applied Radio-TV cement to weak/damaged spots of the toilet-paper cardboard tube of the RF output coil assembly. I restored and reconnected the original RF tap point for the keying monitor (which I had originally built, but it did not survive my youth, except for the speaker). I reinstalled the coil assembly. 5. Today I did these things;

  Replaced the sheet metal screws for mounting the two RF output jacks. I slightly enlarged the four chassis holes so they would pass #6-32 screws. I did this to ensure solid connection to chassis of the shield side of the jacks.   Replaced the broken rubber grommet that was protecting the AC power cord. To do this I had to unsolder and remove the power cord, install new grommet, and re-install power cord.   Made up a test cable for the telegraph key. It includes a DC 0-150 ma meter in series with the key, so I can measure cathode current rather than rely on the pilot lamp. [Aside]The pilot lamp is supposed to be a #1819 (28 volts at 40 ma.), but I remember that the #1819s kept burning out until my friend Vincent Van Pelt, WB4BPS, suggested I put 1k  in parallel with the lamp. Sometime in distant past I eventually removed the 1k and substituted a #1820 lamp (28 volts @ 80 ma.).

 

TEST RESULTS 23 JULY 2011 50 load “Cantenna” attached to RF output.  output.  No connect to keying monitor output. Crystal None 3625 3725 3745 7195

kHz kHz kHz kHz

Dip Current 25 22 25 20

Least Chirp 30 30 30 25

Peak Current 70 40 45 60 70 (not oscillating)

The 80M crystals yield double dips, the 2 nd dip being for the 1st  harmonic of crystal frequency (as expected and explained in original article). The least keying chirp, as observed in SW receiver, occurs when RF tune is slightly clockwise from dip point (more capacitance). With the 3625 kHz crystal, oscillation occurred regardless of the RF tune setting, and DC cathode current did not exceed 40 ma. Therefore the #1819 lamp would not burn out unless the crystal was to be unplugged and transmitter is keyed. With the 7195 kHz crystal, oscillation always stopped when RF tune is somewhat clockwise of dip point. Cathode current rose to 70ma, which would quickly burn out a #1819 lamp. With other crystals the results were in between these two. The article mentions that “flash” of pilot lamp, while tuning RF with key down, is due to excitation trimmer not set correctly. I believe this flash is same as “no oscillation” that I observed above and explains why my #1819s kept burning out. I tried unsoldering a 10 mmF capacitor that was across the trimmer, that I had installed ages ago, but it didn’t make any noticeable difference to cathode current behavior vs. RF tuning. I connected my Fluke digital VOM to RF monitor port of the Cantenna and observed peak of about 130 mV (approx. 1/100 of RF voltage across the 50 load.

 

December 29, 2011

  Rewired the pilot lamp socket to use the #1819 lamp in parallel with a 1k resistor.   Resoldered the AC line connections to the ON-OFF switch. One was loose.   Built spreadsheet power estimator; RF LOAD POWER ESTIMATOR LOAD IMPEDANCE

50. 00 OHMS

FLUKE VOLTAGE READING

0. 128 VOLTS DC

VOLTAGE DIVIDER FACTOR ESTIIMA EST MAT TED PEA PEAK K VO VOLT LTAG AGE E AC ACR ROSS OSS DUMM MMY Y LO LOAD AD

101. 00 12.93 2.93 VO VOL LTS PEA PEAK K

ESTIMATED RMS VOLTAGE

9. 14 VOLTS RMS

E ST STIMA TE D P O OW W ER ER DELIV E ER RE D TO DUMMY LOAD

1. 67 67 W A AT TTS

I do not yet have a tuner to match the RF output impedance to the load, so I am not bothered by the apparent low power delivered to the dummy load.

Key Down, 50 Dummy Load, Tuned, but not Loaded

 

Companion Keying Monitor January 1, 2012- Several months ago I gathered the parts to rebuild the companion keying monitor. I breadboarded the circuit today so that I could experiment with selecting secondary taps for the speaker. The article failed to give a part number for the PNP transistor Q1. I chose an SK3005 that I happened to have. I think I might have used an SK3004 in my original unit. Despite the admonition in the parts list, I must substitute a Thordarson 22S61 universal output transformer for T1, as I no longer have the Lafayette TR-12 transformer.

Keying Monitor Detail from Original Article

 

 

Terminal Strip will Mount to Rear Section of Enclosure Battery+ goes to ground, and battery- must be attached at the RF input. I used a 1.5 volt battery to power the unit for testing. It would not work with a 3 volt battery, and I don’t know why. The unit will not work without the diode in the circuit, even on battery. I don’t know why. I settled on pins 2 and 5 for 5  for attaching the speaker. The output tone messes up if you touch the speaker cone or let it rest on a surface. I don’t know why.  why.  

Test Setup; Apply Battery- at RF Input Point I used ClarisDraw 1.0v3 to make drill templates for the enclosure.

 

4"

1 1/4"

2 1/4"

1 1/4"

  Front Panel 3 7/8"

1 1/2"

1 1/16" 3/4"

2 1/8" 1 15/16" 3/4"

Rear Panel (RF Input Cable, Terminal Strip) January 2, 2012- Here is a photo of my completed keying monitor (next page). The terminal strip is mounted on the backshell. The transformer is mounted on the left side of the frontshell. The speaker grill is plastic window screen material. The monitor sounds great with 1.5 volt test battery. RF test is pending. April 5, 2012- The keying monitor did not work properly unless I greatly detuned Scrounger (i.e., reduced the RF output power). With maximum RF power I measured the rectified RF voltage, at the 1N34 diode, as about -10 volts (a very good indication that Scrounger is delivering significant RF power to the antenna tuner). As I noted earlier in this report, the keying monitor sounded best when the applied DC was around -1.5 volts (for speaker connection to pins 2 and 5). I decided to install a 500   potentiometer (Honeywell #381N500) on back of Scrounger so that I can adjust the RF voltage supplied to the keying monitor. This

 

worked, sort of. The sound volume and tone quality vary depending on crystal and output band (80M or 40M). May 24, 2012- I decided to use secondary tap positions 4 and 5 for the speaker connections. This change made the keying monitor performance acceptable when plugged into Scrounger. See Appendix A for a detailed discussion of the universal output transformer turns ratio combinations. Impedance ratio is the square of the turns ratio.

Completed 2012 Build of Scrounger Keying Monitor

 

Antenna Tuner for Scrounger January 3, 2012- The Scrounger article referenced the September 1963 issue of Electronics Illustrated (which I also used to have, but not anymore) for an antenna tuner design, to be used to match the Scrounger’s Scrounger’s output  output to random-length longwire antennas. I remember that it was just a simple L-network;

ANT.

XMTR GND. (OPTIONAL) I will construct one from parts that I already have; E.F. Johnson #229-203 E.F. Johnson #250-E30

28 H 250 F

Rotary Inductor Air Variable Capacitor

I used a discarded wooden wine case as the enclosure. I left room on the right side of the case for adding a power meter. My Scrounger can sit on top.

L-Network Antenna Tuner Enclosed in Wooden Wine Case

 

Interior View of L-network Antenna Tuner

 

January 26, 2012- Today I tested my Scrounger with the Antenna Tuner. I rigged up an outside long wire antenna, approximately 50 feet or so. One end of it attaches directly to the tuner output. I was able to tune and load Scrounger to about 40mA nd

plate current at 7250 kHz (2  harmonic of my 3625 kHz crystal) as described in the article. My R-174 receiver, with ANT and GND posts shorted, served as a crude field strength meter and indicated that I was delivering significant RF power to the antenna. There happened to be a group of hams on SSB (identifying (i dentifying themselves as “WESCARS” net) slightly below 7250kHz. According to their website, they meet daily at 11 a.m. on 7248 kHz. Update April 19, 2012- Today I determined that my wire antenna is actually 62 feet long. The wire from the tuner to the T/R switch is 3 feet 2 inches, for total antenna length of 65 feet, 2 inches. A ½ wavelength antenna for 7.04 MHz is 468/7.04 = 66 feet, 5 inches. No wonder my antenna is working so well.

 

Station Assembly and Transmit-Receive Control I am almost ready to use my R-174 receiver and Scrounger together. I need only to wire up a DPDT transmit/receive knife switch and an old 12 volt power supply to achieve the antenna changeover and R-174 receiver operate/standby functions, as follows; LONGWIRE ANTENNA

H

R-174 ANT POST

J

 ANT

FROM TRANSMITTER ANTENNA TUNER OUTPUT

F K

A

E

+ A D C

B

MONITOR INPUT VIEW FROM FRONT

R-174 MONITOR INPUT CONNECTOR

RECEIVE

TRANSM IT IT

12 VOLTS DC

E C

February 14, 2012- Since the R-174 monitor input plug is not available, I made three push-on push- on terminals using 3/32” copper tubing. February 16, 2012- My T/R circuit works fine. Now I should be able to communicate. I still need crystals or a VFO for the current CW sub-bands, which are different from what they were in the old days.

My Ham Radio Station, March 16, 2012

 

My Old Crystal Collection My 80 meter band crystal collection is mostly based on the old Novice band from 3700 to 3750 kHz; 3 500 3525

3 55 0

360 0

365 0

37 00

37 50

3 800

3 850

3 90 0

395 0

40 00

EXTRA  ADVANCED         0         1         7         3

        5         2         6         3

        5         0         0         2         3         2         7         7         7         3         3         3

        2         9         8         3

        5         0         4         4         7         7         3         3

GENERAL

OLD

NOVICE

NOVICE BAND

MY CRYSTALS VS. DECEMBER 2011 80 METER BAND ALLOCATIONS

I only have one 40 meter band crystal at 7195 kHz. The old Novice band was from 7150 to 7200 kHz. I can do 7250 kHz by using my 3625 kHz crystal and tuning to the 2 nd harmonic, just as described in the Scrounger article. 7000

7050

7100

7150

7175

7200

7250

7300

EXTRA

 ADVANCED         5         9         1         7

        0         5         2         7

GENERAL

NOVICE OLD NOVICE

7025

7125

BAND

MY CRYSTALS VS. DECEMBER 201 2011 1 40 METER BAND ALLOCATIONS

February 24, 2012- I am trying CQs at 3725 kHz, which is in the voice or CW sub-band. March 17, 2012- First contact was yesterday evening on 7195 kHz. I was trying BK to a group QSO on SSB, and N5DCG responded and correctly read back my call sign. I bobbled his call sign (I thought N5CDG) and was too flustered to operate the key properly. I tried again a short time later but they thought I was someone that they know who was trying to annoy them. I resolved the call sign using FCC on-line database. He is in Longview, Texas, about 1400 miles from here! Second contact this morning was WJ6CM, Tom in Huntington Beach, CA (about 40 miles). He had been calling someone else on SSB, but then graciously responded to my BK by using CW. We had a

 

short two-way CW QSO. I could copy about 60% of what he sent due to both of us a little rusty on Morse code. March 24, 2012- KH6SKY kindly donated a CW sub-band crystal to me. I received it yesterday afternoon. Last night KJ6LB in Coos Bay, Oregon answered my CQ on 7037 kHz. That’s 713 miles according to this site. site. He reported my RST as 339. His was 239, lots of QRM and very difficult copy, but I did manage to copy his QTH and the RST.

I am now all set to work 40M CW with my Scrounger-based ham station, thanks largely to the inspiration and help from KH6SKY.

 

Some QSL Cards from Contacts I made with Scrounger From 1966-67 QSOs (showing front and back);

From Spring of 2012 and my station as described in this report;

 

April 20, 2012- Additional Operating Notes To minimize chirp, I do this; after loading to almost 40 ma cathode current at the dip, I off-tune the plate tank resonance to slightly lower frequency (more capacitance). The cathode current increases very slightly relative to the dip point, but it is worth it to minimize chirp. Since I started paying attention to chirp I have been getting T=9 in the RST reports. The antenna loading procedure using the L-network seems to work OK, as I expected it should for end feed of a ½ wave wire. The antenna tuner settings that seem to work well are as follows; Inductor; Capacitor;

½ turn Clockwise from Counterclockwise Stop 42 on the dial

My antenna wire is strung from slightly west-southwest (WSW) to east north-east (ENE?) direction. Below is a map showing the locations of the first few distant contacts that I have made on 7037 kHz with my station as described in this report. Coos Bay is 713 miles from my QTH.

NOT THE END

 

December 3, 2012 More Testing Results I resoldered a loose connection (the orange wire) on the excitation control trimmer. I connected a 15 watt light bulb to the antenna tuner output for the following tests. This may be the first time that I have attached a light-bulb dummy load to Scrounger, although I vaguely recall having possibly done so using a 7.5 watt bulb in my youth. I tested my five C-W brand 80-meter band crystals with the dummy load, and I also explored the effects of the excitation control trimmer. I used my R-174 receiver to monitor the signal. I discovered that with 80-meter band crystal I can set the trimmer such that Scrounger will remain in oscillation regardless of the setting of the plate tank tuning capacitor. This behavior is consistent with the original article. I have never observed it when using my 40-meter crystals. This could explain why I had to add the 1kW resistor across the #1819 indicator lamp, when the author of the article did not have to do so (He never said that 40-meter crystals would work in this circuit). When the circuit drops out of oscillation the plate plus screen currents rise to about 70 ma which always burns out the #1819 lamp. See also my 23 July 2011 testing notes. The 3720 kHz crystal was more finicky than the others, but I was able to find a trimmer setting for it that also worked OK for the other four crystals. The trimmer behavior was as follows;

  Increasing the trimmer capacitance causes the crystal output frequency to decrease, while decreasing capacitance causes the crystal frequency to increase.   Decreasing trimmer capacitance causes the RF output power to increase (dummy load gets brighter, up to a point), but the 3720 kHz crystal did not like too little trimmer capacitance (would not stay in oscillation when plate tank was off-tuned, or would not double to 7440kHz). After adjustments the antenna tuner coupled enough power to light up the dummy load to about 1/4 to 1/3 of full brightness. I compared the brightness to that obtained using step down transformers, to 30 volts and 46 volts, from the house 120 volt AC line. The maximum brightness could getThe from Scrounger’s RF output was comparable to the 46 I volt case. 30-volt

 

brightness was definitely less than I got from Scrounger. I can only guess at the resistance of the light bulb since it varies from 85 ohms cold to 960 ohms at 120 volts (15 watt rating). If we say 400 ohms at 46 volts, then the power output was 5.3 watts, which sound very reasonable! I tested all 5 C-W crystals at their 2 nd harmonic outputs, 7420kHz, 7440kHz, 7450kHz, 7460kHz, and 7480kHz, which are of interest for other projects I am planning. They all worked fine after setting the trimmer properly.

 

Appendix A Thordarson 22S61 Universal Audio Output Transformer (The Triad # S-62X is identical to this model.) I used the impedance ratio data for this transformer to determine the primary-to-secondary turns ratios for the various secondary tap combinations. I present the data two ways; Way 1: Taps Total Primary To Secondary 2-3 397:1 4-5 125:1 1-2 85:1 1-3 70:1 3-4 56:1 2-4 50:1 3-5 39:1 2-5 35:1 1-4 31:1 1-5 25:1 An easier way is to express tap combinations as fractions of normalized total (1-5)secondary turns, i.e., taps 1-5 = 1; 1-2 .29 2-3 .06 3-4 .45 4-5 .20 1-5 1.00 For the Scrounger’s Keying Monitor, I ultimately chose pins 4-5 4 -5 to attach the speaker. The highest volume I tested used pins 2 and 5, which was too loud, except when using a 1.5 volt battery instead of Scrounger. The Thordarson factory data sheet for this transformer is in my Scrounger manila folder. I do not yet have scanned version of the data sheet.