On Saturday, January 20, the ARRL held its semiannual Board of Directors meeting. On the agenda was the AM power issue. The Board voted to ask the FCC to retain the existing AM power privileges, effectively making the grandfather clause permanent. Reportedly, the vote was 11 to 3 with one abstention. One Director contacted by AM P/X said he was not aware of any serious debate in opposition to this action. Full details will appear in the minutes of the meeting, to be published in March QST. At this time, no details are available on exactly what form the petition to the FCC will take, nor when it will be submitted.

Of course, the battle is not yet won. This is only the first step in the process. We must remember that the FCC does not always grant requests, even from ARRL. You will recall that the League originally requested permanent grandfathering of the 1 KW DC input limit in its initial comments to Docket 82-624. The League also failed to convince the FCC to call off its ill-conceived 10 metre amplifier ban. So far, the League seems to have met with little success on the 220 mhz issue. Nevertheless, we have overcome the initial hurdle; the ARRL Board has committed itself to support us. We hope to have more complete details on the Board's action in February AM P/X.

One note of caution: up to now, discussion of this issue has been essentially confined to the AM community; it has been almost completely ignored by the mainstream amateur radio media. Once this news hits the greater amateur radio community via W1AW bulletins, ARRL Letter and other amateur radio newsletters, and ultimately in QST and the rest of the popular ham magazines, this issue will be out of the closet and the anti-AM element of amateur radio will be alerted, and the League will undoubtedly feel some pressure to reverse this decision. It wouldn't be a bad idea for all AM'ers, ARRL members or not, to contact the Directors of our respective Divisions and express our appreciation for their support. As word of this Board action already began to spread, several non-member AM'ers were heard over the air expressing a willingness to take out ARRL membership if the League follows through on this decision! If you include yourself in this category, why not drop your Division Director and League Headquarters a line and let them know how you feel.

Our first, immediate reaction to A.R.R.L. should be POSITIVE REINFORCEMENT - for two reasons: to assure League officials that there are hams out there who believe they made the right decision, and to counteract any upcoming anti-AM backlash, once this news reaches the mainstream amateur radio press.

This article expands upon my earlier articles in the June and July 1988 P/X on asymmetrical peak limiting and bass phase adjustment. An improved version of the asymmetrical peak limiter (APL) is presented, which offers bass phase adjustment and high quality variable clipping as well.

PEAK LIMITING

This version of the APL has an asymmetry adjustment. The peak modulation level will depend on the waveform and the degree of asymmetry selected. If you set up for 2:1 asymmetry, with 200% positive and 100% negative modulation, then sine waves, sibilants and whistles will modulate to 100% negative, and will typically not exceed 100% positive, because they are symmetrical. With the same settings, an asymmetrical sound waveform such as your voice could, if properly polarized, modulate up to 200% positive.

BASS PHASE AND VOICE ASYMMETRY

A variable phase equalizer in this APL advances the phase of the lowest frequency components of a voice signal in order to optimize the polar asymmetry of the composite waveform. I call this circuit a bass phase aligner (BPA). Fig. 2 illustrates the effect. The BPA is unique in that it can adjust polar asymmetry without introducing harmonic distortion.

CLIPPING

The clipper in this APL is unusual in that it is part of the gain control servo-loop. The clipping threshold is the same as the peak limiting threshold in each polarity, so it can take advantage of the full modulation capability of the transmitter. Waveforms with high peak to average ratios are amplified and clipped harder, while waveforms which already have low peak to average ratios are not clipped as heavily. High frequency distortion components resulting from the clipping are attenuated before being introduced into the audio path, so low pass filtering of the resulting audio is not required.
 

THE IMPROVED ASYMMETRICAL PEAK-LIMITER

Figure 1 - Schematic
Figure 2 - Bass Phase Aligner
Figure 3 - Square wave canting and combined wave clipping
Figure 4 - Direct modulator connection
Figure 5 - Adding asymmetry to a tube-type peak limiter

Refer to Fig. 1. Input gain is set by R1. U4a is a buffer. Bass phase is adjusted by R29 (if S3 is closed); the low frequency phase advance produced by C13, R28 and R29 is doubled by U5a, yet frequency response stays flat. S4 selects input polarity.

U1a can provide over 30 dB of gain. Q1 reduces the effective gain by shunting the input of U1a. R22, R23, R24 and C11 compensate for any leakage in C1 so that there is no DC shift in the output of U1a when Q1 conducts.

The output of U1a feeds U3, which drives T1. R9 and R10 set the gain of this stage. C15 compensates high frequencies, and C16 keeps RF leakage out of U3.

T1 is driven so that the audio voltage at either end of its secondary winding is about two times the audio output voltage from U1a. The output from T1 is full wave rectified by diodes CR1 and CR2. With S1 off (open), the rectifier will be equally sensitive to peaks of either polarity (no asymmetry), while if S1 is switched on (closed), and R34 (asymmetry) is turned all the way up (zero ohms), the sensitivity will be cut in half for waveform peaks of one polarity (2:1 asymmetry). The resulting full wave rectified waveform is peak detected by Q2, filtered by C17, R35, C5, R17, C8 and R16, and fed to Q1.

Q1 (a MOSFET) serves as a variable shunt that works with R3 to divide the audio from C1 before feeding U1a. Think of Q1 as a voltage variable resistor. A MOSFET will shunt low AC voltages with relatively little distortion; Q1 will see about 30 mV p-p audio.

For simplicity, imagine S1 is open. If the audio level at CR1 or CR2 exceeds about 3V peak to peak, the peak detector output causes Q1 to begin to conduct, reducing the audio fed to U1a. Any further increase in input level will only result in a slight increase in output level, producing correspondingly greater gain reduction. Thus the output signal is peak-limited to about 1.5V peak-to-peak.

If S1 is closed, the peak detector will be desensitized to peaks of one polarity, by an amount determined by the setting of R34. S4 (input polarity) is used to select the input polarity so that the higher input peaks are fed to the desensitized peak detector. S2 (output polarity) sets the polarity of the output so that these peaks produce positive modulation.

If R35 (clipping) is turned up, gain control voltage due to instantaneous peaks will immediately charge C17 and pass through to Q1, but this will not immediately charge C5. The result is a much faster release time for peaks, so fast that the peak limiting action begins and ends with the peak. This makes it peak clipping! C17 extends the release time enough that peaks corresponding to frequencies from about 3 KHz and up are not clipped, but are peak limited with an extremely short release time. That attenuates high frequency clipping distortion components and reduces intermodulation distortion of high frequency sounds such as sibilants. It also reduces blocking of high frequencies by louder low frequencies that are clipping. See Fig. 3.

R8 sets the output level to the modulator.

SETUP

Line levels to and from the APL are expected to be about 0.2V rms. Higher input levels can be used by reducing the setting of R1. Higher output levels can be produced by turning up R8. The modulator is expected to have flat frequency response and good phase response following the APL. Setup is best monitored with an oscilloscope, and it can be done over the air if necessary.

Use a voice signal, or another asymmetrical signal, for tests. The APL should be adjusted to match the modulation capability of your transmitter, then the BPA should be adjusted to match the asymmetry of the voice waveform to the asymmetry of the APL.

Insert the APL between your audio preamp and your modulator. Set S1 (polar asymmetry) on (closed). Set S3 (BPA) off (open). Set R1 (input gain) about mid-scale. Set R8 (output level) for about 50% modulation. Set R34 (asymmetry) all the way up to 2:1 (zero ohms). Set R35 (clipping) all the way down (zero ohms).

Now try both positions of S4 (input polarity). One of them should permit larger modulation than the other; this is the setting you want. Set S2 (output polarity) so that the higher peaks are in the positive direction. Set R8 (output level) for maximum positive modulation or 100% negative modulation, etc, depending on your particular station setup.

R8 (output level) affects both positive and negative peak levels, while R34 (asymmetry) affects only the positive peak level. If your modulator is not capable of 200% positive modulation (it probably isn't), then you may want to reduce peek limiter asymmetry by turning R34 (asymmetry) down. It may then be possible to increase R8 (output level).

If the asymmetry of the voice waveform is not adequate, try switching S3 (BPA) on and adjusting R29 (bass phase) for the best waveform. Start with R29 turned all the way down (maximum resistance), and gradually turn it up. See Fig. 3. You may find that you need to reverse S4 (input polarity)!

Now try the servo-clipper by turning up R36 (clipping). Voice signals will sound increasingly louder, with little audible distortion until the loudness improvement is quite substantial. I set R36 (clipping) around the 10:30 position (about 1/3 of the way up). You may want to adjust R8 (output level) slightly, because you will be better able to see the peak modulation level after introducing the clipping.

Note that clipping sounds best on a crisp audio signal. If your audio is bassy or muffled, you should try a bright mike such as a D-104, or add high frequency boost, or both.

Once the APL is set up with a transmitter, you should use only S4 (input polarity) to reverse phase as you change mikes or play with equalization, bass phase and clipping.

Close-miking is advisable, for several reasons: room acoustics are usually bad, room reverberation reduces articulation when clipping is used, directional mikes sound "warm" up close, and RF, hum and noise problems are minimized when you reduce preamplifier gain.

A VERY SUBTLE SETUP ADJUSTMENT

With heavy compression, this APL produces a soft rounding down of peaks in one polarity, increasing or decreasing asymmetry by as much as 10%. The polarity that is rounded down can be selected by moving the connection of S1 between R13/CR1 and R14/CR2. If you change this, you must also reverse input and output polarity.

My own station can only provide about 150% modulation, so I set up my APL so that this effect reduces positive modulation. The effect is subtle; unless you have an oscilloscope, don't bother with it.

IN CASE OF TROUBLE

The audio chain following this audio processor must be of good quality. Significant phase distortion or frequency response variations following the compressor will defeat its purpose. Equalization should be done in the preamp feeding the APL.

If square waves from the clipper are canted due to restricted modulator low frequency response, try decreasing C4 and C10 to minimize the canting. See Fig. 3.

In the crowded HF bands, amateur AM sounds best when it has bright, crisp audio. This is especially true if clipping is used. If your audio is bassy or muffled, try a bright mike such as a D-104, or boost highs, or do both.

If you expect to achieve voice-polarity supermodulation, your transmitter and modulator must be able to handle the large peak powers involved. If they can't, it won't work.

This processor has a lot of audio gain. If you get RF leakage into your audio, if your modulator "talks back" to you, if you have loud background noises, etc, you will have problems. Shielding, filtering, grounding and room acoustics become more important when advanced techniques such as compression and equalization are used.

IF YOUR MODULATOR IS NOWHERE NEAR FLAT...

... you can still have APL by using the circuit shown in Fig. 4. It may also be necessary to increase C17. R8 will no longer control the negative peak levels it will act as a gain control, and distortion will result if it is set too low or too high. You may have to move S1 from CR2/R14 to CR1/R13 for proper APL action.

CONVERTING A TUBE TYPE PEAK LIMITER

Use push-pull peak limiters only. With push-pull audio and common-mode AGC, control voltage changes are balanced out of the audio, permitting the extreme control speed required for good peak limiting and proper operation of the servo-clipper.

Simply add a resistive voltage divider (see Fig. 5) as necessary to reduce peak detector sensitivity to one side of the waveform. You can add servo-clipping by inserting the equivalent of R35 and C17 in series with the main AGC loop filter capacitor.

CONVERTING A TUBE TYPE PEAK LIMITER

Use push-pull peak limiters only. With push-pull audio and common-mode AGC, control voltage changes are balanced out of the audio, permitting the extreme control speed required for good peak limiting and proper operation of the servo-clipper.

Simply add a resistive voltage divider (see Fig. 5) as necessary to reduce peak detector sensitivity to one side of the waveform. You can add servo-clipping by inserting the equivalent of R35 and C17 in series with the main AGC loop filter capacitor.

IF YOU CAN'T FIND A 40673 or 3N211 FET FOR Q1

You can use any N-channel enhancement J-FET, such as the MPF-102, or any single-gate MOSFET, and connect its single gate to R16 and R17.  You may have to adjust the values of R20 and R21 to set the compression threshold, depending on the exact characteristics of the particular FET you use.  If gain is low, or if the compression threshold is too low, raise R20 or lower R21.  If you can not get gain reduction, lower R20 or raise R21.

Ron, WB0LXV Ron Reu, WB0LXV is shown with his homebrew AM kilowatt (the tall rig next to him) and BC-610I (the black rig on the left). Ron operates from Winfield, MO, and can be heard on 160 and 75. Ron is the author of the article in November, 1989 AM P/X (Issue # 77) describing an E.F. Johnson factory modification to replace the hard-to-find 4D32 with an 829B in the Johnson Viking I.

Heber Springs Taken at Heber Springs, Arkansas on May 28, 1988. L to R: Vernon Whites WB0SUJ, well known AM occupant of 1985 khz from Rolla, MO; WD5BMD, Paul Coats from Trumann, AR; WD0AJP, Jody Brown from Wappapello, MO; K5BWB, Malcolm Dillard.

Paul, WA2NFF Paul Kelly, WA2NFF is shown teaching his youngest son Michael (12-1/2 months) the ABCs of AMing.

John, W4KYL W4KYL (ex-K3OWZ) John Martin, Montrose, PA.

You readers have seen my statements get questioned in the OPEN FORUM, so now, I have more unbelievable statements to make on my formulas. However, I'm going to quote you, or your friends, first. Allow minor errors on my part; I was not always copying you well. All of these were in the last year.

I had made changes in my AM transmitter response, and I thought that I had gone way, way too far in my preemphasis. "It should sound terrible." "I'll be laughed off the air." "Oh well, let's try it anyway, before I correct it." Here are your reports to me:

WA3SOT Ryan, "Could have more bass." G2AQH Jerry, "Audio is excellent; Very very nice." VE4BX Doug, "Don't change anything; Nice audio. I like it. Don't get it too bassy. It cuts through QRM. It'll be heard. Don't touch it. Just right. Just dandy." N2AH Art, "Wouldn't say too much highs; could use more deep bass, not all that high frequency boost you used to run." WA3PUN Ed, "Cleanest audio I've heard from you. Clean and natural for the first time."

W1GAC George, "Very soft and beautiful audio; I like it very much." WB2UYJ Larry, "Signal sounds real super on this SSB." W2VJZ Erb, "Sounds good on my 5" speaker." W2SGP, "Excellent." W2QIL Al, "modulation is o.k." WD5EHS John, "Very nice; no problem, very nice signal." W2VJZ Erb (later), "Excellent audio. I like it sharp. Better than average readability." KB2CXJ Al, "Audio really good--great; broadcast quality; I like it; Very natural; Quite pleasant."

WB0YTR Al, "I wouldn't do anything else; your audio sounds like you are right here next to me; You couldn't do any better; Great; Sounds good." W4IVB Jim, "Terrific signal; Sounds beautiful; Don't see many signals that strong; Is balanced crisp audio I'd like to have; Wouldn't want you to change anything; It's better than anything else that's on the air; It's super readable like most of us would like to have." DJ3DP Eder, "Modulation is excellent; You are the strongest U.S. Station."

KE5IS Jim, "Full range; not too much highs, not overmodulated; It's fine; To me you have done what you desired, not too many frequencies out off; not tinny from too narrow; Speech 9+10 gives 9+20 on modulation peaks; You're doing good; Excellent; I wish I could record and capture it all." W8AHB Jack, "Sounds good, lots of highs; You're about the best I've heard you."

N4FS Mike, "Sounds quite good; quite intelligible." WA1JAS Mike, "Sounds good; tremendous amounts of audio; Not overdone; Easy to copy; You could be S2 and I could understand you." WA1WMG Jim, "Sounds very good; Audio clean, well rounded; Sounds nice." WA8ZNX Mac, "You sound great." N8CFU Theo, "I like it. It's great; Excellent audio; Don't change anything." N3GLE Dott, "Audio sounds very good."

W2SGP Mike, "You sound great, beautiful; Real communications voice." N3FLN Doug, "Audio absolutely excellent, beautiful." W3TUB Jim, "Your audio is very good; I see nothing wrong with it; I find no fault whatsoever." K1DKC Darrel, "Your audio is real penetrating." KA2DBD Cliff, "Beautiful, delicious." K2SQI Dave, "Sounds good on SSB."

WB3FAU, Russ "Real good; fine audio; no problem...may lack some bass; Not doing anything wrong; No trouble with readability; You stay highly intelligible with selectivity; Doing a fine job." N8CSU, Theo (again), "Very good, pronounced high frequency audio above mid-band; adds extra details to fully understand; Really brings up understanding; The real spaghetti behind the sauce...It sticks out from the average audio."

There were similar reports from W1FAT Les and N4FS Mike, "Lacks real lows which (makes) all sound like John Wayne...you...very natural..." WB3FAU Russ, "With great selectivity still very readable..." AA4YD Joe, WA3WDR Bacon, "...can put a lot of selectivity on you and it doesn't sound bad..." WB3FAU Russ, "...like nothing else ... one of the best signals I've heard.... cutting through the QRN." N2IFY Jamie (a spontaneous report), "Your audio is very good, too." W2WZQ Ralph, N1DID Jim, "...in 16 KHz position sounds very very nice..." WA1HLR Tim, "...very much readable and clean..." KA2WCC Leo, "...beautiful to my ears."

WHAT DOES ALL THAT MEAN?

All that means that we have been on AM with much less modulation than can be on our carriers, for lo, these many years. This is the year to upgrade.

By my experiments I have found that my voice energy peaks between one hundred and two hundred Hertz, when up close to a microphone. Unbaffled bass is lost at a distance. After that, my peak energy for any higher frequency falls off at a rate in excess of -6dB per octave. If I compensate that falloff by a preemphasis of close to +6dB per octave, I can bring up much of my treble to be heard above the noises that associate with my modulation on the way to your loudspeaker.

SEE IT ALL

Chart 1

See Chart 1. It simplifies the whole possibilities. The solid line "curve" is the typical peak energy of a human voice (males and females use the same frequencies; it is the scan rate "pitch" which sounds different) plotted at a maximum loss of -6dB per octave. Singing would show less loss between 1,000 Hz & 3,000 Hz.

The dashed line shows the overall preemphasis that I can use so as to bring my treble peaks up close to my low frequency peaks. The line of zeros shows the +3dB per audio octave from spectrum noise. The line of XXXX is the theoretical perfect resultant output.

COMPETING WITH NOISE

Masking of sounds occurs when sounds of nearby frequencies are strong enough to cover our ears detecting those sounds. Thus, basically, we do need to have, as a minimum, a preemphasis curve of +3dB per octave, overall, mike to antenna, just to cover spectrum noises.

A little study of Chart 1 shows us that as background noise levels do increase against our signal, our treble becomes masked, by those noises, for all frequencies above a frequency which becomes a lower frequency with more masking noise strength. This is why signals in heavy noise can be heard to be talking but little intelligence comes through.

BEFORE NOISE, NOT AFTER

Once the ratio of treble to noise is set, there is no immediate way to bring the intelligence out from underneath that noise. We've lost the game. Instead, what we can do is to bring up the energy in the treble to nearly the same peak levels as in the bass tones, below 500Hz, before it enters the arena of noises. That, we can see, approximates a +26dB (400:1 in power) preemphasis at 3,333Hz, compared to 150Hz.

We can see that "high fidelity" at 8,000Hz is down to -36dB (4,000:1 power), and can not be appreciated far from the transmitter. All manufactured transmitters had their high voice frequencies rolling off well below 3,333Hz, and the resulting poor readability of them, in noisy conditions, helped stimulate experiments with SSB.

What is realistic is to carry audio from 150Hz through 3,333Hz and to cut off beyond those. Then we put in a +6dB per octave preemphasis, and our listeners with their receivers will slice our preemphasis and that much accompanying noise back to near normal. Then, our 400 times power increase at 3,333Hz, has done us a good job.

STRAIGHT LINE

What does not work is to use an irregular frequency response. I have tried many non-straight line responses. It was only my straight line "curve" which provoked the above reports. I have not left out any negative comments of any reports, (except that I am readable in some telephones). In the previous Part of this series, in the AM Press/Exchange, I showed a simple circuit that could use up spare gain to produce almost that much preemphasis. An outboard stereo equalizer, of 10 octave bands (Realistic No. 31-2008, 10 watts, Radio Shack) (usually at a cut price early in the year) can do better, but costs about $100 more.

What is needed with the 10 Band (10 octave) equalizer is: an output phono plug from (B) output on a cord to fit into your microphone socket on your AM transmitter; a phono plug to phono plug, jumper, to take the output of one (A) side to the other (B) input; and a phono plug at the (A) input back to a socket for your mike.

PRECAUTIONS

Keep this equalizer unit away from strong r.f. fields such as hot tuners or open antenna leads. Shielding can be completed by rubber cementing foil to the inside of the fiber panel of the case. I put a 130V line surge protector directly across the transformer primary. I bypassed both sides of the A.C. line to the chassis through short lead 0.001 ufd ceramics. I grounded the chassis and kept it away from the transmitter and the unshielded tuner. I averaged unity output through the equalizer.

Be sure to slide all the way down on the knobs for 30, 60, 8,000 & 16,000Hz. Now put both 4000Hz at +15, both 2000Hz at +12, both 1000Hz at +9, both 500Hz at +6, both 250Hz at +3 and both 125Hz at 0. Try it. There is a button on the front to put it in and take it out. Watch out for overmodulation, but expect some increased modulator current for 100%.

CHECK IT OUT

Monitor with headphones on your own receiver with the transmitter into a dummyload. If your audio squeals, cut every control down by the same amount. The two sides add their dB. You may need lower gain on every control, and you may even need ferrite beads on hot leads.

If you have an original curve that is down at both ends of our 150-3,333Hz spectrum, you may wish to also compensate for that, but be cautious.

There are a number of common designs that do not allow the full 100% modulation on either end of our 150-3,333 range. Just pushing in more equalizer gain will only increase distortion and splatter.

With your dummy load and headphones, test each gain slider to see if you are close to a distortion level on any slider. Back off at least -3dB from any such level, no matter what. There are ways to correct all those limits. It's a complete linearity program, by formulas, something worth its time, another day.

Don't take one or two on-the-air reports as conclusive. Test with many station operators to see how far you can go. If they are nearby, have them tune off by about +10 KHz and listen for splatter when you run your 1,000 & 2,000 & 4,000 sliders up and down.

RESULTS

The 8,000 and 16,000Hz sliders being fully down will begin cutting back at around 4,000Hz, so that you will peak near 3,333Hz. Never go to full power with any 30, 60, 8,000 or 16,000Hz settings above the bottom. Some transformers or capacitors can not take the extra signals. Play it safe; keep all those 8 sliders all the way down.

When you like what you hear, put your preemphasized AM on the air. You could be readable against up to +20dB more QRM & QRN (100 times the wattage) than you have been. You may be on the air, in lightning static, when the SSB stations can't get out of town.

As a "demonstration experiment", slide down the 125, 250, 500 and 4,000Hz sliders. You will then be needing phonetics, just like on SSB.

Remember that voice readability does not come by volume, but by how much of the voice spectrum is heard against the noises. We do need 150Hz through 3,333Hz to be intelligently heard by our contact above all the noises that are out there. This is one way to do it well. You will definitely get better reports if you do it well, too.

Editor, AM Press/Exchange:

I've been a S.P.A.M. member since June 6,1984, under my old callsign, which was KA7GXF. Due to the fact that I have had constant TVI problems and have changed my QTH several times, I haven't been too active for the last five years. But my love for AM and all the good things that the mode stands for is still strong in my mind. Hopefully, our numbers will grow, and the AM mode will become a mainstay on the bands for years to come. I've been an AMer since I was 14 years old, operating on the air with old army and navy equipment that I would manage, somehow, to get going. Now I am 23 years old, and I still love my military WWII equipment, and that will never change. With the right presentation, young people will carry on the great tradition that has lasted since the first AM transmitter: an enjoyable, "user friendly" mode!

Steve Davis, KD2NX

Picture of switch and wallplate

If you have any switches or cover plates like these lying around, which you are willing to sell or trade, please contact AM P/X Editor and Publislher Don Chester, K4KYV, 2116 Old Dover Road, Woodlawn, TN 37191.

I recently bought a Ranger I for $30 with PTT, and factory wired. However, the modulation transformer was shot. Having a basket case Harvey Wells TBS-50, I scrounged what I could from it, and came up with the modulation transformer. I laid the transformer on its side and slipped it in between the meter switch and the 1614s and under the meter case. It can be fastened along the side of the chassis with whatever you can find in your junkbox scrap hardware. I used two bakelite strips. The former owner had an outboard shielded cable to a modulation transformer to the rear. I brought the cable back through one of the holes near the 1614's and made connections with the replacement mod xfmr. I did have to bend back the long screw guide bracket so the screw would go past the bottom of the xfmr. This mod. xfmr works FB and is a bit huskier than the original one. See figure 1 below.

Figure 1

A bit of nostalgia from the late '30s is the crystal VFO using a razor blade. Bliley Electric Company made the VF-1 crystal holder with the knob on top. The homebrew crystal holder I made had a half of a double edge razor blade eased in between the back plate and the crystal, which changed the frequency. You can move quite a bit on 20 metres when using a 160 or 80 metre rock. See figure 2.

Figure 2

If you want to replace the three can electrolytic capacitors in your DX-100, here is where you can get them. The 20-20-20 uf at 450 v. can be bought from Antique Electronic Supply, 688 W. First St., Tempe, AZ 85281. The two 125 uf at 450 v. can still be bought from Heath. The part number is 25-34. They are also used in the SB-200 linear amplifier and cost $5.25 each. I had a heck of a time trying to find them. Even the biggest capacitor company was no help. Special thanks to Richard Bell who helped me locate these capacitors. I thought I would pass this information for anyone who is repairing or rebuilding a DX-100. (These capacitors might be useful for other equipment as well. -Ed.)

Marty Drift, WB2FOU/5

WANTED: Urgently need final tuning condenser for Viking Ranger (under the chassis). Also "U" shaped shield for under Viking Ranger.

FOR SALE: Johnson Ranger I, $90. Elmac AP 68, $50. Agast time delay relays heavy duty high current $20 ea. 80 & 40 mtr High Gain traps 4 ft. long $40 for pair. 866 rectifiers $4 ea. 805 $20 ea. 807 $3 ea. 19 inch rack modulation indicator scope mfg by Design Inc. N.Y. $50. Millin rack mounted modulation monitor scope $40. Inquire on tubes not listed above.

WANTED: Little DC motor/generator that was shown in text books about 4" in diameter and was used in high school labs.

FOR SALE: Hallicrafters Mod. SY-117. Two xtals missing. With manual. $60.

FOR SALE: Collins 75A-4 receiver with 3 mech filters (mint condx) $450. Drake RR3 commercial general coverage receiver (mint condx) $850. Johnson Invader 2000 transmitter (mint condx) $700. Johnson Pacemaker transmitter $150. Johnson Desk KW (pedestal only) $1400. Johnson KW matchbox with directional coupler $175.

WANTED: Top cover for R-388/51J receiver; front panel for RCA AR 88 receiver; NRD 525 receiver; Johnson 500 transmitter.

WANTED: B&W 40 TVH and 80 JCL plug-in coils.

WANTED: 6 meter AM transceivers from 1950s and 60s. Please send description and price.

WANTED: Help! I need one low voltage transformer for my Johnson 500 model no. P.30316-46-SN. Will pay top dollar for good transformer.

WANTED: Communications receivers-- S-38D, S-53A, Collins, etc. Working or restorable.

WANTED: My first bought station. Hallicrafter HT-6 transmitter & Howard 430 receiver. Also looking for antique transmitter and receiver, and books & magazines.

FOR SALE: R390A receiver manual & schematics (original) $35. 5/8" hardline connectors (to VHF 259) $10 each.

WANTED: Will pay a premium for early ham equipment and Federal radios, parts and restoration information.

FOR SALE: Johnson Viking Valiant in excellent condition. Ready for your mods. Includes B&W coax relay and manual. Pick-up or will meet half way in N.E. area. $200.

WANTED: Transformers U.T.C. LS-18 and LS-49. Also old style pushbutton light switches (110 volt a.c.), and covers for same (metal or bakelite).

HAMFEST: The Ashtabula, Ohio Amateur Radio Club will hold their hamfest at the Human Resource Center, Mill Street, Conneaut, OH on March 11, 1990. -TNX W8VYZ

This is the AM PRESS - An amateur radio publication dedicated to Amplitude Modulation.

This is the AM EXCHANGE - Offering FREE ADVERTISING to enhance the availability of AM equipment and parts.

Subscriptions $10 per year (12 issues) in North America. Classified ads (AM related) free of charge to subscribers and nonsubscribers. Display advertising and foreign subscription rates on request.

Edited and published by Donald Chester, K4KYV

NOTICE: The purpose of this publication is the advancement of Amplitude Modulation in the Amateur Radio Service, and there is no pecuniary interest. Therefore, permission is hereby expressed for the use of material contained herein without permission of the publisher, with the exception of specifically copyrighted articles, provided that The AM Press/Exchange is properly credited.