Hammarlund HQ-140-X

Discussion in 'Amplitude Modulation' started by KA4KOE, Jul 17, 2019.

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  1. WA5VGO

    WA5VGO Ham Member QRZ Page

    This is bound to stir controversy, but my experience has been the TRW constructed VFO used by Heathkit is just as linear and more stable than the Collins.
  2. N2DTS

    N2DTS Ham Member QRZ Page

    Its not hard really, my 2nd homebrew receiver drifts a lot less then my 32V3, even from a dead cold start.
    I think one of the big things about the pto is they could adjust the tracking to maintain dial accuracy.

    Seems to me, when you use powdered metal as a core, and/or wind wire on a cheap form, you wont get stability unless you compensate.
  3. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    The main problem with the Collins-type PTO is that the ferrite material in the moveable slug deteriorates with age, slightly reducing the permeability. That means that for a given physical movement of the slug in and out of the coil, there is less change in frequency. As a result, for example with the 75A and R-390 series, 10 turns of the shaft don't quite change the frequency a full 1000 kc/s. IOW, the PTO develops too much bandspread. There is a little adjustable slug-tuned inductor in series with the main coil that spreads the tuning range out a few percentage points. As the slug ages, this inductor can be adjusted to less inductance, which brings the tuning range back in. Problem is, in some PTOs the eventual loss of permeability is enough that adjusting the little coil to its minimum inductance still doesn't bring the tuning range back in. The fix is to open the PTO and remove a turn or two from the little coil, reducing the fixed series inductance enough to bring it back in line.

    Even with the tuning range brought back to spec, linearity of tuning may be affected. There is an adjustment for linearity by sliding over the metal wafers that form the adjustable cam followed by a roller, but this is not an easy adjustment. I have found that zeroing the end points at 100 kc/s in from each end of the tuning range limit, @ 0.100 - 0.900 instead of @ 0.000 - 1.000, nearly always results in better overall tuning linearity across the band.

    I believe the Heathkit LMO is more like a classic VFO with a variable capacitor instead of variable inductor. They probably had access to precise temperature compensating capacitors. Another rig that I found had extraordinary stability with an analogue VFO was the Ten-Tec Century 21. I never owned one, but I repaired a few for other hams and was always impressed with the stability. If they had made them to cover 160, I might have acquired one to use as a VFO for my transmitters. As it is, I use a highly modified T-368 master oscillator, which has good enough stability that I have never bothered to use one of the DDS VFOs I picked up over the years from hamfests.

    I believe frequency stability and operating zero-beat with other stations in a QSO are equally as important with AM as with other modes.
    N2EY likes this.
  4. N2EY

    N2EY XML Subscriber QRZ Page

    The Heathkit LMOs are indeed variable-capacitor oscillators. They tune 5 t0 5.5 MHz with some overrun at the ends.

    The Heathkit LMOs were all supplied by contractors as a wired, tested component. General opinion seems to be that the TRW ones are the best.

    Depends on what you mean by "zero beat". In a CW contest, an offset of a 200 Hz will often mean the other station won't hear you because you're outside the receiver passband (250 Hz with excellent shape factor). 200 Hz on AM.....

    Digital modes have even tighter frequency requirements.

    73 de Jim, N2EY
  5. K9STH

    K9STH Ham Member QRZ Page


    Specific individual TRW LMOs might be more stable and more linear than specific Collins PTOs. However, in general, that is not true. There have been MANY more Collins PTOs manufactured than the TRW LMOs and the percentage of LMOs that are more stable / linear than certain Collins PTOs is VERY small.

    In addition, any of the Heath LMOs, those manufactured by TRW as well as those manufactured by other companies, are VERY prone to the "warble" problem which requires disassembly, cleaning, and then modification.

    I do have a number of Heath SB-Line equipment with the TRW LMOs, as well as several spares, and quite a number of Collins equipment with various PTOs. In addition, over the decades, I have seen a LOT of both Collins PTOs and Heath LMOs. I can safely say that, in general, the TRW LMOs are NOT as stable nor as linear as the Collins PTOs. The LMOs are very good but not as well made as the PTOs and the performance does show this.

    I do have a Collins 708A-1 unit that is over 65-years old and the calibration is rated as being no more than a maximum of 30 Hz "off" at 4.2 MHz. That is, one can "dial up" the frequency to no more than 30 Hz "off" and the stability is considerably better than 30 Hz at 4.2 MHz. Such accuracy and stability is just not possible with the TRW LMOs.

    The "fix" for the LMO "warble" can be found at the following URL:


    Glen, K9STH
  6. WA5VGO

    WA5VGO Ham Member QRZ Page

    Like I said, certain to raise controversy. The 70K2 VFO was simpler and smaller than the TRW, but in terms of actual performance, I’ll stand by my observations.
  7. W8KHK

    W8KHK Ham Member QRZ Page

    VGO: I share your satisfaction with the TRW LMOs in the Heath line. My experience with the SB-100, SB-101, SB-102, SB-300, SB-301, SB-303, and SB-401 has been consistently excellent. Assuming the dial has been installed carefully, the fiduciary pointer stays vertical and rarely needs to be touched across the entire range of the oscillator, indicating consistency in frequency linearity throughout many units. This linearity is primarily a function of capacitor rotor shape, and it is a constant. The variable capacitor has double ball bearings, and a substantial spring-loaded gear train with minimal backlash. Frequency settings can be approached with accuracy from either direction, and turning friction is minimal. The enclosure is heavy-gauge boxed aluminum which is very stable and almost totally immune to vibration. Both the vacuum tube and solid state versions have extremely minimal drift after a short warm-up period.

    SHT: I also have spent a great deal of time using the 75A3, 75A4, 51J4, 32V2, 32V3, and have spent countless hours while in the USAF using, and servicing, many R388s and R390s. Although stability after warm-up is good, I cannot say the same for linearity or dial accuracy. I am not alone, as Bill Orr comments that dial linearity should be at least within 1 KHz across the dial after maintenance (in the 60s no less). http://www.collinsradio.org/wp-content/uploads/2012/04/New_Life_for_51J_PTO.pdf I have never seen any with 30 Hz accuracy anywhere on the dial. On the 75A3 that my father bought new, I recall he always had to reset the fiduciary pointer when moving 100 KHz, I have that receiver now and nothing has changed. I have found this to be typical of all my Collins PTOs. Yes, they are accurate within one KHz between calibration points, but for any better accuracy, interpolation is needed. In addition, I have found that it is almost always necessary to approach a frequency setting from the same direction every time to avoid backlash error, which was never necessary with the LMO. I realize there is about a ten year difference in vintage between the two products, but these issues were present years ago as they are now. The PTO worm drive has considerably more resistance to motion, requiring more force to turn, compared to the relative ease of the LMO dial rotation. Maintenance and lubrication address this, but it is still inferior to the turning force required for the LMO. I could go on with more details, but I think it is sufficient to summarize that the LMO is an order of magnitude more pleasant to use day-to-day. I even converted one of the 5.5 to 5.0 LMO units to 6.9 to 7.4 MHz for use as a 40 meter VFO, and it retains both the linearity and stability of the original product. My mileage does not seem to vary.
    Last edited: Aug 10, 2019
  8. K9STH

    K9STH Ham Member QRZ Page


    The 708A-1 and 708A-3 are not the garden variety units. There are like 4, or 5, component differences between the 2-units. Basically, they are identical. I have one of the 708A-1 units. Attached is a photograph of my unit plus scans of the manual page where the accuracy is shown plus the cover of the manual.


    freq accy-1.jpeg


    The TRW LMOs are, generally, excellent. However, over the years, they do become problematic. Not only is the "warble" situation extremely common, the 5-tabs, that are on the tuning shaft that limit the tuning range to just over 5-turns, become jammed and the entire 500 kHz range cannot be tuned. Correcting that problem requires cleaning and lubricating the assembly to restore proper operation.

    I have seen a LOT of the various Collins PTOs and the Heath LMOs over the decades. It is my opinion, based on my experiences, that, in general, the Collins PTOs are better. The TRW LMOs compare favorably to the Collins PTOs but the Collins units, again generally, have a slight edge.

    Both units are accurate to the nearest kHz and both units definitely require interpolation between the kHz marks. There are situations where interpolation is just not that accurate. Most of the time, such accuracy is not needed. However, I have found situations where getting the "exact" frequency is very difficult. For example, during DX contests where the DX station is operating, usually on 40-meters, below the U.S. phone band and is calling out a very specific frequency on which he / she is listening. Often, because of QRM, if you are more than around 100 Hz away from this frequency, the DX station is going to have problems receiving you and, quite often, you are not going to be able to make the contact.

    When operating "split" with my Collins S-Line, I usually receiver the DX with one of my Collins 75S-1 receivers and control my 32S-3 with my 75S-3A receiver. This allows me to know what is going on with the DX station's receive frequency. I finally added one of the AADE frequency readouts to my 75S-3A. This readout is accurate to at least the nearest 10 Hz and allows me to get on the "correct" transmitting frequency with a minimum of effort.

    I do have Heath SB-Line equipment on my main operating console including a SB-110A, SB-301, SB-401, and an SB-310. For HF, I don't use the Heath SB-Line equipment as much as my Collins S-Lines. But, when I do use the equipment, I definitely do not have any problems with frequency readouts. For 6-meters, my SB-110A is my primary piece of equipment.

    Glen, K9STH
  9. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    "Zero beat" means that the carriers of both transmitters are set to the exact same frequency, regardless of mode. With SSB, the reference frequency is the suppressed carrier. Listening in AM mode with an envelope detector, the VFO is moved until the heterodyne drops in pitch to "zero" cycles per second.

    I think it is poor operating practice for two stations to operate CW off-frequency with each other to the audio offset. Zero beat the station with your VFO, and then tune your receiver off frequency to the desired heterodyne pitch. A third party monitoring the QSO should hear both stations' signals on the same frequency at the same audio pitch.

    With the newer generation of hams, zero-beating is apparently becoming a lost art. I frequently run into operators on the air who don't know how to do it.

    Once when I was overseas, I was given a discarded 708-A that had been in the store room of a defunct ham club. It looked in good shape, but had no documentation. I was unfamiliar with it, and it was too big and heavy to affordably ship back home. I parted it out and shipped home some tubes and three (IIRC) PTOs. I think I still have them, but they never covered a useful frequency for anything I ever tried to build.
  10. N2EY

    N2EY XML Subscriber QRZ Page

    I've understood all that for 52+ years.

    I've understood all that for 52+ years too.

    Exactly! I've operated that way since I got a VFO, soon after upgrading to Advanced from Novice in 1968.

    With a good CW receiver that has a sharp filter, one simply tunes the desired signal in properly (centered in the receiver passband) and then turn on the transmitter VFO and tune it to the center of the receiver passband as well.

    The point I was making is that if CW ops using sharp filters in their receivers aren't zero-beat to within 100-200 Hz or so, they won't hear each other. But with AM, the carriers can be hundreds of Hz off and the stations can still hear each other.

    One big reason is that, for most HF hams, separate receivers and transmitters were replaced by transceivers way back in the 1970s or earlier. Many, many hams with less than 40-50 years licensed have never operated separate tx/rx; they've never needed to.

    Here's another reason:

    If an HF transceiver has a sharp CW filter, and is set up properly, all one needs to do is tune the received signal to the center of the receiver passband and the transmitter will automatically be zero beat. This is why I started building CW transceivers 40 years ago - doing S&P CW contesting with separate tx/rx was MUCH easier, and produces a MUCH higher QSO rate, than running separates.


    If an HF transceiver does not have a sharp CW filter, but instead just has the stock SSB bandwidth filter, the operator must set the receiver to the "right" tone in order to be zero beat.

    For example, suppose a transceiver has a nominal "1000 kHz" IF, and covers the 40 meter band by conversion with a VFO tuning approximately 8000 to 8300 kHz.

    Suppose that for SSB the SSB IF filter has a passband of 1000.3 to 1003.0 kHz, and the carrier oscillator is at 1000 kHz. The SSB signal received and transmitted will have an audio range of 300 to 2700 Hz.

    Suppose that for CW the CW IF filter has a passband of 1000.6 to 1000.8 kHz, and the carrier oscillator (BFO) used for receiving is at 1000 kHz. The CW signal received will have an audio range of 600 to 800 Hz - 700 Hz being the center. In order to transmit zero-beat, the carrier oscillator used for transmitting will need to be at 1000.7 kHz. By centering the received signal in the passband, the transmitted signal will be automatically zero with the received signal. Easy peasy lemon squeezy.


    Suppose that for CW the SSB IF filter with a passband of 1000.3 to 1003.0 kHz, and the carrier oscillator (BFO) used for receiving is at 1000 kHz. The CW signal received will have an audio range of 300 to 2700 Hz! If the carrier oscillator used for transmitting is at 1000.7 kHz, the operator will need to set the tuning for a 700 Hz received tone, in order to be zero-beat, even though the receiver passband is much wider. Anything other than 700 Hz in this example will not be zero with the received signal.

    This is why a good sharp CW filter is very desirable in a modern HF transceiver.

    73 de Jim, N2EY

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