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Knight T 60 Transmitter 20 meter crystals

Discussion in '"Boat Anchor" & Classic Equipment' started by KD1RT, Jun 28, 2019.

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  1. KD1RT

    KD1RT Ham Member QRZ Page

    I would appreciate comments from anyone using or experimenting with 14.xxx MHz crystals for operation on the 20 meter band instead of the the specified 7.xxx MHz range crystals. Typically the oscillator's buffer/ doubler stage in this TX makes the conversion from 7 to 14 MHz for use by the final RF amp.

    The 14 MHz crystals seem to work fine during my limited testing into a dummy load producing about 40-45 watts output. In addition to the primary 14 MHz signal an additional very weak signal can be heard at the corresponding (doubled) 28 MHz frequency on a local receiver. Lacking any test equipment I cannot determine actual strength of the 28 MHz signal, but it is very weak into the receiver located only a foot away from the transmitter. The monitoring receiver has about a three foot long coax attached which is shorted to ground at the antenna (coax) selector switch.

    Although clean when used on 40 meters, the 7 MHz crystals tend to have a small tone chirp when used on 20 meters while the 14 MHz crystals produce the expected clean tone.

    It is suspected that due to the close proximity of the monitoring receiver the second and third multiples of any 3.5xx, 7.xxx, or 14.xxx MHz crystal will be heard and is therefore not a good general indicator of TX spectral purity. I would be grateful for your thoughts.

    73- Roy
     
  2. K5DH

    K5DH Ham Member QRZ Page

    I suspect that you'll hear a signal on 21 and 28 MHz if you use a 7 MHz crystal on 14 MHz. There will always be harmonics. It's a matter of how strong they are. You should be fine.
     
  3. KD1RT

    KD1RT Ham Member QRZ Page

    Dean, thanks for your reply.

    Back in the 1960's perhaps the 14.xxx MHz crystals in the FT -243 holders were not as readily available as are the CR49 crystals today? This may have influenced the Knight TX design to use the doubling and tripling harmonics. Heathkit also used frequency doubling in their contemporary HW-16 transceiver.

    I am continuing to experiment.

    73- Roy
     
  4. KE4OH

    KE4OH Ham Member QRZ Page

    FWIW, I lightly modified a Globe Scout 680 to work on 30m. I couldn't find any FT-243 crystals in the 5 mHz range that would double to get me what I wanted on 30m. But I could get rocks in the 10 mHZ range just where I wanted them. They work fine.

    I have to be careful where I tune the oscillator, as the unmodified osc circuit in the GS will peak at both the fundamental frequency and on a harmonic. That may or may not be an issue in your T-60. I'm not familiar with its oscillator circuit.

    73 de Steve KE4OH
     
  5. KD1RT

    KD1RT Ham Member QRZ Page

    Steve,

    Thanks for your comments.

    The T60 uses a pi-network after the multiplier stage to tune the operating frequency being delivered to the RF amp grid. It appears that this network does not care if the frequency is fundamental from the crystal or doubled by the multiplier. The inductor value in the network is established by the BANDSWITCH and brought into resonance with the variable capacitor TUNE panel control.

    The 14 MHz crystals seem to work fine directly supplying the desired frequencies for 20 meter operation. In fact their transmitted tone is cleaner than the normal 2x 7 MHz Crystal's which do exhibit a slight chirp.

    I am still experimenting before committing to routine on-air operation.
    73- Roy
     
  6. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Are you using true FT-243 type 7 MHz crystals or the HC/49U crystals mounted in old FT-243 holders sold by AF4K?

    If the latter, you definitely can have problems especially if you peak the multiplier / driver tuning. Even with true FT-243 crystals by slightly detuning the circuit you can get rid of a chirp. With the T-60 the oscillator operates "straight through" and the next stage is the last stage before the final amplifier.

    You might also tighten all of the machine screws and nuts in the transmitter. All of the grounds, in the transmitter, are made through these connectors and they definitely work loose and / or become corroded over the years. Bad grounds can cause all sorts of "strange" problems.

    Glen, K9STH

     
  7. KD1RT

    KD1RT Ham Member QRZ Page

    Glen, thank you for the suggestions.

    Up to now I have been using my assortment of HC49s and HC49u crystals. Initially by combing through them some 7 MHz crystals were found to perform much better than others in terms of chirp and drift. Some were quite close to the near perfect tone of an original CR6 crystal in an FT-243 holder and others very poor. I think the smaller physical size of the HC49's is what is leading to the unstable operation when operating in the old tube oscillator circuits.

    I found that adding a resistor in series with the crystal has greatly reduced the small amount of chirp exhibited by the best of my crystals. This same modification was also reported by others in the past. Reducing the current flow through the HC49 crystals minimizes heating and the resulting frequency chirp and drift.

    The chirp condition is being assessed by "ear" and visually on the audio scope of my Icom 7300. Even small frequency changes can be seen as a deviation in the associated waterfall display.

    Following the resistor mod I am quite happy with the T60 using the HC49 crystals while my search continues for original FT-243's. On-air reports also support the minimal to no chirp condition being heard at the other end of the QSO.

    An additional step would be to improve the voltage regulation of the power supply. I'll save that for a winter project.

    Thanks again for everyone's comments.

    73- Roy
     
  8. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    The HC-49/U crystals just don't have the mass that the old FT-243 crystals had. As such, they can definitely chirp especially when used with tube-type oscillators.

    Glen, K9STH
     
  9. N8AFT

    N8AFT Subscriber QRZ Page

    Why sweat a Slight Chirp?? That rig sounds like the vintage gear it was back in the day when it was new...
    I have 4 or 5 OO "greeting cards" complaining of my vintage gear having a slight chirp...
    Regulating your osc screen grid voltage will go miles in ridding the chirps over all. An 0a2 tube works great.

    I have tons of A4FK rocks and it pretty much depends on the rig whether they chirp any, a little or a lot.
    My 2 Conar 400's are pretty bad with about all my rocks save a couple. One likes 80m the other works best on 40.
     
    KE4OH likes this.
  10. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    AFT:

    The Conar 400 chirps to high heaven disconnected from the antenna, unplugged, put in a box, and stored in the farthest reaches of your attic!

    Yes, I do have a Conar 400!

    Conar-1.jpg

    And want a Conar 500 receiver!

    Glen, K9STH
     

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