My First-Ever QSO -- 50 Years After Passing Ham Test

Discussion in 'Ham Radio Discussions' started by G3EDM, Aug 27, 2021.

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  1. G3EDM

    G3EDM Ham Member QRZ Page

    Finally the modification suggested last month by Karl-Arne, @SM0AOM: adding an RC filter to the sidetone generator to block the RF.

    I had been dreading this alteration because it involved fairly extensive deconstruction of the transmitter. The added pair of VR tubes, and their bracket, had to be removed to get access to the sidetone generator. Space is tight in there.

    [​IMG]

    Anyway the new circuitry was added. The orange capacitor is the "C" part of the filter, the carbon film resistor near the left edge of the picture is the "R" part.

    Result: Nada. The key clicks are just as loud.

    Karl-Arne suggested that in that case, increase the value of C. I started out with .01uF. So I tried .1uF. Result: key clicks just the same, but now, there is "chirp" on sidetone (Karl-Arne warned me this might happen).

    End of experiment. I removed all of the new circuitry and screwed the bottom plate back on.

    Enough is enough. I don't know what the problem is with that sidetone circuit, and I now strongly suspect that it never worked properly in 1968 unless you were prepared to ignore the deafening key clicks (and I cannot).

    The plan now is to build a new, external sidetone generator. It is required because I use a regenerative receiver, and they are unable to monitor a local on-air signal (they get overloaded). I may even decide it is worth using a keying relay, or rather two of them, in separate, shielded boxes (one to key the TX, the other to key the sidetone generator). Yes, the relays are noisy, but they could never be as noisy as the key clicks I am hearing now, and my sending is slow enough that the relays should be able to keep up!

    73 de Martin, G3EDM
     
    Last edited: Oct 13, 2021
  2. G3EDM

    G3EDM Ham Member QRZ Page

    So that was the story today. It may sound negative, but actually, troubleshooting is a process of elimination. Having eliminated all the obvious candidates for solving my problem, the problem is still there.

    Sometimes you just have to give up trying to make the flawed equipment work and choose a better solution altogether!

    Much of what I did today was good in its own right. The key-click filter at the key is a good idea and will stay wired in. Shielding all of the audio interconnect cables was obviously a good idea anyway, and should help reduce issues including mild AC hum that I have been hearing.

    73 de Martin, G3EDM
     
  3. SM0AOM

    SM0AOM Ham Member QRZ Page

    Since you, by process of elimination, have investigated all "probable" causes, it is now time for the "improbable".

    The sidetone multivibrator uses the voltage drop over the resistor R4, which is about 6 V at the 5763 operating point. An RC filter with C across the supply terminals of the multivibrator did not help, which rules out RF travelling on the leads from the resistor.

    To find out the cause, which I more and more suspect is direct induction coupling from the pi-filter coil into the germanium transistor circuitry, remove the crystal and key very briefly. If the clicks still are there, it is sparking at the key contacts that is the problem, if they vanish, it is direct RF coupling. Sparking does however seldom contain so much energy that it can affect multivibrators directly.

    The germanium transistor base-emitter junctions are very efficient RF rectifiers, and a transient will throw off the operating points of the transistors. Shunting the base-emitter junctions with a 1000 pF or so small ceramic capacitor may help.

    Another way is to move the sidetone oscillator out of the transmitter chassis, and provide some RF filtering on the key leads.

    However, this is a lot of work. When faced with problems like this in my professional practice, I reach for a "magic wand"; the opto-isolator or MOSFET opto-relay.

    By using this method, all RF sensitive circuits become isolated from the environment by a low-capacitance barrier, and can be decoupled and shielded to "your heart's content".

    Place the LED across a small resistor in series with the key-line so a few mA of current flows through it when the key is pressed, and the photo-transistor in series with a small battery supply which in turn feeds the multivibrator. I have used the Clare LCA110 and PLA140 in previous projects, but there are other makes.

    The very "tough" PLA140 was used to solve an annoying reliability problem in a 10 kW Collins amplifier which had the bad habit of welding the grid-bias keying relay contacts together when a flash-over occurred in the PA tube.

    upload_2021-10-13_19-12-28.png

    73/
    Karl-Arne
    SM0AOM
     
    G3EDM likes this.
  4. G3EDM

    G3EDM Ham Member QRZ Page

    You suggested that before and I tried it then, and again today. Removing the crystal makes no difference to the key clicks.

    I did consider adding extensive shielding (aluminium barriers) between the tank circuit and the sidetone oscillator. That would be possible, but time-consuming in terms of mechanical construction, and made quite a lot harder by the existence of some added VR tubes in that area. I ruled it out as too much trouble, but: do you think there is any percentage in giving it a go?

    Seems to me the obvious solution is to build a simple, self-contained external oscillator. Can be used with this rig ... and with others in future, if there is an issue with local reception, or if "split" operation is desired.

    I have one circuit in front of me from ARRL in early 60s ... it uses a neon bulb as the oscillator!

    73 de Martin, G3EDM
     
  5. G3EDM

    G3EDM Ham Member QRZ Page

    I may try that. Ironically, I've already built it: it is the code practice oscillator from "How to Become a Radio Amateur" (identical circuit). I have it here in front of me (this photo was taken while I was building it about seven years ago). So no need to rip out the existing circuit, just disable it!

    [​IMG]

    73 de Martin, G3EDM
     
  6. G3EDM

    G3EDM Ham Member QRZ Page

    @SM0AOM: I guess I need to run two leads out of the TX, from either side of R4, to power the external oscillator?

    (I don't see a way to take it direct from the keying line.)

    73 de Martin, G3EDM
     
  7. SM0AOM

    SM0AOM Ham Member QRZ Page

    That there is no difference between keying with or without the crystal makes me wonder
    if there could be a parasitic oscillation or something similar around.

    If this is the case, it is not very effective to try shielding, as the oscillation may be at a frequency
    widely separated from the fundamental signal.

    73/
    Karl-Arne
    SM0AOM
     
    G3EDM likes this.
  8. SM0AOM

    SM0AOM Ham Member QRZ Page

    If you intend to use the voltage drop over R4 in its present position for powering,
    two wires are necessary. However, you can place R4 wherever you want in the circuit.

    Nothing prevents you from placing it between the key arm and ground.

    73/
    Karl-Arne
    SM0AOM
     
    G3EDM likes this.
  9. G3EDM

    G3EDM Ham Member QRZ Page

    OK so short R4 in the transmitter and place it outboard. What does that do to the metering??

    Presumably not much, the main shunt is R9? Should R9 be increased to 11k?

    (Not a big issue right now. The main thing is to find out if moving the sidetone oscillator outboard makes any difference to the key clicks. The details can come later....).

    73 de Martin, G3EDM
     
  10. G3EDM

    G3EDM Ham Member QRZ Page

    Apologies to those reading this who are seeing references to particular components in the schematic. As an aid, here is, indeed, the latest schematic:

    [​IMG]

    73 de Martin, G3EDM
     

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