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

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

ad: L-HROutlet
ad: l-rl
ad: HRDLLC-2
ad: abrind-2
ad: Left-3
ad: Left-2
ad: MessiPaoloni-1
ad: L-MFJ
ad: L-Geochron
  1. SM0AOM

    SM0AOM Ham Member QRZ Page

    If you short out R4, there will be no metering, as there will be no voltage drop to measure.

    But a few volts more or less in the cathode circuit will make very little difference, so you can add another external resistor of a few tens of ohms between key arm and ground, and use the voltage drop over it for powering the oscillator.

    A better solution is however to use an opto-coupler to key the supply voltage for the sidetone multivibrator, as it makes the circuit completely galvanically isolated.

    G3EDM likes this.
  2. G3EDM

    G3EDM Ham Member QRZ Page

    OK will try out the "outboard" option tomorrow. Obviously it is a key part of the troubleshooting!

    73 de Martin, G3EDM
  3. GM3ZMA

    GM3ZMA Ham Member QRZ Page

    The key click thing,
    I am sure that I have mentioned this before, but connot find where it is written here on the ZED.
    I think that the problem is due to the way that the sidetone oscillator is powered.

    The oscillator is powered by the volt drop across resistor R4, OK no problem with that.

    But when the key is "up", the whole sidetone oscillator circuit is sitting at some high voltage.
    What is the voltage across the key (J1) when it is open?
    When the key is closed the sidetone oscillator sits at zero volts ie connected to chassis.

    This rise and fall of what is effectively common mode voltage at the oscillator, will charge and discharge capacitors C16 and C17.
    I suspect that this charging and discharging of capacitors is what is creating clocks in the headphones.

    Jim GM3ZMA
    N2EY and G3EDM like this.
  4. G3EDM

    G3EDM Ham Member QRZ Page

    I just measured it at 65V on the Simpson VOM.

    73 de Martin, G3EDM
  5. GM3ZMA

    GM3ZMA Ham Member QRZ Page

    Right, I suggest that here lies your problem of the key clicks that are not key clicks and don't respond to all the usual key click suppression methods.

    Jim GM3ZMA
  6. G3EDM

    G3EDM Ham Member QRZ Page

    @SM0AOM, @GM3ZMA.

    OK the key-click puzzle is basically solved, although the final resolution still needs to be figured out.

    I moved the sidetone oscillator outboard. This was easy because I already had a duplicate (my code practice oscillator, whose circuit is almost identical).

    Also disabled the transmitter's internal oscillator (by disconnecting it from R4).

    Powered the external oscillator with a 50-ohm resistor (two 100-ohm, half-watt resistors in series) in series with the key, giving 2.5V which is more than adequate to drive this oscillator.

    Didn't have a spare 10K potentiometer for the volume control, so stole the antenna attenuator pot from the T/R switch (it doesn't get used much any more because there is now an inboard antenna attenuator pot on the new receiver front panel). For those who are wondering, it is not in the transmitter's schematic but I will add it to the next posted version. Basically you replace R12 with the pot, and the wiper is connected to C17. This already existed on the inboard oscillator, I just never bothered to add it to the schematic.


    Powered up. Result: Key clicks almost as bad. Not totally deafening like before, but still objectionable enough to cause major operator fatigue.

    Then I tried disconnecting the receiver from the oscillator, and plugging in high-impedance headphones into the oscillator. Result: No key clicks at all, perfect tone. This is new, it was not the case with the internal oscillator. (I had tested it before the alterations, by plugging the high-impedance headphones directly into the transmitter's headphone output with the receiver disconnected from the transmitter. Also, I had to suffer through several weeks of key clicks when using the 3-transistor receiver which has a high impedance output.)

    Now, remember that this sidetone generator is connected at the (high impedance) junction of the first AF and the second (final) AF of the subminiature tube receiver.

    My next test was to connect the output of the sidetone oscillator only to the AF deck (the bottom deck) of the subminiature-tube receiver. This means it was working straight into the push-pull final AF (as usual) but without any wiring connecting it to the first AF stage or the earlier RF stages.

    Result: no clicks (well, if you listen hard there is a click but nothing to be concerned about).

    Edited to add: I just had another listen. Actually, they are not even clicks, they are just a rather clean "attack" at the start of the pulse, to a background of total silence. I could probably shape it to be softer with a bit of messing around with component values, but who cares.... Basically, it sound exactly like my good-old code oscillator.

    So what's almost certainly happening now is that sparks from the key are being picked up in wiring in the RF deck of the receiver, and somehow getting through to the AF deck despite being disconnected by the muting relay. The relay is a small reed relay and the RF must be crossing the contact gap, or, just as likely, simply the leads around and to the relay are picking up RF. That top deck is essentially unshielded apart from a grounded front panel. So no surprise really.

    We are finally in conventional "track down the RF issues" territory. I am now sure it can be fixed.

    A one-step solution, possibly, would be for the muting relay not merely to disconnect the detector from the first AF stage (which is what it does now) but to kill the RF and first AF stages more effectively. It's a SPDT relay with "receive" being the un-powered position. Powering the relay for TRANSMIT opens the circuit.

    Or, I could try moving the muting relay from where it is (between the detector and the first AF) to be, instead, between the two AF stages. Not sure whether that is a good idea or a bad one....

    Whether it was actually necessary to move the sidetone oscillator outboard I am now not totally convinced. But: I like it that way, in particular, I like having the volume control easily accessible. I have temporarily placed it inside a grounded biscuit tin, but will probably mount it in a proper shielded case. (Also: I think the clicks may have multiple causes, and having the oscillator mounted inboard a couple of inches from the PA tank coil was asking for trouble.)

    Various things I have already tried, none of which made any difference:
    • Increasing the distance between the key and the oscillator.
    • Increasing the distance between the key and the receiver.
    • Inserting the ARRL choke-and-resistor RF filter at the key.
    In case you're wondering, the T/R switch in TRANSMIT position doesn't just activate the receiver's muting relay (disconnecting detector from first AF), it also shorts the receiver's antenna terminals.

    I am going to be spending some of the next few weeks doing boring but necessary work regarding shielding, grounding and so forth. Adding a cabinet to the top (RF) deck of the receiver is one of those things.

    73 de Martin, G3EDM
    Last edited: Oct 14, 2021
  7. G3EDM

    G3EDM Ham Member QRZ Page

    Sorry, obviously in the 5th paragraph of the previous post, I got 50 ohms by connecting two 100 ohm resistors in parallel (not in series, as I mistakenly wrote).

    73 de Martin, G3EDM
  8. G3EDM

    G3EDM Ham Member QRZ Page

    Finally the problem of key clicks on the sidetone has been solved.

    First, I removed the sidetone oscillator from the transmitter and made it outboard. I estimate this reduced the loudness of the key clicks to 75% of what they were before.

    Next, I moved the muting relay in the receiver. It used to be between the detector and the first AF, I moved it to be on the "lower deck" of the receiver, so it is between the 1st AF and the 2nd AF. Result: the loudness of the key clicks is reduced to about 25% of where it was before.

    That's good enough for me. The loudness of the key clicks is fixed and is independent of where you set the sidetone volume control. But the clicks are much quieter in the first place and that's what matters.

    If you manually break the circuit between the "RF deck" of the receiver and the "AF deck" then the key clicks vanish altogether. So my theory is that because my muting relay is small (reed relay) the RF from the sparks at the key is travelling to the relay (via the RF stages of the receiver) and then leaking across the open contacts.

    In an effort to eliminate clicks altogether I went further and shielded all the audio signal cables between the first AF and the second AF. It did not make any audible difference to the clicks, but it was worth doing just as a process of elimination -- and I suspect it may help with other issues such as the (very mild) AC hum I hear from time to time.

    I also did an experiment by shielding the entire radio (cardboard box lined with grounded aluminium foil) which made no difference at all to the clicks. What it did do is greatly worsen the performance of the receiver (and in my experience this is not unusual for a regenerative radio).



    Anyway this particular saga (the "key clicks") is over.

    Other receiver news:
    • Took delivery of the new silver mica capacitors and installed them, so can now calibrate the dial to the new variable capacitor.
    • Took delivery of the panel bushing and the plastic shaft for the regeneration throttle and installed them.

    But the best thing is that there are now no obvious equipment barriers to just operating, rather than messing around with modifications to the equipment.

    I do not have to put up with the deafening key clicks anymore.

    73 de Martin, G3EDM
    Last edited: Oct 14, 2021
    KP4SX likes this.
  9. K4NWD

    K4NWD Platinum Subscriber Platinum Subscriber QRZ Page

    Persistence pays. Carry on, OM!
    G3EDM likes this.
  10. G3EDM

    G3EDM Ham Member QRZ Page

    The dial is now calibrated for the new tuning capacitor.

    Those frequencies may seem arbitrary, but they are actually those of my "known good" FT-243 crystals.


    73 de Martin, G3EDM

Share This Page

ad: Alphaant-1