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Hum in vfo/exciter???

Discussion in 'Amplitude Modulation' started by N2DTS, Nov 24, 2021.

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  1. N2DTS

    N2DTS Ham Member QRZ Page

    I have a home brew VFO/buffer/exciter I built years ago, its a 6C4 osc that runs all the time and I key the buffer
    and 6146 output tube.
    Hum seem to be 35 db down from the signal within the vfo/6c4.

    Its not on the high voltage, I changed the tube, I even ran the filaments off dc and the hum is always there.
    I added a choke to the power supply, added a lot of capacitance, zero change.

    I am unsure if the hum was always there or it started at some point, I used the exciter (on 40 meters) for many years but only a month ago got a report of hum on the signal.

    I do not understand how I can have hum on it when running everything on DC...

    The power supply:

    No matter what I do the hum seems to be on the output of the vfo, even before the buffer tube (6AU6)!
  2. W7TFO

    W7TFO Premium Subscriber QRZ Page

    RF is getting into it's own works. Either a feedback problem or a parasitic.

    If the hum is neither 60Hz or 120Hz (as previously stated), then is a heterodyne.

    It can be a real bugbear to find...

    Got a spectrum analyzer?

  3. N2DTS

    N2DTS Ham Member QRZ Page

    Its 60 cycles....
  4. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Have you tried looking at the output waveform on a scope; is the hum visible on the trace? Have you been able to get any signal reports whether or not the hum is audible at a distance on your OTA signal? I sometimes hear a strong hum in my own VFO signal when spotting for zero-beating purposes, but it is not audible on my transmitted carrier. It could be a ground loop problem. At times I can connect a clip lead between the receiver cabinet and the VFO chassis and the hum is greatly reduced or even goes away. Other times I can perform the same exact test and the hum increases!

    The Globe King 500 and BC-610 are notorious for hum on the carrier when using the built-in VFO. This is not due to an inadequately filtered plate supply somewhere; it's frequency modulated hum, amplitude modulated, and either the receiver slope-detects it in AM mode or else it's some kind of intermodulation with a.c. mains voltage. With the BFO turned on, FM hum modulation will be clearly audible as a 60~ fuzz on the heterodyne.

    My hum as heard on the receiver is annoying when I try to monitor or spot my own signal, but since it isn't transmitted over the air I don't worry too much about it. There must be a jillion ways 60~ a.c. can modulate RF from local sources before the signal gets to the receiver. Maybe from small 60~ voltage differentials from chassis to chassis amongst various pieces of equipment in the shack. Or an imperfect connection somewhere has corroded and produces rectification. Sometimes successful measures to silence the hum from one piece of station equipment increases it to another.
  5. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Do you measure this hum when the VFO is in complete standalone mode on the test bench with nothing more than its power supply connected or do you observe this when it's in-service and connected to other equipment like a transmitter? If the latter I'd look for ground loop issues with the interconnected gear and might try a short low impedance ground strap between your VFO and the transmitter it will drive.

    If the VFO supply is independently grounded (e.g. via the AC cord) you might try single point grounding it to the grounded transmitter instead of having multiple ground connections via the VFO power supply, interconnect cables to the transmitter and transmitter power supply if not other grounds.

    Maybe all that checks out or the hum has nothing to do with ground loops but given what you've posted those are things I'd look into.
  6. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    And BTW Brett, does that frequency counter modulate your VFO signal? I tried connecting my bench frequency counter to the VFO output by inserting a BNC tee-connector in the coax line from VFO to transmitter, but it modulated the signal with a lot of spurious rubbish. Like the hum, it may not have been audible on the transmitted signal, but I still disconnected it.

    I run my VFO oscillator stage filaments off a regulated DC supply. Not because of hum problems, but even with the Collins type PTO, small changes in a.c. line voltage (even less than 1 volt) vary the filament voltage enough to cause a noticeable drift, which becomes problematic when I try to operate 40m CW, since the VFO signal on all bands is multiplied from a 160m fundamental.

    I run my 75A-4 receiver off a miniature Sola constant voltage transformer for the same reason.
    The transformer was originally used to run an office server, and unlike every other CVT I have ever tried to use, it produces very little acoustical buzz, not enough that I can hear it unless I put my ear right next to the transformer.
  7. K5UJ

    K5UJ Ham Member QRZ Page

    I run 3 counters off VFOs but the line from the T section ends with a small 10 pF 3 KV ceramic cap at the counter BNC jack. They don't need much RF to work and I didn't want to blow the counter input with high p. to p. RF voltage which some of the old VFOs produce. One rig has no physical connection to the counter; it's driven by a couple of banana plug leads hanging inside the rack where the RF PA is, the other ends plugged into one of those banana plug to BNC female adapters.

    As always, really nice professional looking homebrew work there.
  8. N2DTS

    N2DTS Ham Member QRZ Page

    Well, thanks for the idea's guys.
    The freq counter has no direct connection to the vfo, its just a probe into the vfo box, it picks up enough signal.
    I tried everything, running just the vfo tube (6C4) and not the buffer I seemed to have the hum.
    I then thought the RF stages should not really pass 60 Hz, its made to pass 7 Mc stuff, not audio!
    I had a 6kv .004 uf disk between the 6146 plate and the pie net (dc blocking) and thought I would reduce that and put in a .001 uf cap.
    That or something else I did seems to have reduced or eliminated the hum on the carrier.
    I asked a number of people with good receivers if they heard any hum and no one did.

    So was it the switch to .001 or was it something else?
  9. N2DTS

    N2DTS Ham Member QRZ Page

    We have some of those sola CVT power supplies (48 volts 10 amp) and they make a huge racket!
    We do not use them anymore, now most equipment has super loud fans....

  10. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    Most of them do. But mine is something like 75 VA, just barely enough to feed the 75A-4... 115 volts, < 1A, and it's barely audible.

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