Hummmmmm

[N2DTS]

I have various transmitters and various exciters and notice some have a lot of hum on the carrier, its not the high voltage supplies to the RF decks or to the plate modulators, it seems to be on the exciters more then anything else.

Those seem well filtered on the power supplies, choke input, plenty of capacitance.
Where is the hum coming from?

I just built a vfo to replace the Collins PTO in one exciter and it works well, I need a bit more drive to the 6146, but it seems loaded with hum.
How can an RF exciter at 7 Mhz have a hum on it?

[Pete, WA2CWA]

Could be an issue with shielding or close proximity of components to each other or grounding or by-pass caps. A scope should be able to track it down.

[steve_qix]

This is probably a dumb question, but how do you actually know there is hum?

It does seem unlikely given the good power supplies and all . . .

[WD5JKO]

Those seem well filtered on the power supplies, choke input, plenty of capacitance.
Where is the hum coming from?

One possibility where there is more of a 120hz buzz is the diodes in the power supply. The reverse turn off time of the diodes keep one diode on for a few microseconds when the desired diode is already on. This temporarily shorts out the transformer. Then a microsecond or so later the reverse biased diode finally turns off. Then the transformer like a snake, puts out a high amplitude back EMF spike. This spike goes straight through the filter choke (turn to turn capacitance), and then is hardly even seen by the big filter capacitance since the big cap is inductive such that a microsecond or so wide pulse passes straight through. This happens 120 times a second. These pulses are hard to see on a scope due to the low repitation rate and narrow pulse width. With a decent scope they can be seen.

The cure when I run into the scenario above is to follow the choke input filter with an RC where the R is perhaps 100 ohms, and the C is a 0.1uf polypropylene. Some folks instead put a 0.01 uf across each diode, or use a fast recovery diode instead. The 1N400X (X=1-7) is famous for this problem.

I am sure there are those that think what I am saying is totally nuts. Perhaps! 

Jim
Wd5JKO

[KD6VXI]

I have the exact same problem in my shack.   Nice hum modulation on anything I listen on.   SDR,  receiver,  yeasu or Kenwood,  even a CB radio (either base or mobile,  tried both).

Everything is bonded,  gfi outlets at every plug so no leakage.   Even TX into dummy load.

On air recordings sent to me are beautiful.   No trace of a hum at all.

Friggin ground loops / pin1.


--Shane
KD6VXI 

[Pete, WA2CWA]

Reduce the low end frequency response; no need to sound like Barry White on the radio.

[W7TFO]

The same effect always happens in broadcast AM co-located stations...the detector is overloaded (biased into non-linearity) and responds with hum that misleads engineers.

The fix?  Back off the RF into anything listening, or swamp the simple diode.

73DG

[flintstone mop]

If you are monitoring on a receiver with just a wire connected its input; then that can cause the RF hum. All of the RF equipment should be connected using coax. The monitor receiver needs to be isolated from the T/R relay with an additional T/R relay so you don't fry its front end. Most vintage table top receivers aren't going to have enough shielding to stop pick up of the stray RF in the room

[w1vtp]

The last two respondents may have something. So what are you using to detect this hum?  A simple swamped diode circuit is my suggestion.  Has anyone else in a QSO told you had this hum?  I have never heard you with one

Al

[MikeKE0ZUinkcmo]

My Humm was pretty simple.   A ground loop.   I built up a control box to allow switching Key, sequencer leads, mute and mic audio between 6 different rigs, and when I got done, the guys complained about AC hum on the audio.

After a little pondering, I found the problem, and added an isolation transformer out of an old grief kit phone patch, easier said than done I might add because I had to add another wafer switch section and rewire a few control cables, but the hum's gone.

[N2DTS]

Thanks much for the tips/idea's.

Here is a very odd thing:
I replaced the PTO in my exciter with a vfo because there is no good way to use a pto without the dials and scales that go with it, plus it drifed a good amount and never seemed stable.
The VFO allows me to change the frequency between bands in the vfo, plus I get to use a cap with a dial I can mark with frequencies.
Anyway, 6c4 osc, 12by7 buffer tube, 6ca7 driver, 6146 output tube, standard pie net.
Works great on 80 meters, low plate current, low hum.
On 40 meters it takes a lot of plate current to drive the grids of my 813,s and has a VERY loud hum on the output.
If I put it into the antenna I have no problem getting 40 or 50 watts out, with the LOUD hum.

Something is very wrong on 40, and it worked great on 80.
It drifts under 30 Hz over a half hour of a qso so that part is good.


As far as how I can tell about the hum, I can hear it in the mod monitor when its loud and see it on the sdr when its not, and other people I have worked on the air (with sdr's) can hear and see it.

I now have so idea's to try, but need to figure out the 40 meter problem first, maybe something is tuned to a harmonic or something...
Why that would cause hum is beyond me though...

[Opcom]

If you are monitoring on a receiver with just a wire connected its input; then that can cause the RF hum. All of the RF equipment should be connected using coax. The monitor receiver needs to be isolated from the T/R relay with an additional T/R relay so you don't fry its front end. Most vintage table top receivers aren't going to have enough shielding to stop pick up of the stray RF in the room

seen this lots. was annoying until figured out. same for the diode post above it.

[N2DTS]

Got it fixed, or at least the hum is way down, -45 db below the carrier.
1st I had some sort of very strange parasitic in the drive control on the 6AG7 driver tube.
The diagram of the exciter is in the 1967 handbook, page 189, a 75 to 120 watt 6146 transmitter.
I had to put a .01 mylar cap at the pot  hot side.
Without it the thing was transmitting a strange broad signal at 7.4 something MHz and the carrier at 7290.
I noticed the swr was way off with the thing connected to the antenna and tuned around with the sdr to find it.

I got good power out but still had the hum and needed to put a .47 mylar cap on the 6146 screen voltage adjustment pot.
It seems these screen circuits will pick up all kinds of trash very easy.