The AM Forum

Anyone want to lend a hand here.  Used this king for 5 years of 75 and 40 meters. Today, I loaded it up on 15 meters.  Big hum on signal.  Traced it to the VFO/exciter.  I can hear/see the hum on the FT101EE/IF scope sitting next to the king when I tune into the VFO signal.

I loaded the king back on 40 meters where its been for years, No hum. Dead silent full power. 

Here is the schematic.  I hope I dont have to go inside the VFO. Its hot braised to the chassis and a royal PITA to get inside. 

Guess I am stuck at 20 watts from the FT101EE on 15 for now.  Thinking of bringing in at least a ranger :(

C

Sounds like a ground loop somewhere in the cabling maybe. Disconnect everything and run into a dummy load and test with a CW carrier.
If its inside the rig you might need a real scope.

Clark, There is a big difference between observing hum in a monitor receiver, and what you send down the coax to the antenna. I've had hum induced issues countless times when my transmitter was clean. A dummy load is useful here too if the shack is "too-hot" with RF. So divide the possibilities in two to determine if you are actually transmitting a carrier modulated by hum, or if your FT-101 monitor is behaving as if there is a ground loop when you are on 15M.

Jim
WD5JKO  

Edit, Sorry Carl..We posted at about the same instant.

Thanks guys.

The Rig was into a Bird Dummy load.  The HV OFF and not transmitting.  Just the VFO turned on which also keys up the BUFFER stage.  You tune the osc.  Then you tune the buffer and set the Grid.  At this point, I can see the HUM on the IF scope of the 101 and hear it on the ICOM and FT101. 

When I do turn the HV on, I can see the ripple on the scope and Timtron mentioned it on the air.  He said it was strong.

At that point shut things down and went back to the dummy load and retuned the rig back on 40 meters where the signal from the VFO is clear on teh 101 IF scope and ICom.  No ripple. I switched back to 15 meters and tuned the VFO and the hum was back. 

So the hum is comming from the VFO it self or the OSC which is the top deck of the rig.

My Friend Jim mentioned to me that it could be there on 75 and 40 but very low and when its multplied up to 15 meters, the hum is heard there.  I guess thats possible.

Should I try a Xtal now?
C

Yes, try a crystal. That will bypass the VFO and help you focus on the proper circuits.

  Absolutely! I used to have a beat up King 500B and used it on 160-10m without hum. If the crystal fixes the hum problem, then there might be an issue with that VFO shielded cable between the decks that is band dependent. Also, I recall having a copper strap to bond the three decks together. It's been a while since I used it (1985?) so I cannot remember why I had to put that strap in there.

  The VFO runs on 40m for 40m, and runs on 40m for 10m (X4). I still don't trust a monitor receiver, and on the air reports are pretty subjective.

Don't you have a "Clean RF, Variable RF sampler"? Use that with a real scope.

http://www.nu9n.com/am.html#monitoring

Jim
WD5JKO

Yes. I have the Clean RF sampler and a Tektronics scope. I have been using it. It sames anything that leaves the shack.  But what it wont see is just the VFO tune on with the HV off.

So I ran a test. I found a 6990 Xtal in a junk bin. That got me around 21.000.  I tuned the VFO to 2100. No hum on icom. Then used the Xtal, No hum on VFO either. WTF?

I guess Jim and Carl where right.  If I turn the FT101EE simply ON, The Hum is there super loud.  If I turn the FT101EE OFF, The hum is gone. WHAT the hell?

I was using the FT101EE as a receiver and switching the coax switch on 15 meters with the king 500.  The damn SX28A slipped the dial cord and is now way off Freq so i used the FT101EE as the receiver.

I dont think there is anything wrong with this King after all !

C

Dont forget that as you go up in frequency lead/cable length becomes more and more critical. What was once a ground is now an antenna.

Just the internal chassis harness wiring could be leaking enough RF to either get back into the audio or even back into the VFO. You should also check to see if you are getting any FMing, that is a sure sign that you have stray RF getting back into the VFO. A simple oxidized connection at one of the harness plugs might not be much of an issue on 75 & 40, but could cause a real problem on the higher bands.

Stray RF becomes more and more of a problem as you go up in frequency.

Also take a real good lookie at your scope, use the envelope waveform and NOT a trapezoid and see if there is another frequency mixed in with your operating frequency. the hum could be something that the lower stages are putting out mixing with the fundamental frequency. This could also show up as FMing.

Jim's idea of tying all of the chassis together with a good non-inductive ground strap could be a good place to start.

Did some work on the station today. The Hum is from the Yaesu YO100 Oscope and not the FT101EE. The YO-100 has a two wire cord on it.  I flipped the cord around and the problem is gone.  Very simple solution to a complex problem.  Why this shows up on 15 meters is beyond me. The GK500 never had any trouble.   

C