The AM Forum

  ???  Thanks for Looking and Happy New Year. I ran into a small HUM problem on the RCA BTA-1MX transmitter. It don't matter what I have CHECKED , Changed , tighten,re tighten ect, I still have a slight hum when in transmit with or without audio hooked to it ..Got any Ideals on what I am missing or thoughts  ??   Tim...WD4TC...

Don't you LOVE it !!

Tell us more:

What type of hum is it (frequency - 120 / 60 / something else, "sound" of the hum, etc.)?

How are you hearing the hum (mod monitor, local receiver, etc.)?

Have others reported the hum with off-air reports?

Let us know !

Regards,

Steve

Yes and don't forget to check out the RF hum. You maybe getting it from your mic cord or any other convient source that you or I can't think of right now. Good Luck!

It may even be  a ground-loop hum only audible on your local receiver.

Thanks men for all the input. I have already broken down All the Caps,removed Gnd system, added Gnd, ran into a Dummy Load, changed exciters, hooked or unhooked All Audio Input devices, and on the Air and in remote rec,and scope there is this Gremlin,,I will keep looking and I will find it and it will be just a Little problem but a pain in the Neck to find.I was hoping to have it on the air tonight but I may just have to run the GK.500A ..Tim..WD4TC..

Yes, but what kind of a hum IS it  ;)

Regards,

Steve

Hello guys,
I have fought the same "hum" problem before. It could be RF leakage in the shack. I hear this hum when using the dummy load. Once the TX is part of the T/R system and all grounds are common, the hum will usually disappear.
My mysterious problem sounded like 120 a cycle hum.
If the transmitter is ready for the air...........ax the others in the QSO if they hear your hum. Announce your test to the others and disconnect your audio chain for about 10 seconds, hopefully someone will not key up. If there is a Ham op close to your QTH, you can ask them to work with you while the band is "dead" OR before high traffic starts. I 'm assuming you are trying to get this system up for 160M.
North Carolina is a little too far from me for an early evening (4 or 5PM) attempt.
Fred

Hi Tim,
Try to isolate it a bit if you can.  Unplug the modulators and see if you still get the hum.  If you do, then focus on the RF side of things.  Are all your plate voltages normal?  Sometime those filter reactors can short and power supply ripply will increase and modulate the carrier.  Steve's question about the frequency of the hum is a good question since it will give you a clue where to look.  Full wave rectification produces ripple voltage at 120 htz.  A good Simpson VOM on the AC scale when place across the output of the various power supplies will allow you to measure the ripple voltage.  I prefer a scope AC coupled so you don't have the big DC offset.  Remember oil filled caps seldom fail, but aluminum electrolytics are prone to failure.  Even one on a low voltage supply can modulate that stage with hum and pass it along to the output.   
If you find its in the audio stages, start unplugging tubes down the audio chain to see when it goes away.  Sometime high leakage between filament and cathodes can produce hum as well. 
It's a big subject, but maybe some of my idea's will help you zero in on it...
Be very careful when working with those high voltage supplies!  Sometime you don't get a second chance...
lGood luck,
joe W3GMS     

Dunno where the carrier is generated, but you might want to start there??

Internal - power supply regulation
Extrernal - same thing AND ground loopiness

  Just a thought.

               _-_-WBear2GCR

  Update to this little bug, I have Not had a lot of time to look into the problem or I would answered some of the Questions about the hum .I do thank all of you for the advice and hints. It is Not coming from the exciter,or audio drive.I did have time to run the Bias down on the modulators and NO Hum,Gone,. Nice Clean Carrier on the Scope,and dead air on the monitor receiver. I did find a couple of bad solder joints,and changed some caps, and a bad feed thur Insulator in the audio section. The filter reactor are OK, so the search goes on. Tim...WD4TC..   ???

That's good news Tim,
An Rf hum would have been easier to track down. So, you will have to start isolating the audio driver stages to find the hum, it could still be in the "low voltage supply" or the bias supply for the mod pubes. A 'scope can help out a lot ............be careful with those high voltages! :o
Keep us informed. I want to hear this beast on the air soon.
Fred

Fred, It is close to being there . I have not had it but 9 months or so,  it is right on my time Of 1 year for a 1 week chore. I did have it running before this and I thought when I moved it to the new building everything was going to be Lug and Play,but NOT, anyway I will Get it ! It did not take but 3 or 4 months to track down a broken Feedthur  in the home brew 3-400 transmitter ,
   New thing here is   If it don't glow it's got to Go ...Tim.....WD4TC...
   

  Is it not the Bitch to dig into a transmitter and check everything and think you got the problem solved and see this ?? :-

Been there and done that Tim. I know the feeling and it sucks! This is where you have to get into some creative thinking. If it is possible to make the hum go away ie pull a tube out of the speech amp (did you measure it there or at the grids of the modulators?) Sometimes conventional wisdom is not always the answer. Good luck and let us know what you find.
PS: I'm secretly jealous of you since I would love to have a BTA-500MX or -1MX ;>)

  ;D   Problem Solved....If you look long and DEEP enough you WILL/CAN find the problem..After looking into the wiring for the Audio section on the RCA and replacing this small oil filled cap and 1 foot piece of  wire the  HUMMM is GONE....Now to get it hooked back up and on the Air...
  Thanks for All the help and hope to hear you on 1885 soon...Tim.....WD4TC

Most excellent results, Tim! I hope to hear you on 1885 as well.