Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: Prev 1 [2]   Go Down
  Print  
Author Topic: 811-H and RF feedback?  (Read 5947 times)
WG7X
Member

Posts: 349




Ignore
« Reply #15 on: February 25, 2011, 11:17:44 PM »

Charlie,

What type of connection do you have at the antenna feed point?

Is there a balun up there? I ask because I had similar problems with a balun that would overheat when the core saturated and then the VSWR would increase as it heated.

This sounds similar to what you are talking about.

Just curious.

Gary
Logged
KF7CG
Member

Posts: 1215




Ignore
« Reply #16 on: February 26, 2011, 07:02:12 AM »

Check nuetralization on amp. If it is marginal, than the amp can run for quite a while until something just barely changes and then break into self-oscillation every time it is keyed, or even intermittently. Used to build my own transmitters, nuetralization was always a problem. Grounded grid helps but isn't a cure all. Back in my old CW separate transmitter days, final amplifiers were often run as doublers to get around tuned grid tuned plate oscillation.

If changes at your antenna are letting to much of your amplifier output get back to he input then you could have a big problem.

KF7CG
Logged
KH6AQ
Member

Posts: 7998




Ignore
« Reply #17 on: February 26, 2011, 11:28:52 AM »

The problem is the transceiver and not the amplifier.

The transceiver, with no audio into the mike, goes full power. The amp ALC voltage goes to maximum. This is a symptom of RF feedback.

What experiment might help prove or disprove this hypothesis? When the transceiver/amp display the problem switch the transceiver to receive. Advance the amp LOAD to drop the maximum available RF output power. Switch the transceiver back to transmit and see if it goes into transmit.

Another experiment is when the problem occurs switch the amp into a dummy load. Does the problem occur?
Logged
KB3BTO
Member

Posts: 41




Ignore
« Reply #18 on: February 26, 2011, 04:28:21 PM »

  Howdy, boys.
  Let's see... the dipole antenna doesn't have a balun. It's fed with ladder line.
  I'm not familiar with "neutralization." I'll have to research it and check that aspect. Thanks.
  I'll have to double check to see if the meters peg just by hitting the PTT or if it only happens when I begin to speak. I "think" that it's only when I begin to speak. It's just a rapid reflex action: push then speak, so I'm not positive on that at the moment.
  I don't understand why, if this is a transmitter problem, does it only occur when I'm using amplification and not when I'm running barefoot? Wouldn't it be a constant condition if it were the transmitter?
  I didn't have much time to get on the air today but I did work a few stations during the Mississippi QSO party this afternoon. These were the short, signal report, 73, type of QSO's. I had the amp connected but not turned on and was running 80 watts. I had the TS-940 running for well over 2 hours and there was no sign of the excessive meter deflections and I was getting 5/9 reception reports.
  I don't have an "EE" degree, but is there any high percentage chance that a wonky transistor would be effected by the heat that develops after the amp has been turned on for a while and would somehow cause this problem? I'm probably reaching a bit with this but it's all that I have to offer right now. (Okay, okay. I'll admit it. I'm getting desperate.)
  As always, thanks for your constructive input.
  73
  Charlie.
Logged
KH6AQ
Member

Posts: 7998




Ignore
« Reply #19 on: February 26, 2011, 04:41:28 PM »

An RF feedback problem requires that a certain threshold be passed. If that takes more power than the transceiver outputs it will not occur until the amp is amplifying.

How about if you go try it with the amp?
Logged
KB3BTO
Member

Posts: 41




Ignore
« Reply #20 on: February 27, 2011, 09:17:19 AM »

  I found it! It was the mic. Yep, the mic. (I know, I know...)
  I use a MC-60A mic and, early this morning when I was mentally going through this mess for the umpteenth time, I had narrowed down what I had left to change or check before I boxed up the amp and sent it back to Ameritron. I reasoned that I would replace the mic, the coax coming from the Bird 43 to the amp and the from the rig to the Bird, and check to see if the feed line had shorted out, somehow. I was desperate.
  Anyway, I had also posted this question to the Yahoo/Ameritron group and a ham wrote back that he had a friend that had this problem and it turned out to be a shorted wire in his mic. I thanked him and told him that I had just had the same idea and would try that solution. That was the magic bullet.
  I worked several DX stations this morning running 400 watts, driven by 21-22 watts, and had none of the excessive meter deflection problems and had good signal reports; therefore, I must assume that the mic was the cause.
  I, for one, shall remember this event for a very long time and will file it under "lessons learned."
  I would like to once again thank all of you who provided constructive input. Your help was greatly appreciated.
  73
  Charlie Shaw
Logged
KD8GEH
Member

Posts: 620




Ignore
« Reply #21 on: February 28, 2011, 06:57:07 AM »

Congrats Charlie! You found the gremlin.

Welcome to the Kenwood mic world. You may have to add a secondary ground to get rid of it. Interesting.

73  Dave KD8GEH
Logged
KH6AQ
Member

Posts: 7998




Ignore
« Reply #22 on: March 01, 2011, 06:51:28 AM »

It's good you didn't send the amp in for repair. None of the symptoms pointed to the amp as the problem yet folks had you opening up the amp, poking around, and getting ready to send it in for repair.


Logged
KB3BTO
Member

Posts: 41




Ignore
« Reply #23 on: March 01, 2011, 01:23:22 PM »

  Well, Dave, I figure that most of those folks meant well and were trying their best to help me and I'm thankful for their assistance. It's difficult for most folks to diagnose a tech problem just by reading what someone wrote in a brief overview.
  Now that the crisis has passed, I find it rather humorous as to how seemingly unrelated conditions once again arranged themselves in such a manner as to point in one direction when the real problem was back behind me. For instance, how the condition only surfaced when the amp was being used and not when I was running barefoot. Talk about a "red herring!" It surely pointed to the amp for me.
  I never could figure out the path from the amp and back into the rig, not once. That's because there never was one. It was so obvious and yet I overlooked that one major fact again and again. Well, no one was born with the knowledge. Everyone had to learn it somewhere, sometime, and this was quite a learning experience for me.
  Happy hamming to you.

  73
  Charlie.
Logged
Pages: Prev 1 [2]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!