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Post by gator7 on Dec 27, 2008 9:14:45 GMT -5
My mark III has a dislike to cold weather. Well, cold for Florida that is. When we have 30-40 degree temps. The xmiter will not hold a steady carrier. After i key up the RF drops to almost zero, then comes back to the normal dead key. This takes place for at least 30-45 Min's after power up. Duration of carrier fade is 2-3 seconds. Not sure, but I think it happens either dead key or under modulation. I understand that the old tube rig needs to warm up a bit. And yes the warmer it got the less it did the carrier drop. So if I had let it get up to operating temps before transmitting, after a hour or so, I would not have noticed it. But, what I am wondering is, what would cause it to do that besides the fact that it was cold? Tubes, Power resistor breaking down? Any ideas?? I have not pulled the tubes yet. Thought I would ask 1st.  |
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Post by gator7 on Dec 27, 2008 9:17:17 GMT -5
I forgot to mention. My shack is the garage. As most of you have figured out. 73's  |
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Post by Tombstone (R.I.P.) on Dec 27, 2008 10:20:16 GMT -5
Maybe test the tubes first.
Tombstone
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Post by Sniper..Unit 305 on Dec 27, 2008 12:31:30 GMT -5
Sure sounds like a tube issue ! Sniper
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Post by gator7 on Dec 27, 2008 22:15:26 GMT -5
Ok, I'll do that. My buddy has a tester. Can't take them to the corner drug store to test them anymore.
73's
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Post by 2600 on Dec 28, 2008 1:12:28 GMT -5
Checking tubes is always a good place to start.
But for problems that "come and go", I take a more physical approach. Keying the transmitter and (gently) rocking each tube in its socket while watching the power and modulation can reveal a socket with oxidation on the spring contacts, or on the pins of a tube. If you don't have a 'scope to watch the transmitter's output, listening on another radio will do.
Tap on the relay to see if this affects the power while it's keyed.
A radio that lives in an unheated space will acquire a film of condensed moisture whenever cold metal meets warm, humid daytime air. This will lead to "crunchy" volume controls and mode selectors. In a Mark III SSB transmitter, this also leads to catastrophic breakdown in the AM/USB/LSB selector on the transmitter. A layer of dust on that selector will become conductive when you add enough humidity. When this happens, circuits with 400 Volts on them will start leaking current into the meter, and into low-voltage stuff that will get smoked in a decisive way. At the very least, it can produce a layer of oxide on the contacts that cause functions to drop in and out.
Most on-again, off-again problems have a mechanical basis, so just worrying the controls, relays, and the tubes in their sockets can narrow down the source without reaching for any fancy test equipment.
Might just be "terminal crud" on metal contact surfaces.
73
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Post by BladeRunner now OLD FLASH!!!!! on Dec 28, 2008 10:50:11 GMT -5
My treansmit drifts just a bit for the first hour it's warming up but after that it's rock solid. I agree with the others that it very well could be a physical or tube issue .
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Post by gator7 on Dec 28, 2008 11:37:17 GMT -5
Thank you 2600. I will try those things. Along w/ getting the MK III into a conditioned space.
73
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