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T-4XC problem
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I'm looking at a T4XC for a friend. It needed new output tubes when he got it, they showed overheating marks on the plates and the cathode resistors were burnt out. With replacements fitted and the neutralising adjusted it seemed to work fine.
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A few weeks on he's reported a problem; transmitting on SSB ok, but on releasing the PTT it sometimes stayed in tx and the only thing he could do was turn the power off. I asked if he had the VOX on, he said thought not, and operated the switch to check. It went straight onto tx and now goes to tx the moment power is turned on, regardless of the VOX switch or mic gain settings.
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Has anyone seen this problem before? Any suggestions on possible culprit circuits?
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Thanks and 73,
Steve G8GSQ |
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Try it with the mic disconnected to make sure it's not the mic that has a problem.? You can switch it to x-cw without the mic to see if the relay sticks.? The relay can get sticky.? You can remove it and GENTLY take the clear cover off so you can spray some deoxit in there.? Be careful putting the relay back in because you can easily bend the pins on the relay or the blades in the socket.? I had this same problem with my TX-4Cw when I first got it and cleaning the relay with deoxit fixed the problem.
On Tuesday, January 21, 2025 at 09:22:11 AM CST, Steve G8GSQ via groups.io <g8gsq@...> wrote:
I'm looking at a T4XC for a friend. It needed new output tubes when he got it, they showed overheating marks on the plates and the cathode resistors were burnt out. With replacements fitted and the neutralising adjusted it seemed to work fine.
?
A few weeks on he's reported a problem; transmitting on SSB ok, but on releasing the PTT it sometimes stayed in tx and the only thing he could do was turn the power off. I asked if he had the VOX on, he said thought not, and operated the switch to check. It went straight onto tx and now goes to tx the moment power is turned on, regardless of the VOX switch or mic gain settings.
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Has anyone seen this problem before? Any suggestions on possible culprit circuits?
?
Thanks and 73,
Steve G8GSQ
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Most of the time, it¡¯s a matter of adjusting the VOX and antiVOX gains, which can be a bit touchy.? Note also that the VOX settings for CW and phone are separate.? Steve Wedge, W1ES Time flies like an arrow. Fruit flies like a banana. Sent from for iOS On Tue, Jan 21, 2025 at 10:41, Barry KJ5GQM via groups.io <chasbg@...> wrote:
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Steve (G8GSQ); I bought?my first brand new C-line in 1974.? A year or two later I had exactly?the? problem you describe with the "auto-tx" on power up. Change the VOX tube....6EV7.? Try several if need be.? That cured the problem, though back then I went back to the local dealer to find the cure.? Worth a try. 73 Peter VE7PS On Tue, Jan 21, 2025 at 8:21?AM Steve Wedge, W1ES/4 via <w1es=[email protected]> wrote:
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At last, the promised follow-up on the T-4XC which was sticking on tx. It turned out to be the relay switching tube (6FQ7 in this set).
On the tester (I have an AVO VCM Mk3) the original showed as slightly leaky/gassy, probably not enough to worry about in many applications but the T4 has high resistance feed into the grid and leakage of a few uA is enough to drag the 'off' bias voltage down to where the tube conducts enough for the relay to pull in.
It took an age for the replacement to arrive but all seems good now.
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I also looked at the relay, the PA switching contacts had a lot of debris around them and are well worn. I've suggested the owner starts saving to get a replacement (around $80 over here).
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Again, thanks for all the suggestions, great bunch of .people here.
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73, Steve G8GSQ |
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