FT-401B Questions

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FT-401B Questions

Postby Kd5pda » Wed Feb 01, 2017 1:53 am

Hello everyone,
I recently aquired a Yaseu FT-401B that needs a little TLC. The Radio is all original and has the 6KD6 tubes. That being said, the radio receives just fine, but has an annoying hum. Also, the transmit is non existent. I don't have any way to test any of the tubes except with a voltimeter, any suggestions on where to start for the issues? Also, all the tubes visually appear fine, however we all know looks can be deceiving. Any help appreciated!

73
Kd5pda
 
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Re: FT-401B Questions

Postby N8YX » Mon Feb 06, 2017 5:36 pm

Kd5pda wrote:Hello everyone,
I recently aquired a Yaseu FT-401B that needs a little TLC. The Radio is all original and has the 6KD6 tubes. That being said, the radio receives just fine, but has an annoying hum.

I'd look at replacing every filter capacitor in the power supply for starters. You may also want to consider recapping the entire radio as a preventative measure, as I'll wager many of the various electrolytics have begun to dry out with age.

Also, the transmit is non existent.

There is no Heater switch on a '401 (as there is with the later series hybrids) so you can rule that out. Are the driver and final tube filaments lit?

I don't have any way to test any of the tubes except with a voltimeter, any suggestions on where to start for the issues?

The very first thing I would do is to get hold of a decent 30+MHz, 2CH scope plus probes. The second might be the acquisition of a B&K, Tenma or similar "All in one" test center. These combine a frequency counter, DMM, capacitance meter, function generator and DC power supply into one desktop instrument - handy for someone whose workbench space is limited.

Or buy separates if you have the budget and room. A voltmeter will not tell you much in the way of signal characteristics. You need proper test gear (or a buddy with such) to diagnose problems other than the "evident".

Also, all the tubes visually appear fine, however we all know looks can be deceiving. Any help appreciated!

See my comment regarding lit filaments. How about the Accessory plug in the rear of the radio - is it present? If not, the driver and PA tube filaments get no power - at least with the later model FT hybrids.
N8YX
 
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Re: FT-401B Questions

Postby Kd5pda » Thu Apr 06, 2017 10:47 am

Ok quick update. I recently went to a Hamfest and picked up a Heathkit Tube Checker (TC-3), 2 6KD6 tubes, and a giant box of loose tubes all under $100. I found that the final tubes were 6LF6 sweep tubes, and upon testing them found they were dead as a door knob, not to mention the wrong tubes. This past weekend I tested every tube and found a 12at7, and a 6ba6 were defective (the 6ba6 immediately upon turning on the tube checker started arching). Now the next step is replacing the caps, my question is should I also replace the large filter capacitors? The hum is still present, but at least the audio is better! Thank you for your suggestions
Kd5pda
 
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Joined: Tue Jan 31, 2017 12:14 pm

Re: FT-401B Questions

Postby N8YX » Wed Apr 12, 2017 5:38 pm

Kd5pda wrote:...The hum is still present, but at least the audio is better!

Is it present during receive, only on transmit or both?

A scope will really help here. Look at the B+ (low and high voltage) sections of the power supply with the scope set to AC coupling and see how much ripple is present.
N8YX
 
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