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Author Topic: Atlas 350 XL  (Read 472 times)

VE3VTG

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Atlas 350 XL
« on: June 28, 2022, 08:38:55 PM »

Hi guys,
 So i recently acquired a Atlas 350 XL with matching power supply.  Made a few cw contacts and no complaints from anyone.  When i use the hand mic to work phone, I key up the mic it acts like I'm sending a carrier.
So typically you would have to speak into the mic PEP to get any power output, in this case i key the mic and the meter is showing 100 w output without speaking into it.
I have never seen anything like this happen. There is also a hum in the signal and noise coming from the power supply.
So the obvious is theirs something wrong with the power supply, But why the carrier?
Any help would be greatly appreciated.
73 de VE3VTG
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WB6BYU

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Re: Atlas 350 XL
« Reply #1 on: June 28, 2022, 09:04:28 PM »

There typically is one (or more) internal adjustments to balance
out the carrier for SSB.  If one of those has drifted, or a diode
has shorted, then the modulator won't be balanced, and the
carrier will pass through it.

To operate CW, there may be a switch to unbalance the modulator
to generate the carrier.  If something is wrong with that switch,
then it could also generate the carrier in other modes.

(I'm assuming you are switching to LSB or USB to work voice
rather than leaving it in CW mode.)

You could also have feedback in the audio circuit that generates
a tone (or the CW sidetone oscillator) that is modulating the
audio even when you aren't speaking into the mic, or AC hum
picked up on the mic cord.


I don't remember the details of that particular radio, but  you
might try setting it in LSB or USB mode and keying the radio
with the microphone unplugged to see if you still get the same
symptoms.  Some rigs had a separate SEND/RECEIVE switch
that will force it into transmit, since you no longer have the PTT
switch on the mic, or there may be some other way to make it
switch.  If you have the pinout,  you can jumper the contacts
in the mic socket to do so.  If the problem is audio pickup on the
mic cable, this should eliminate the carrier.

WD8OTT

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Re: Atlas 350 XL
« Reply #2 on: June 29, 2022, 07:17:53 AM »

I had a 350xl at one time. Nice radio. First things first, get a schematic of the radio/ power supply. http://atlas.wireless.org.uk/Atlas350XL.pdf     I like the last paragraph of WB6BYU. Take the mike/cord out of the audio chain. Quick and easy. Can you power the 350xl from a known GOOD power supply?  How about a 12volt battery? Now go to that pdf manual, it's pretty good with circuit description and theory. the troubleshooting guide isn't bad either. Definitely check out pg 17 circuit theory. Also check out pg 27, 43, and 47. If not the mic/cord/connector then I suspect a switching transistor from cw to ssb, maybe the voltages that control said transistor. See page 27, top right, board PC-650. Or as WB6BYU said, a diode in the balanced mixer. Good luck.
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VE3VTG

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Re: Atlas 350 XL
« Reply #3 on: June 29, 2022, 06:13:27 PM »

Thanks to WB6BYU and WD8OTT for the advice.  I think i will need to adjust the carrier for ssb.  Yes every time I'm testing i switch  from whatever mode I'm in cw,lsb,usb. Your assumption  is correct.
As far as feedback in the audio circuit, I realized today that the speaker wire coming from the power supply was wrapped around the power cable, Going from the power supply to the radio. Since the matching power supply also has a build in speaker.
I quickly decided to take WD8OTT advice and swapping out the matching power supply for an Alinco power supply. I went for easy first! :)

What i discovered quickly was that  the humming sound i thought was coming from the power supply, was not coming from the power supply but rather the speaker in the power supply.  This is when i decided to untangle the speaker wire from the power cord and i installed a ferrite choke with about 5 turns.

The improvement was  immediately noticeable  with the hum, although not gone.  I decided to put a ferrite choke to the power cable with about 3 turns.

This basically eliminated any hum that i was picking up through stray RF.  After doing a few quick test i have noticed that when i key up the mic in LSB the carrier is now only about 10 watts, Where before it was 100 watts keying down the mic without speaking.  Seems to have improved but problem still exist.

Although it still continues to have a 100 w carrier in USB when i key the mic on 20 and 40 meters without speaking. 10 and 15 meters  seem to act as they should. Power output on 80 meters is low. I assume this radio needs a good alignment.

I will take your advice Dale and have a look at the SSb/CW switch.

I will also take a look at adding some more ferrite chokes in different positions with different amounts of turns, to see if that corrects any issues.

I wish i had more technical knowledge to tackle this problem. Regardless I appreciate the help that i have received over the past few days.

Nice to see the Ham community as i remember it.

 
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WB6BYU

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Re: Atlas 350 XL
« Reply #4 on: June 29, 2022, 09:05:32 PM »

Check PA0FRI's website:  under the red tab that says NEW, he has
some notes on repairing a 350-XL.

Then click on the British flag to get the English translation.

Among other things, he found that the drive stage would oscillate
under some conditions, and that would generate drive for the final.
He shows a simple fix, adding one resistor and capacitor, that
eliminated the oscillation in his unit.

There may be other conditions, such as any sort of audio hum
picked up on the mic lead or in the audio stages.  But this certainly
could be contributing to the problem.

VE3VTG

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Re: Atlas 350 XL
« Reply #5 on: June 30, 2022, 06:35:28 PM »

I will have a look at that site..
Thank you...
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