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Author Topic: ART-13 carrier hum  (Read 4008 times)
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SA2CLC
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« on: October 22, 2016, 02:28:44 PM »

Hello,
Finally got the ol '13 to deliver some rf after a lot of work tracing wires, aligning the autotune and installation of the J108 I got from Jim WA2MER. This would definately classify as a parts set, but I was lucky to find it here in sweden. Had some issues getting any grid current reading and thought the switch was faulty, but turned out the 837 didn't oscillate. Some more wire tracing revealed that the wire grounding the cathode of the 837 had been cut at the keying relay. Fixed that and was able to load up 120w into the dummy load. Tuned a receiver to it and was surprised to hear that the carrier was heavily modulated with mains hum. Noticed that it's there whether im on cw, mcw or am, which should rule out the filaments on the 811's, but still leave the 813 as a possible cause. Plugged my headphones in the monitor jack, and it sounded really good, no hum whatsoever, so that should leave out the 400v supply. Maybe the new filter caps I bought for the HV rectifier is bad. Have not yet hooked up the scope to check the HV, feeling a bit nervous with 1300v around the probes. The 28v supply is a 13 amp meanwell switcher, got a friend who uses that exact switcher with his 13 and that works FB. Any ideas other than the HV supply?
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WB6NVH
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« Reply #1 on: October 22, 2016, 04:42:31 PM »

What frequency is the hum?  I notice that no one ever seems to ask this.

I assume you have 50 Hz mains.  If the hum is 50Hz, I would suspect something like common mode hum coming up the power line and "swamping" the receiver right next to the transmitter.  This is a poor description of what actually happens but I wasted time chasing hum on Heathkit HW-16's that turned out to be from that and bypassing the AC line to ground at the power supply entrance with something like .01 mfd safety capacitors cured it.  Turned out the hum was not audible at distant stations without them anyway.  Just a possibility.

If the hum is 100 Hz, then I would be looking at the filters in the power supply B++ section.  
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Geoff Fors
Monterey, California
SA2CLC
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« Reply #2 on: November 02, 2016, 04:32:05 PM »

Ok guys, time for an update on this. Tried a bunch of things with the HV supplies. Connected the capacitor stack from my 813 amp and added a big 10h choke to the 1250v supply, added a 5H choke and a caps to the 400v supply to form clc filters on both supplies, no improvements.. Still heard the buzz on the monitor receiver in the shack. Got tired of messing around and tuned up on the cw part on 80m and got nice reports, except for some chirp Smiley
Listening around on saturday I heard some AM stations, called them up the '13, and none of them had any complaints about rectifier or mains buzz of any sorts. A mains filter is ordered, and if it doesnt change anything I'll let it be as it is.  
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SA2CLC
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« Reply #3 on: November 02, 2016, 04:39:05 PM »

Another question just popped up to those with ART-13 experience. I noticed that there is a fair amount of "talkback" from the modulation transformer. I find it quite annoying. Any solution to this other than to replace the transformer, or just to live with it?
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N8ETQ
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Mort


« Reply #4 on: November 02, 2016, 06:12:37 PM »



   DO NOT HOOK YOUR SCOPE PROBE to the HV.

  IT sounds like you may be running a string
of caps with EQ "R"'s.  If so you can scope
the "Bottom" Cap, (The one hooked to ground
or the IP shunt. should be 1/4 or 1/3 of the
full Ep.

    If you see ripple multiply by the number of
caps and you have total V ripple.

    Carbon Mic or dynamic?


/Dan

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KD6VXI
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Making AM GREAT Again!


« Reply #5 on: November 02, 2016, 07:49:55 PM »

I bet he has the same hum I have,monitored locally.  Even my sdr receiver, has a hum.

It was pointed out to me, on this forum, that it's rf of the common mode nature.

Choke it, and be done.

I also found it to go WAY down when I removed the station ground from the test bench equipment.

I have NO hum on my carrier or audio if listened to anywhere but in my own shack.  That sounds like what he's fighting as well.

--Shane
KD6VXI
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