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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: May Mon 01, 2017 7:01 pm 
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Joined: Jan Mon 12, 2009 7:38 am
Posts: 978
Location: homestead, fl
Or the stock supply is just running out of gas. Seen that in cheap 70's audio gear. Lousy PSU = increased noise.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: May Mon 01, 2017 10:15 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18704
Location: Warner Robins, GA
True.

I've often found a large filter cap usually takes care of that for the most part for things that only need a little help, but if that don't work a more capable power supply is necessary.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: May Tue 02, 2017 1:56 pm 
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Posts: 12576
Mostly hum can be traced to the RF in AC power lines being picked up by receiver... This is usually why some radios may have issues and not others... Carefully orienting transmitting antenna away from power lines often reaps great rewards...

For P/S it's easy enough to look B+ with scope and see if it's dirty...


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: May Tue 02, 2017 5:09 pm 
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Joined: Jan Mon 12, 2009 7:38 am
Posts: 978
Location: homestead, fl
35Z5 wrote:
Mostly hum can be traced to the RF in AC power lines being picked up by receiver... This is usually why some radios may have issues and not others... Carefully orienting transmitting antenna away from power lines often reaps great rewards...

For P/S it's easy enough to look B+ with scope and see if it's dirty...


good point on scope, gonna try that. im gonna be a bit busy this week so wont have much time to experiment but this TH thing has been a longterm project with me. have 5 of them here so no fear if i trash one accidentally while hacking.

if we could only get a schematic to these darn things...


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Oct Sun 01, 2017 2:52 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1165
Location: Tucson AZ
There is a fellow local to me that has 3 new 4.6 units for 50 each. I'm really tempted to get one.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Oct Fri 20, 2017 3:17 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1165
Location: Tucson AZ
Cleaned up A LOT of hum by disconnecting a ground wire that connects between the ground pin on the AC plug and the rectified output circuit board in the wall wart for the unit.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Oct Fri 20, 2017 4:11 pm 
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Sometimes hum is less without a ground...

I wired the ground for one of my tube units thru a switch, can flip it back & forth to check what's best... It can vary day by day depending on how wet the roof is, what's in operation etc... Since I moved transmitter out to garage I rarely switch it, rainy day and a little hum, not worth getting wet to correct... Maybe one day I'll wire it to a relay controlled by a garage door remote receiver... Ummm, naah that ain't likely...


Tom


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Oct Sat 21, 2017 1:36 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Warner Robins, GA
35Z5 wrote:
Sometimes hum is less without a ground...

I wired the ground for one of my tube units thru a switch, can flip it back & forth to check what's best... It can vary day by day depending on how wet the roof is, what's in operation etc... Since I moved transmitter out to garage I rarely switch it, rainy day and a little hum, not worth getting wet to correct... Maybe one day I'll wire it to a relay controlled by a garage door remote receiver... Ummm, naah that ain't likely...


Tom


Agreed.

Also depends on the radio.

My R-390 hums less with the ground lift switch one way and my RCA K-80 console in the same room hums less with the switch in the other position.

I also took mine and installed a transformer so I could have a balanced or unbalanced line connected and used one of those isolated 1/4" jacks so that the only ground is through the ground prong on the power plug.

That seemed to tame hum somewhat.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Mar Thu 01, 2018 1:10 pm 
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Joined: Sep Mon 10, 2012 10:41 pm
Posts: 3879
Location: Phoenix, AZ
I just got a TH 4.6 and was disappointed with the audio and hum.
found this thread.
tried several caps on c301 and settled on tacking on a .015uf.
I noticed (accidentally) with the unit sitting on the metal drawer of my bench that when I moved it around sometimes the hum disappeared slightly. Did some experimenting and found that if I put a 3 prong to 2 prong adapter on the ac adapter (as some have tried), and then grounded the case of the TH to my bench frame (which is not grounded), then restarted the unit, the hum was gone. currently my antenna is parallel to the bench, and attached to a handle on a filing cabinet.

_________________
Everyone you meet is fighting a battle you know nothing about.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Mar Thu 01, 2018 3:59 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18704
Location: Warner Robins, GA
My advice is this. Try using a 1:1 audio transformer between the source and transmitter and use as short of an audio cable as possible between the transformer and transmitter.

Sometimes the audio cable to the source plus whatever power cvable the source has can cause an increase in hum and the audio transformer eliminates the cable's ground connection to the transmitter. Plus it ensures that the only path to ground is the transmitter's ground itself.

I had a lot more hum issues until I installed the audio transformer in my two tube transmitter and used an isolated 1/4" jack since my case is metal and the transformer is mounted internally.

Some radios hum less with the transmitter ground connected to the power outlet ground and some hum more with the transmitter ground connected to the power outlet ground.


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