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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Mon 16, 2019 10:45 pm 
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Location: Raleigh, NC area
Pauls, if you can't hear any hum, even with zero input, then you have an unusual magic unit, and I wonder what the magic is...

KC1LML on that notch filter...

You might be right about values needing to be very close. But maybe if the Q isn't very high, we still get enough reduction to prove a concept.

I found some 5% (both R and C) 1206 SMT parts to try, if I can get them soldered together well enough just for a quick test on a little bit of copper clad. Not sure if I'll be able to do it tonight. Late on getting a Christmas tree!

Thinking about this a bit more: does it need to be a notch at all? Wouldn't a simple high pass work? I keep thinking about the low end, but there is no low end on the antenna, except the stuff we don't want coming in.

Edit: high pass, not low pass


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Tue 17, 2019 2:23 am 
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Location: Raleigh, NC area
I've been reading old ARF threads after a search for "AM transmitter hum." It seems there is a mode whereby the signal from the transmitter gets onto the AC wiring, which then re-radiates it, along with the hum.

Given that I have the hum with nothing on the input, it seems reasonable that this is what is going on. In that case, no antenna filter will matter. And the only workaround is antenna position/location.

Anecdotally, there are plenty of people who claim no issue with hum at all. Maybe they have the special combination of location, grounding, and personal "hum tolerance." It appears also that hum on AM transmitters is nearly universal. There are some super low power transmitters which are reported to not have much hum, but that might be simply because the power level is so low, it cannot get onto the house wiring so easily (these transmitters need to be within a few feet of the radios.)

With this theory, it would make sense that the TH is the one usually reported to have much hum, because it is also one of the most powerful transmitters. Yes, it is Part 15 compliant, supposedly, but A/B measurements by an ARF member showed that it was a higher output level than the SSTRAN, which was flat at 100mw. But the SSTRAN does not have the range of the TH. And it still can have hum issues, as reported by members here. They also have a hum reduction paper on their site (google search).

So, I might try to find an AC-quiet place in the house to try this again. I don't really think there are "good" and "bad" versions of the same design. That is highly unlikely. The much simpler reason for the differences is location variability. My house is two story, not very large, small rooms. I expect there is not a quiet place anywhere. If this effort fails, I'll get rid of it and maybe look into the very low power versions, or not at all.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Tue 17, 2019 3:03 am 
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DaveInNC wrote:
That is good information if I decide to input a signal from a ground-referenced source. But right now, I am not. I'm using an iPod.


Doesn't matter, because the audio cable ground and anything in the iPod connected to the headphone jack ground terminal in the iPod will still be part of the antenna ground circuit.

I've discovered that with some radios, they hum more if my transmitter is grounded and others hum less if my transmitter is grounded.

Grounding especially at RF frequencies has to be done certain ways to be proper and not introduce any hum, ground loops or any other circuit issues.

Picture the ground circuit in the transmitter as an antenna ground plane.

Anything added to the ground circuit of the transmitter adds to the ground plane which alters it. That an either have a positive or negative effect on the transmitted signal.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Tue 17, 2019 1:53 pm 
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Even though my hum is fairly low, I like to experiment - any thoughts on placing the standard transmitter antenna outside maybe 25-50' away from the house using coax? I was planning on leaving the transmitter un-grounded at the AC power outlet and instead grounding the coax to my ground rod.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Tue 17, 2019 2:52 pm 
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Location: Raleigh, NC area
I would think outside antenna placement would be fine, with a coax. But I thought I read that if you use the external antenna input, that the auto-matching (CAL) would not run any more? And at least on my unit, the switch for indoor/outdoor antenna is mechanically fixed to the indoor side. I don't know why they did that.

I had considered placing the whole unit outside, on a battery, in a watertight plastic bin, and broadcasting from there, so that the strongest signal wouldn't get on the AC lines. It might work. But I then said to myself, "Really, Dave? Really?" And answered myself, "Yeah, right. All to play some music on a few old radios. Not worth it!"

I also need to keep peace in the family. My wife asked me to turn off the radio last night as I was experimenting, because she couldn't stand the hum.

I'll be listing it on eBay later this week. Probably will lose money on it with the shipping, but I don't have time for lost causes.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Tue 17, 2019 3:13 pm 
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Here is the information on the optional accessories and external antenna:
https://www.dropbox.com/s/oisvwysuytlct ... 5.pdf?dl=0
note that it can transmit up to 1/4 mile.

Here's the patent with a schematic:
https://www.dropbox.com/s/0r5l2aq26kpv7 ... m.pdf?dl=0

Here's the 5.0 manual:
https://www.dropbox.com/s/4x9t7ao0113u5 ... l.pdf?dl=0

Here's is the current manual (or was I can't be bothered to confirm):
https://www.dropbox.com/s/bj14qkly2gdus ... l.pdf?dl=0

When it asks you to login or create an account click "no Thanks, Continue to View"
Then Click Download in the upper right corner.
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You're Welcome

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Tue 17, 2019 4:04 pm 
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Thanks Flinx, never had the manual, maybe not the exact correct for my model (4.6) but will work for me.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Tue 17, 2019 4:29 pm 
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Thanks Flinx! The schematic looks more complex than I would have thought, but a lot of it has to do with message recording. I suppose these would work much better in a vacant, noise-free house that was for sale!

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Wed 18, 2019 4:43 am 
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Location: Raleigh, NC area
Thanks... Already knew about the schematic in the patent, but they've made mods since then and hard to read the reference designators on the PDF. There is also a ton of extra "stuff" in the design, complicating the picture.

But I'm moving on. It's packed up for sale. There are a few long time members who have had the same experience with TH5 as I have, namely, totally unacceptable hum, in every situation and configuration. Then again, maybe my house is just a very inhospitable environment for this device (and maybe other AM Tx devices). Clearly many folks are very happy with their TH5's.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Sat 21, 2019 12:05 am 
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I bought a TH 5.0 from Marvin. The infamous hum from this transmitter is really disappointing. I tried many modifications but could not get rid of it. I was about to give up when I saw that there is a 1nF capacitor (C348) at the output pin 7 of the op-amp IC301 (LM358N). Since this op-amp can drive at most a 50pF load, C348 is too big and probably causes the op-amp to become unstable. Removing this capacitor greatly reduce the hum. I think this capacitor is the culprit.

A GE superadio II is used to test the TH. With the TH off, I find a vacant spot on the dial and orient the radio such the noise heard is minimal. I turn on the TH feeded by a CD output, slowly tune its frequency to the same frequency as the radio and wait for it to tune the antenna. I slowly re-orient the radio the minimize the hum. With C348 in place, the hum is heard regardless of the radio's orientation. With C348 removed, there is a radio's orientation where the hum is reduced to a large degree.

I didn't like how the a line input is amplified. The TH is designed to take a microphone input. With a line input, the signal is reduced by 46dB (ratio of R231 or R232 to R230) and then amplified by 42 dB by IC201 (BA3313L) for a net gain of -4dB. It is amplified by another 38dB before it reaches the base of transistor TR304 (BC337-40). The total amplification in the chain is 84dB or about 16,000 - a huge number. That was my first suspicion. A sensitive amplifier with a poor layout is susceptible to pick up a lot of noise and hum.

With the help of the schematic diagram in the patent, I managed to trace almost the entire circuit of the TH. I used LTspice to obtain the frequency response of the audio chain. It doesn't look very good to my liking. I played with the components' values in the simulator until a better frequency response was obtained. I actually tested the modified circuit with a real audio source and the TH sounds much better.

Below are the frequency responses of the original and modified audio circuit. Also shown are the modified schematic pages pertaining to the audio circuit. The modified components have their original values shown in parentheses. Some op-amps and transistors are not actual but equivalent models for simulation purpose.


Attachments:
TH5_sm_Original_FreqResp_PAOut_to_LeftIn.jpg
TH5_sm_Original_FreqResp_PAOut_to_LeftIn.jpg [ 150.7 KiB | Viewed 620 times ]
TH5_sm_Modified_FreqResp_PAOut_to_LeftIn.jpg
TH5_sm_Modified_FreqResp_PAOut_to_LeftIn.jpg [ 156.65 KiB | Viewed 620 times ]
TH5_sch_sm1_LiveInputs.jpg
TH5_sch_sm1_LiveInputs.jpg [ 210.86 KiB | Viewed 620 times ]
TH5_sch_sm2_AudioPreAmp.jpg
TH5_sch_sm2_AudioPreAmp.jpg [ 260.13 KiB | Viewed 620 times ]
TH5_sch_sm4_AudioPowerAmp.jpg
TH5_sch_sm4_AudioPowerAmp.jpg [ 210.98 KiB | Viewed 620 times ]
TH5_sch_sm3_VoiceRecordPlayback.jpg
TH5_sch_sm3_VoiceRecordPlayback.jpg [ 175.29 KiB | Viewed 620 times ]

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Sat 21, 2019 4:29 am 
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Location: Raleigh, NC area
That packed-for-shipping TH5 of mine may be coming back out of the box after this post. Nice work, Binh!

I wonder if the ground shapes could be improved to further reduce hum? And if unused parts of the circuit could be removed with stategic cuts on the PCB and component de-pops...and if that would also help?

To be honest, I'm already looking into the homebrew LM386 transmitter designs. The main (only?) advantage held by the TH5 is frequency agility. And antenna tuning, assuming that it actually works and doesn't just make noise ;) It's in a fairly large, unattractive box, for what it does (and poorly).


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Sat 21, 2019 2:16 pm 
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Wow - Binh. You have been busy. I think I follow most of your modifications on the schematics, but I'm not sure about your notation for R208 and R209. Does that mean you removed them?

Also, what do you think the purpose of that brown wire running from ground to ground serves?

Many thanks

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Sat 21, 2019 2:52 pm 
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Thank you, Dave.

I hope that you or someone else could remove capacitor C348 to verify that the hum is reduced. Regardless of whether the issue is solved or not, C348 should be removed. Putting a large capacitive load without an isolation resistor at the output of an op-amp is generally not a good idea unless the op-amp is designed for it.

The ground distribution and the component placement could be improved. For example, the ground of the pre-amp, which is closer to the power supply ground, is separated from that of the power amp by a long distance. The signal between these two amps has to return by the same path. I could shorten this path by a short wire but unfortunately force the large ground current from the modulator to flow through the pre-amp's ground path. The power supply should have been placed closer to the modulator and power amp than the sensitive pre-amp. The TH's power supply placement is a big violation in any power amplifier design.

Here are some other modifications that I did.

I removed the brown wire on the top of the board connecting IC301's ground to the power input jack JK300's ground. The idea was to shorten the antenna ground path to the power input jack's negative side which is connected the neutral wire inside the AC adapter. Somebody had the idea to route this top wire through the sensitive pre-amp IC201 to make it look neat, another big design violation. Since IC301 gets its 18V supply from a charge pump, it's better to connect its ground by a short wire on the bottom side to the charge pump ground. A convenient place is the negative side of C334 which serves a power supply tank for the charge pump.

As far as I can see, there is only one big capacitor for the 5V supply rail. This cap is C207 near the pre-amp IC201, far from the voltage regulator REG300. I increased C207 to 220uF. I replaced the ceramic cap C330 near REG300 with a 220uF cap and paralled the original ceramic cap on the bottom side.

I opened up the AC adapter to see what is inside. There are some EMI filters on both sides of the step-down transformer and the DC negative side is indeed connected to the neutral wire. However there is no capacitor after the bridge rectifier. I'm not sure if this absence matters but I don't like the big AC ripple current flowing through the output wire. I put a 220uF 35V capacitor at the rectifier output.

The pre-amp uses about the same components as the example circuit given in the datasheet for the BA3313L. However the example circuit has a lot of bass boost (see figure below). To flatten the low frequency response, it's sufficient to make the time constant R212*C209 the same as R211*C208. Without changing the gain, R212 has to be the same value. This makes the new value for C209 to be R211*C208/R212 = 6.8K*0.1u/0.047K = 14.47uF - use 15uF or 10uF + 4.7uF.

I guess that there was an unsuccessful effort to resolve the hum problem. That may explain to low value 10nF of C301 to kill the hum (~5dB total gain at 60Hz). With the hum resolved, it's better to use 1uF for C301 to restore the low frequency response.

It's interesting to note that the modulator operates on the same principle as the one I designed a year and a half ago. However I took great care to resolve the storage time of the switching transistors (TR305 and TR306) and to have 50% duty cycle to lower the high frequency harmonics.
https://antiqueradios.com/forums/viewtopic.php?f=12&t=340999

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Sat 21, 2019 2:58 pm 
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Marvin, R208 and R209 are used to attenuate the pre-amp output before it reaches the power amp. On the board R209 is not populated and R208 is replaced by a wire jumper.

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Last edited by bb.odin on Dec Sat 21, 2019 4:48 pm, edited 1 time in total.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Sat 21, 2019 2:59 pm 
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Frequency response of the original pre-amp IC201: the first with C209 = 47uF; the second with C209 = 15uF


Attachments:
FR1.jpg
FR1.jpg [ 142.57 KiB | Viewed 590 times ]
FR2.jpg
FR2.jpg [ 140.51 KiB | Viewed 588 times ]

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Sat 21, 2019 4:14 pm 
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Why not completely ditch the circuit that drives the final modulation amp and redesign it for a line input?

Just input the signal at U301B.

Loses the AGC though, but I've never been fond of an AGC that I couldn't control.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Sun 22, 2019 2:03 am 
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All righty, in the Name of Science, I pulled the TH5 out of the packing box for experiments.

I'm sorry Binh, but I prepared a very careful A/B comparison, with and without C348. I could detect no difference in the hum.

Method:
- fix the position of the TH5 box and all wires, including the antenna.
- case top removed in both tests.
- line in jack from the ipod, but nothing playing.
- power up, step away, allow antenna tuning.
- set portable radio at location and orientation of maximum hum.
- volume of portable radio set at 1/2 point.
- note the hum level (I did not measure it)
- remove PCB enough to get to the backside of C348. Desolder it.
- reseat PCB, and all screws, nuts as before. No case top as before.
- power up, step away (verified no change in antenna tuning.)
- check noise level at the radio, same volume level.
- no difference noted.

I could use a sound meter, but the hum before and after was the same to my ears. Very loud and annoying ;)

Edit: I will even go so far as to say the hum is worse without C348. With C348 in place, there were radio orientations where the hum could be almost nulled. Without C348, there is almost no nulling. The wildcard here it the power-up time. The unit could get worse the longer it is powered. I will replace C348 and try again.

Edit2: replaced the cap. Tested again, able to find some lower hum orientations, but not as well as in first test. Still, with C348 in place, the hum is not worse than without the cap.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Sun 22, 2019 5:34 am 
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Location: Medford, Or.
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...Following. I don't have hum problems per say, just crappy sound as befits a $2.00 radio. No comparison to SStran 3k, or my FM transmitters, (which is a given). I have 2 THII's but the boards say THIII, and I also have a TH 4.6. It sounds like a $3.00 radio. Pretty disappointed. I wish I had a contribution for a fix, but don't so far.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Sun 22, 2019 7:48 am 
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Try feeding the signal in at U301B.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Sun 22, 2019 1:54 pm 
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I did some tests with/without C348 and got some interesting results. Here's my setup. I know this part is boring, but I think it's important. We all know the TH is not a star performer, but I think the environment has a lot to do with whether it performs at a "good enough" level.

My TH is on my wife’s desk. A laptop with WiFi, power supply for laptop, external monitor & its power supply, all within 24” of TH. The antenna for TH runs vertical to ceiling. All of this is on an outside wall in the middle of the house. Our cable modem/WiFi router is about 8’ away in the same room. There is also an electronic piano & its power supply about 6’ away.

The TH is connected to my wife’s laptop headphone jack via a ground loop isolator and is playing Pandora. The frequency on the TH is set to 1010kHz. I’m using a small Sony portable to pickup the TH in the living room about 35’ and two rooms away. It’s too cold and snowy outside.

I set the Sony’s volume to a comfortable listening level while receiving the TH. Standing there in the living room, I rotate the Sony for maximum signal and minimum noise/hum. At maximum signal, I can’t hear any noise/hum in what Pandora is playing. That’s with the Sony’s internal ferrite rod antenna broadside to the TH two rooms away. However, there is a lot of hum/noise if I rotate the Sony to any other position.

Next, I go back to the TH and unplug the line input cable from the TH. I was wondering if there was hum/noise I couldn’t hear due to Pandora playing. There isn’t any. If I turn volume to max, I can hear a little whine, but I think that’s the Sony. As before, there is a lot of hum/noise if I rotate the Sony to any other position.

Next, I turn the TH off and go back to the living room. The only thing I notice is now is there isn’t any hum/noise no matter how I rotate the Sony. So, obviously the TH was sending out the hum/noise, but the Sony only picked it up if it wasn’t broadside to the TH. That seems very strange. I had suspected the hum/noise was something else in our house but not so.

Next I lifted a leg on C348. The setup is still the same. The first thing I notice is I have to turn the volume up on the Sony. So the signal (modulation?) is weaker. The next thing I notice is the hum/noise that I would hear on the Sony if it wasn't exactly broadside to the TH is almost gone. I would say disabling C348 eliminated maybe 80% of it.

The big mystery to me is the hum/noise I get with C348 installed when the Sony isn't broadside to the TH. I have a feeling that some radios pick that up all the time.

I did test each of those 4 TH's I sent out with my Sony portable. I couldn't detect any difference or hum/noise between them, as long as the Sony was oriented broadside. This Sony is my goto tester for tracking down MW band noise in our house.

With that said, I got out my Tecsun PL-880. I had almost forgot it receives AM, since I use it for shortwave DXing. Guess what - it has what I'll call unacceptable hum no matter how I orient it. And that's with C348 disabled. Makes me think the receiving radio has more to do with this situation than the environment!

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