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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Oct Tue 29, 2019 12:45 am 
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Tube Radio wrote:
Would it be even better if the talking house was grounded straight to a ground rod like some of us do with our communications gear?


I am not sure, I only know that it seems to work better and hum is reduced considerably if you give it a ground plane. like a horizontal antenna and wire connected to the chassis 3 or 4 feet below it.

I have not done more experimenting than that.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Oct Tue 29, 2019 1:07 am 
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I've not experimented with that, but I do know with my two tube transmitter, I use a grounded power cord with a ground lift switch and some radios hum less with the ground connected and some hum less with the ground disconnected.

The biggest hum reduction method I've found is to use a 1:1 audio transformer which isolates the transmitter's ground from the audio ground, otherwise the audio cable and whatever it is connected to becomes part of the ground plane and can cause hum. I discovered that when I had a source connected without the transformer and I noticed the hum changing as I moved the audio cable. With the transformer, hum was reduced and moving the audio cable did not affect the hum at all.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Oct Tue 29, 2019 6:21 pm 
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fifties wrote:
1988bluebird wrote:
Just a followup, I made the c301 capacitor modification to my Talking House 4.6 this weekend, and it sounds radically better! The low and midrange is much more pronounced and music now has a "warm" sound. I might be using the Talking House a lot more now since it sounds so good :D

Does yours have a background hum? That's the reason I got rid of mine, even after doing the cap change out.


Yes, its still has a slight hum in the background, but it's not too annoying. I tried the 3 to 2 prong adapter, but didn't make a difference.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Oct Tue 29, 2019 7:31 pm 
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Try a 1:1 audio transformer on the audio input.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Sun 08, 2019 11:06 pm 
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I picked up a TH 5.0 this morning from a local realtor and have been fooling around with it. At first I was getting way too much hum but eliminated a lot of it by putting it on a 2-wire isolation transformer and grounding the transmitter to the ground rod right outside the window in my lab. That took care of maybe half the hum and noise. I've been using a portable radio for reception testing and have nothing connected to the transmitter's audio input.

I'm no stranger to hum and noise in this house. I have to use an outside antenna and ground for my restored radios. They do work nicely that way. Anyway, I decided to try connecting my outside long wire antenna to the TH outdoor antenna connector. Having an amateur radio license, I'm aware that's a no-no. But this was just a quick test, and the nearest radio station is 25 miles away.

Anyway, that took care of the other half of the hum and noise. Obviously, that's not a practical solution, but does anyone know why getting the transmitter's antenna outside the house would reduce hum and noise on a radio inside the house? Maybe it's just a stronger signal (even though it has no audio input) that causes the receiver's AVC to turn the gain down? I would take the transmitter outside and use its regular antenna if we didn't have 27" of snow in the yard.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Mon 09, 2019 12:49 pm 
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I now have it working pretty well using the factory supplied antenna inside the house. As someone mentioned antenna placement is everything. I ran the wire vertical to the ceiling. That works much better for me. I'm also using a 3 to 2 wire AC adapter. So no ground at all. For some reason, that reduces maybe 80% of the hum and noise in my case.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Sun 15, 2019 1:24 am 
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I'll add my 2c into the mix here. I bought the "used" (it was new) unit on ARF classifieds.

The Talking House is a very poor design, folks. I did a series of tests today, very carefully, A/B comparions, to isolate the issue. Bottom line: the antenna picks up 60Hz noise and that gets into the modulator.

I hooked the thing up to my 18V drill battery, used an iPod on a short Line In cord, and took it out into the woods behind the house, with a portable AM radio. 60Hz hum gone, but still had high frequency whine coming from the unit itself. Not from the ipod. I even pulled the Line In out of the iPod and out of the radio. Anyway...

Back inside, I got the antenna positioned such that much of the hum was gone. Still present, still annoying but at least the music was louder than the hum, which it was not, before. This was running from battery.

Then, without touching anything else...

-- disconnect the battery, plug the wall wart into the wall, connect the wall wart to the TH: NO CHANGE in the hum. NONE.

-- disconnect the Line In from the iPod (music stops). No change in the hum.

-- disconnect the Line in cord from the TH. No change...

-- switch to the pre-recorded message. No change...

-- turn down the volume on the radio. Then turn on the TH internal speaker. No hum at all, coming from the internal speaker.

-- turn off the speaker, turn the radio volume back up (hum returns). Now grasp the antenna wire with my hand, and the hum reduces.

This all tells me the SOLE contributor to hum in the TH (5.0 version, at least), is the antenna itself. The wall wart doesn't affect it. Meaning, if you have a quiet place in the house, maybe it will work for you.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Mon 16, 2019 3:53 am 
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Any reason why something like this, mounted close to the PCB, wouldn't work to knock out the 60Hz pickup on the antenna wire?

https://forums.parallax.com/discussion/ ... terference


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Mon 16, 2019 1:36 pm 
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I pulled out the PCB for a better look. Left the display/CPU board in place for now.

That brown wire, upper left, is connecting two parts of the ground plane together.

Passive 60Hz notch filter on the antenna input (prior post) seems like it's worth a try. On this unit the antenna is the primary noise input, as I posted before. Wall wart vs battery has no noticable difference, unless the antenna gets close to the wall wart and pulls more noise in that way.

There are parts of this unit which we don't need for our use. The big chip in the middle is for recording voice. Might be able to pull that out of its socket, if the CPU isn't upset that it cannot talk to it over its SPI bus. The speaker also isn't needed, nor is the audio amp circuit to drive it (but that circuit is clean of hum). The chip/line-in switch can be jumpered to always line-in. The control lock out can be jumpered and jack removed. A power on/off toggle switch can be added in its place, but it would be better over at the DC input jack.

On my unit, there is a clear plastic piece, mechanically preventing the indoor/outdoor antenna switch from operating. But the outdoor antenna input is there. The clear plastic piece can be removed. I haven't yet.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Mon 16, 2019 3:08 pm 
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I got curious what my scope would show, as far as noise. The carrier appears clean to me. That's with the mode switch set to line input and nothing connected. I think the hum is getting into the signal during modulation. That's with the mode switch set to the message chip, but I'm sure it would be the same with something connected to line input.

I'll do some more experimenting later today. Maybe I'll try forcing hum into the antenna by laying it close to something noisy.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Mon 16, 2019 4:13 pm 
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Well, this thread is interesting, opened up my unit (Ver 4.6) just to see what is what. When I first received my unit it had a broken tuning mech that would make a clicking sound (the break caused it to jam) , but with a little glue and it started working. Other than that repair no other mods, YET. I am going to bench test for audio modulation bandwidth as forumuser did in previous post.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Mon 16, 2019 4:26 pm 
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If you get hum with an external input connected, try a 1:1 audio transformer between the audio source and input.

If you don't have a 1:1 audio transformer an audio output transformer from an AA5 radio or pretty much any output transformer will work. The secondary is where the source connects and the source output should be set to a lower volume since the transformer will step up the voltage.

The reason is because with the source connected to the transmitter directly, the audio cable, source and whatever else is connected becomes part of the antenna's ground circuit. The transformer keeps the source ground isolated from the transmitter ground which prevents the audio cable, source ETC... from being part of the ground.

I discovered that problem one day a few years back when I moved the source audio cable while listening to the transmitter and noticed the hum level changed.

After adding the transformer being sure to use an isolated 1/4" jack since my transmitter case is metal, the hum was reduced and moving the cable no longer affected the hum level.

I use the following transformer to couple the audio source to my two tube transmitter and that eliminated most of the hum. I used the 15K to 15K transformer

https://www.edcorusa.com/wsm_series

For those who have a stereo source and want to feed this transmitter while providing the necessary isolation, Edcor also makes a stereo to mono transformer

https://www.edcorusa.com/wsm6400

I used one to feed a mono HI-FI radio and it works good.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Mon 16, 2019 4:53 pm 
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I was setup to make an audio test but realized I don't know how to use the line input with continuous transmit modulation. I can record a message then have the unit transmit the message clip, but how do you get it to send the line input continuously. Unfortunately I don't have the user manual..
I will go ahead and record tones and then transmit, but I was hoping there is a way to transmit the line input continuously.
EDIT: WOW, slapping my head hard, there is a switch that says "Live Radio" !!! What an idiot I can be sometimes.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Mon 16, 2019 5:25 pm 
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Here are my results, obvious if I want to play music with improved fidelity I need to modify.(Ver 4.6)
EDIT: Need to redo this test, looks like I may be clipping, need to reduce the line input.
2nd EDIT: No hum detected at AM receiver or RF signal with input to line at 0. (Siglent generator output set to 0, connection direct to transmitter)


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Mon 16, 2019 6:38 pm 
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Tube Radio wrote:
If you get hum with an external input connected, try a 1:1 audio transformer between the audio source and input

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. That, along with my -very- careful A/B comparisons of battery vs. wall wart, is how I can say with good certainty that (on my unit at least), the vast majority of the noise enters via the antenna.

When I did my battery vs. wall wart comparison, I didn't touch the antenna. I stood in the same place for each. I laid the wall wart cord where it would go. Next to the outlet (so it could not radiate anything), and the end of the cord next to the input jack. I did the same with my battery cord: next to the jack.

Then, battery first. Plug in. Allow to CAL. Note noise level. Remove plug from jack.
Plug wall wart into the wall. Plug in the jack. Allow to CAL. Note noise level. Difference not discernible.

Now of course that was without instrumentation to measure the noise. Sure, there is probably some noise introduced from the wall wart vs. a battery, and perhaps a difference amongst various wall warts, and transformer vs. switching as well. But in this case, the noise just coming in from the antenna was absolutely the first thing to address. Perhaps the ground "plane" can be modified. It's a single layer PCB. Couldn't have gotten any cheaper.

Pauls:ironhorse, it's not clear to me what you are measuring or demonstrating... Will you please add some detail?

Any thoughts on using that passive "twin T" notch filter on the antenna?

- If the 60Hz noise is entering there, it should block it.
- Seems the modulated carrier should have no problem going out the other way.
- At first glance, it doesn't seem like there would be an issue matching it (during CAL).

I would have tried it last night, but don't have any 3.9M resistors, nor three 1000pF caps that actually measure that. I have two... Getting proper cap values is probably the most difficult part.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Mon 16, 2019 7:03 pm 
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Some more observations - I removed the main board and display from the cabinet and am running them bare bones on my workbench. I left the speaker and mode switch disconnected. The sound chip is removed. Yes, the TH works fine using line input without those things. I did check how well it was working before and after removing those things. I couldn't tell any difference.

Without a schematic, I'm not sure what the internal difference is between the external antenna jack and the regular antenna connection. I do know that the circuitry checks the regular antenna to make sure it's connected and the right length and gives an error if something isn't right. It must be checking SWR somehow?

However, the external antenna doesn't seem to check anything. I did briefly connect my outside antenna to it a few days ago, and it really puts out, which would be unwise for anything but a quick test. That does make me wonder if running a coax up to the peak of the attic or outside and attaching the regular antenna to the end of the coax would help get the antenna away from AC hum/noise inside the house?

EDIT: must jumper back 2 pins on mode switch connector.

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Last edited by moallen on Dec Mon 16, 2019 7:17 pm, edited 1 time in total.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Mon 16, 2019 7:14 pm 
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DaveInNC wrote:
Any thoughts on using that passive "twin T" notch filter on the antenna?

- If the 60Hz noise is entering there, it should block it.
- Seems the modulated carrier should have no problem going out the other way.
- At first glance, it doesn't seem like there would be an issue matching it (during CAL).

I would have tried it last night, but don't have any 3.9M resistors, nor three 1000pF caps that actually measure that. I have two... Getting proper cap values is probably the most difficult part.

I checked my spare parts and do have the those components. Unfortunately the resistors are 20% and not close enough. An accurate test will probably take precision resistors. I would think R1 and R2 would have to be very close to each other, as well as their ratio to R3. I'm not sure the caps would have to be precision.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Mon 16, 2019 10:18 pm 
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Quote:
Pauls:ironhorse, it's not clear to me what you are measuring or demonstrating... Will you please add some detail?

Just showing the actual RF output (at 1 Mhz, close to middle of BC]) at the antenna, showing modulation envelope. One showing modulation at 1 Khz audio, the 2nd showing 400 Hz audio (both using line input at 1Vpp), demonstrating the low audio band is poor.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Mon 16, 2019 10:31 pm 
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pauls.ironhorse wrote:
Quote:
Pauls:ironhorse, it's not clear to me what you are measuring or demonstrating... Will you please add some detail?

Just showing the actual RF output (at 1 Mhz, close to middle of BC]) at the antenna, showing modulation envelope. One showing modulation at 1 Khz audio, the 2nd showing 400 Hz audio (both using line input at 1Vpp), demonstrating the low audio band is poor.

Ok, thanks. It wasn't a hum evaluation then.

For low freq response, have you increased C301 (center of the board)? That trick's been out there a while. Believe it's feeding that LM358 op-amp nearby. I need to trace some of this stuff out.

Edited: typos


Last edited by DaveInNC on Dec Mon 16, 2019 10:47 pm, edited 1 time in total.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Mon 16, 2019 10:39 pm 
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Quote:
Ok, thanks. It wasn't a hum evaluation then.
For low freq response, have you increased C301 (center of the board)? That trick's been ut there a while. Believe it's feeding that Lm358 op-amp nearby. I need to trace some of this stuff out.

I cannot detect any hum with the signal gen at zero output, none at radio receiver, none on RF carrier.
I have not yet made the modification. Right now I have no music source to couple to the unit, so until that happens I will pass on the mod for now.

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