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 Post subject: Retro Radio Shop AM transmitter?
PostPosted: Jan Mon 11, 2021 3:44 pm 
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Location: Saratoga, NY
My sstran AMT3000 died some time ago. I checked everything, no go. The output is a low hum no matter what.

Has anyone tried one of these?

https://www.retroradioshop.com/collecti ... e-radios-1


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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Mon 11, 2021 8:00 pm 
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Location: Carmel, Indiana
Unlike the AMT3000, this unit is fixed frequency transmitter.

Blow up photo of the PC board and all of the components: https://cdn.shopify.com/s/files/1/2932/ ... 1598834246

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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Mon 11, 2021 11:21 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: SoCal, 91387
Not many components for the money, and it requires being placed near the receiver, so the range can't be all that good, if you like to hear it on radios in other parts of the house.

AES sells a two tube kit for a little less, but tubes and P/S don't come with it;
https://www.tubesandmore.com/products/p ... n-bob-heil

There's also a thread on using an LM386 as a transmitter, which would seem to be a budget option.
Here's some prints;


Attachments:
LM386 Transmitter.jpg
LM386 Transmitter.jpg [ 201.03 KiB | Viewed 520 times ]
lm386n101112.jpg
lm386n101112.jpg [ 388.88 KiB | Viewed 520 times ]

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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Tue 12, 2021 12:57 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5450
Location: Black Hills, SD 57745
jth877 wrote:
My sstran AMT3000 died some time ago. I checked everything, no go. The output is a low hum no matter what.

Has anyone tried one of these?

https://www.retroradioshop.com/collecti ... e-radios-1



Hmm, I haven't had that type of fail on the SStrans. Usually it's the surface-mount processor chip that either goes or needs resoldered. When bad it causes distorted output and wonky control action. Have you checked any of the transistors?

On the link for the mini-broadcaster, it doesn't seem to state what frequency. That could be an issue with crystal control if it ends up on top of some strong station local to you.
-Ed

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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Tue 12, 2021 3:46 pm 
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Joined: Sep Fri 30, 2011 3:55 am
Posts: 276
Location: Saratoga, NY
Ed in SoDak wrote:
jth877 wrote:
My sstran AMT3000 died some time ago. I checked everything, no go. The output is a low hum no matter what.

Has anyone tried one of these?

https://www.retroradioshop.com/collecti ... e-radios-1



Hmm, I haven't had that type of fail on the SStrans. Usually it's the surface-mount processor chip that either goes or needs resoldered. When bad it causes distorted output and wonky control action. Have you checked any of the transistors?

On the link for the mini-broadcaster, it doesn't seem to state what frequency. That could be an issue with crystal control if it ends up on top of some strong station local to you.
-Ed



I rechecked my amt3000 based on what you said and found two unsoldered joints on the RCA (center) input jacks so I fixed that *So ashamed*. I couldn't find the 18v AC wall brick for it. Must have thrown it out by accident. I tried an 18v DC source because the instructions state you can use 20v-24vDC as power. Low and behold I got a weak signal and a high pitched squeal. I think it's alive!


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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Tue 12, 2021 10:15 pm 
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We can call that progress! A shame to trash an SStrans, 'specially since they're unobtainium new. And a bit spendy even when bought used.

Now to find why low output and a squeal. Double-check inductor settings, antenna+placement and the antenna trimmer. Wouldn't hurt to try a second radio too. A dead processor chip would be a deal changer, not sure if those can still be sourced as an exact replacement.

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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Tue 12, 2021 11:28 pm 
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Try three 9 volt batteries in series. Your results seem to indicate not enough power.

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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Wed 13, 2021 5:26 am 
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I used a 24v dc supply today and peaked the antenna. Got a strong signal but the squeal/hum/screech is still there. The music is audible under the awful noise.

Will try a good ground next. Maybe the DC supply is noisy. Would a 24v ac transformer work or is that too.much juice?


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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Wed 13, 2021 5:33 am 
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I think AC would be the wrong juice; the circuit wants to see DC. I had suggested three batteries because that would be the quietest supply.

The oscillation could be caused by a variety of things. If you have a capacitor tester, you might try checking the Electrolytics.

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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Wed 13, 2021 4:48 pm 
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OK, I'll check that. Unusual that an electrolytic would go bad after 7 years? Most of it not being used?

The amt3000 comes with a 18v AC transformer. There is a rectifier in it.

I'll make a video and post the link of what it sounds like.


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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Wed 13, 2021 7:00 pm 
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Where are the controls set? I have modulation and compression set low, about 9 o'clock. Gain 3/4, about 3 o'clock.

I'm afraid I'm suspecting the processor chip, the SSM2166A Microphone Preamplifier. A later version is available on Mouser for 7 bucks and change: https://www.mouser.com/Analog-Devices-I ... epZ1yyh4l4

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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Wed 13, 2021 8:10 pm 
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jth877 wrote:
OK, I'll check that. Unusual that an electrolytic would go bad after 7 years? Most of it not being used?

The amt3000 comes with a 18v AC transformer. There is a rectifier in it.


The Electrolytic's would be my first passive component suspects. Yes, you can use any AC transformer, but would need a rectifier circuit downstream, to convert to DC. That's simply a Diode or bridge, and an Electrolytic cap across the + and - rails.

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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Thu 14, 2021 2:23 am 
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Fifties, I thought you owned an SStrans?

I own two myself and have built or repaired several for an ARF member or two. All the fails I've dealt with involve that pesky processor chip.

It's designed to operate with an AC wallwart, An 18v, 500ma wart is what was supplied with the kit.

From the manual:

"The power supply section consists of an RF filter section, a
bridge rectifier, a +15 volt voltage regulator (U1) and a +5 volt
voltage regulator (U4).
The wall transformer supplies 18 volt RMS AC power to the
AMT3000 at the power jack (13). Inductors L3 and L4 can
optionally be included in the path or not by opening or shorting
jumpers S2 and S3. With the jumpers in the open position L2
and L3 prevent transmitter RF from being coupled back to the
AC power lines to help alleviate hum problems that are
sometimes encountered due to interaction of the AC power
lines with transmitter RF coupled to the power lines through
the wall transformer. For most installations jumpers S2 and S3
can be kept in the shorted position to take advantage of the
additional ground path provided by the AC lines.
The full wave bridge rectifier, consisting of diodes D1, D2, D3
and D4, converts the AC input to pulsating DC, which is then
smoothed by filter capacitor C4. Each diode is bypassed with a
0,1 uF capacitor to assure a low RF impedance across the diode
when the diode is in the non-conductive reverse-biased portion
of the AC cycle. This prevents possible 100 Hz hum
modulation that can be induced when the RF impedance to
earth ground through the power lines varies with the on/off
state of the diodes. Voltage regulator U1 supplies regulated +15
volts to the RF output and modulation stages. It also feeds
voltage regulator U4, which provides regulated voltage at +5
volts for powering the ICs."

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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Thu 14, 2021 4:01 am 
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Joined: Sep Fri 30, 2011 3:55 am
Posts: 276
Location: Saratoga, NY
Back in business. I had to open all the jumpers except for s7. Still have a slight high pitched hum. Like a tone for a hearing test. That could probably be anything.

I'm using a 24v dc supply. Do you think an 18v ac wart would the eliminate the hum?


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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Thu 14, 2021 4:22 am 
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Ed in SoDak wrote:
Fifties, I thought you owned an SStrans?


What makes you think I don't?


Attachments:
SSTrans.JPG
SSTrans.JPG [ 767.37 KiB | Viewed 198 times ]

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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Thu 14, 2021 4:24 am 
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jth877 wrote:
Back in business. I had to open all the jumpers except for s7. Still have a slight high pitched hum. Like a tone for a hearing test. That could probably be anything.

I'm using a 24v dc supply. Do you think an 18v ac wart would the eliminate the hum?

Glad to see that you got it going again. The way to either totally defeat, or at least decrease the hum, usually lies in the antenna and ground. If you have the antenna near electrical lines inside the wall, for example, that can cause it. You simply have to experiment with placing the antenna, and using or not using a ground.

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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Sat 16, 2021 5:27 pm 
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Joined: Sep Fri 30, 2011 3:55 am
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Location: Saratoga, NY
Final report. I bought a new 18v AC transformer off amazon. It's for door bells. It eliminated the annoying tone and the sound is loud and very clear now.

My conclusion is that you can't use cheap/modern AC/DC switching transformers with the sstran. It also helps to not throw away the original ac transformer by accident.


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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Sat 16, 2021 5:38 pm 
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Congratulations on getting it working! The reminded about those jumpers is also helpful.

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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Sat 16, 2021 7:26 pm 
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jth877 wrote:
Final report. I bought a new 18v AC transformer off amazon. It's for door bells. It eliminated the annoying tone and the sound is loud and very clear now.

My conclusion is that you can't use cheap/modern AC/DC switching transformers with the sstran. It also helps to not throw away the original ac transformer by accident.

So you are using an AC supply directly, W/O any rectification to DC?

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 Post subject: Re: Retro Radio Shop AM transmitter?
PostPosted: Jan Sat 16, 2021 7:43 pm 
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That's great news! Thanks for the followup. I didn't think the AC 18v would be such a particular requirement. Must be the DC wart you tried was deficient in some way or being a switcher was the problem. I'd guess an old-style "analog" DC wart with iron transformer would work fine.

I always save old wallwarts, got a drawer full of 'em. Old 2400-baud modems often used an AC supply as well. I was thinking of mentioning that, but I doubted you'd have one stashed away.

The SStrans was the Gold Standard for AM transmitters. A bit fussy to set up sometimes, but usually they just worked and worked well. Too bad the guy who produced them just dropped it; a long, sad story. It's worth the effort to keep one working.
-Ed

Fifties: That's why I was wondering if you owned one, and why I copied that snip from the manual a few posts above. It's expressly designed for that 18v AC supply and has the on-board rectification, regulation, filtering, etc. to use it. Doesn't yours also use the supplied AC wart?

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