PRC 25 T (Tadiran)Buzzing Tone while Transmitting

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Charcoal01

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Hi There,
I bought an Prc 25T on Ebay,equicked with the PRC 77 Battery Holder(10 D Cells).The Batteries are new,Receiver is working fine,Transmitt Power 3.5Watt,but when I Transmitt,the other Station remarks a lout Buzzing Sound while I speak.
I checked the Frquencys with FQ357 Measure Tool from my Ham equiqment and they wont went of at any Channel. Transmittèr ist working Fine!
I also changed the A50 modul,the Buzzing keeps on.We Tried some Frequencys and ist always there.
Has anyone an clue for me?Is this a Typical Problem?
Best Greetings from Westerwald,Germany
 
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papakb

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It's the 150hz tone squelch getting into the audio. Normally it's modulated onto the signal to open the squelch on the receiving radio. If it's a 25 try changing the A24 squelch module, if it's a 77 you want to change the A54 module. They perform the same function. BTW, There is no A50 in a 25.
 
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Charcoal01

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Hello,Yes its a prick 25.ok,nur is tue 150hz tone hearable on tue other receiving Radio(normaly)?the other Radio was a normal cb radio,Frequency are at some channels um to 100hz equal,yesterday we tried a RT -67 receiver,but the buzz is still there,Not so strong as in the cb radio,but good to hear.
Thanks for your help,i will Change the a24 Modul and give you a Report.
Best regards,have a nice Day
Timo
 

cagosti

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Non army radios do not filter out the buzz. I think you should test with someone that has a modern army radio before you try to fix anything
 

Charcoal01

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Ok,this must be the reason Why it so strong on a normal cb radio.is There a Way to cut it off?on my RT-67 the buzz appears much weaker then in the normal cb radio...at this point i start wondering.
I'd like to use the prick also for cb,Not Problem for a few Frequencies,but some Way i had to remove the new squelch tone?
Best regards,
Timo
 

Charcoal01

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I fixed it:)!you can turn in module A23 the potiometer to the left until the buzzing noise while transmitting complitly disappears.i will test it to a greater distance later today and give you a report.
Thanks for your help,great job!!
Best regards,have a nice day from central germany
Timo
 

papakb

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Guten tag. There are some things you should understand about the military radios. First of all they are wide bandwidth radios where most civilian radios are narrow band. They are also FM rather than AM radios. The PRC-25 and the 77 operate from 30 to 75 Mhz so I'm not sure how you ever talked to a citizens band radio that operates in the 26-27 Mhz frequency range and while the RT-67 operates from 27 thru 39 Mhz you'd only be able to talk between them from 30 to 39 Mhz. Communications would be possible but the RT-67 uses carrier squelch the PRC 25 uses tone squelch and they aren't compatible. You'd have to turn off the squelch on both sets and let the speaker hum to be able to talk between them. With the squelch turned off the 150 tone isn't generated and that's why you don't hear it.

By turning down the pot in the A23 module you've effectively turned off the 150 hz squelch tone and while that will get rid of the buzzing it means that you won't be able to use the squelch feature when talking to another PRC-25, 77, RT524 or R442.
 

Charcoal01

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@ Papakb :Ok,I understand what you say,I just`killed`the 150hz Tone,I couln`t make a Test Yesterday if it works now,but i do that later today and give you a Report.
So I´m not sure if the Squelch on my RT-67 was on,but when it was:wont the incoming strong Signal from my 25 open it?otherwise,when the Squelch on the 25 is on,wont the Signal from the67 open it?I`m not familiar with the Newe Tone Squelch,German Army Radios have not this System,the work like Usual CB Radios,So my SEM25 for Example.Squelch open by Signal Strength.
I Tested al Channels of the Prick 25 from 26 to28 MHZ with a standart Frequency Counter and put down the Results;only few Frequencies in 27Mhz were slightly driftet,but not more then 80hz,for that,some nearly alsolutely on cb Channels.
 

Charcoal01

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@ Papakb :Ok,I understand what you say,I just`killed`the 150hz Tone,I couldnt make a Test Yesterday if it works now,but i do that later today and give you a Report.
So I´m not sure if the Squelch on my RT-67 was on,but when it was:wont the incoming strong Signal from my 25 open it?otherwise,when the Squelch on the 25 is on,wont the Signal from the 67 open it?I`m not familiar with the Newe Tone Squelch,German Army Radios have not this System,the work like Usual CB Radios,So my SEM25 for Example.Squelch open by Signal Strength.
I Tested all Channels of the Prick 25 from 26 to28 MHZ with a standart Frequency Counter and put down the Results;only few Frequencies in 27Mhz were slightly driftet,but not more then 80hz,for that,some nearly alsolutely on cb Channels.
 
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Charcoal01

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It`working[thumbzup] I Tried a few Channels on a 15km distance CB Radio Station and Iwas read Loud and Clear with strong Signal,I was working on a Gainmaster Antenna with 3,5Watts Power.But Please explain me the working Feature of this Squelch,and how could I aligne the Frequencies in the 27Mhz area.
Thanks for all your Help,
 
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papakb

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The term "squelch" or "squelched" means that between transmissions the radios speaker is turned off and you won't hear that "rushing" sound. In the "squelch off" position you will always hear it and it gets old after a while. This is true with both the PRC-25/77 and the RT-67.

The PRC 25 would normally send a signal with the 150 hz tone on it to open the squelch on another of the "new family" radios like the 25 or 77. With the A23 module tone turned down in the 25 the way you did the signal it sends out will still have a carrier without it and even with the RT-67 in "squelch on" position the 67 will see the carrier signal from the 25 and it will open the squelch and you'll be able to hear the signal. Now if the 25 is in "squelch on" mode it probably won't hear the 67 because it's looking for that 150 hz tone to turn the speaker on and the 67 doesn't generate one.

I'm still puzzles how your CB radio on 27 Mhz hears your 25 when it's lowest frequency is 30 Mhz. And wherever you got your 25 you got a good one. Normally they run about 2 watts out.
 

Charcoal01

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Good morning,
Ok,I understand,the Squelch in my 25 probably just works with the 150hz Tone.Inspite of this,i will make a try to "adjust" the pots in the Squelch modul;marking the Start Position in Case it doesn`t work.
This 25 is the Israeli "T" Version build by TADIRAN,I knew that the american Version starts at 30Mhz,I found This one in Ebay at a Antique Seller`s Shop and he has Just in the Moment another one to sell.(Ebay Germany,if your interestet).In untested condition,from Depot.I found it great,because I could work on 11m,10m,and 6m Band.
Thanks :) ,I also found it´s a real good one!By the way,how can i align the Freqencies,I found nothing in the Internet.I would like to align the Lower Band a bit.when ist possible....IMG_20180717_092847[1].jpgIMG_20180717_093258[1].jpg
 

tennmogger

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Charcoal01, you seem to have your PRC-25 working nicely for your purpose now. Neat radios of old technology are fun to play with!

Are you familiar with SEM-25 and SEM-35? I have been lucky enough to find those on this side of the pond and enjoy those too. The '25 and '35 also cover down to 26 MHz, making them usable on 10m. These radios were imported along with Unimog radio trucks that were brought in.

For CB, I am intrigued that you have success with an AM CB talking to an FM PRC-25. How does that work?

Maybe your CB radios have FM capability that 'ours' don't?
 

Charcoal01

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Hello tennmogger,Yes,ist really fun,specially your 150hz Tone Squelch;we don`t have such a technic in our German Radios :).I have a PRC 9 also,but it doesn`t work anymore,thats my my next Project.I got the SEM25 in my Garage,powered with two 12V Batteries and the Original Antenna with the "AGAT" in his bottom.This Radio is nearly not compatible with CB Radios;It works absolutly on the Channels Frequencies in each Channel and ist 50khz Channel steps makes it nearly useless for cb.The 35 i knew from my time at the Bundeswehr,and it was a Desaster...Even in Line of sight sometimes your can`t communicate...Waving your Arms was better then Talking:-D!!No Kidding!
I Tested last Days a lot with my 25,and your right,I can hear Radio Traffic on AM,but I´m sure the other Station couldn`t read me loud and clear.I only use it in FM Mode,Am wouldn`t work for sure!
Ist works on FM Very Good,even when Frequencies are up to 90 khz different.I´m not sure if our "europe" CB Radios be different to yours in the US.Thats a good Question!
Let me say,I´m intregued by your List of Vehicles-very very Nice!!That old Unimogs are very expencive here in Germany,they are still,after all that Years very Popular!I knew same Guys that still work with them in Forrest:)
 
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