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Help fixing old walkie talkie

ATA

May 12, 2016
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Finally the coil arrived. It is too big to fit but extended the legs and got it fitted on the board. Well, Allthrough the volume is turned way up, I can only slightly hear a transmitted signal. But ok, it's there.
The big problem is that when transmitting it is creating a very loud and schrilling noise along with the speak.
There was no problet at all before the coil was inserted. So why is this coil affecting the trnasmitting signal?
 

73's de Edd

Aug 21, 2015
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Sir ATA . . . . . . .

At that time that coil was the only one being offered and to be found . . . .in all of the world . . . . I do believe.
That was a transformer for a CB RF amplifier stage.

It looked like a direct drop in, size wise for the old unit, but it now is seeming to be a wee bit larger.

The only other option for the original unit, would be to take a 10.7 IF transformer and decrease
its windings turn count ~2/3. . . .not so easy nor much fun.
Your new unit should work fine.

I am considering two situations on your present dilemma.
You will have to refer to past posts schematics.
Using your post #9's photo of P1030690.JPG


p1030690-jpg.26790



You can see the involved feed thru holes of the transformer.
Three associated with the primary and two associated with the secondary.

Now refer to post # 19's schematic.

dwQplxg.jpg


You can see TR1 27Mhz RF amplifier and using what I like to call upside down and backwards circuitry.
Most commonly you see an NPN amplifiers being fed with + voltage.
In this case it is being positive but having to be referenced to the - 11v found on the emitter.
Also take note that this transformer uses a matching tap just up from the cold (lower) terminal of the transformer primary.

Your new transformers offering, does NOT have that matching tap on it, so you are going to have to place a small jumper wire between that terminal and the lower cold connection.

I would be using a single strand of fine copper wire from AC line cord wire.

THEN you will be getting DC power to that stage, in order for it to now work.

With the great amount of RF gain occurring in that stage, you can negate the non use of a tap of the winding.
Since you WILL have plenty of gain from that stage, and as you can see, there is even provision for diminishing that stages gain, via the use of the 1k trim pot.
Now if that does not solve the problem, the only other question was if the extension leads did ALSO have two extra ones for the metal shields side tabs to get them connected to ground ?

Talk to me . . . .

73's de Edd
 
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ATA

May 12, 2016
13
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Well, seems a bit complicated to me. But yes I could try and find one 10.7 IF in 7mm size. But it is so tiny to fiddle with.
The only one I can find it this http://www.rf-microwave.com/en/shop...-if-coils-transformers/2536-B-10.7-F-7-N.html

Most I find are 455mhz and 10mm. Could I decrease one of them?

I don't know what I'll do. As it is just an old cb that I'm not gonna use much, I may just let it sit in the drawer and wait til either a coil like the original one comes up on Ebay or I find a similar model and take it from that. Or perhaps any model or other old radio.
 

73's de Edd

Aug 21, 2015
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Sir ATA . . . . . . .

No no no . . . . .I was mentioning that was the ONLY option that there was, until THAT unit was found.

As I said, all that I see that you need to do is confirm that the now floating, corner pcb foil which is being cold terminal (#19 on the Sams schematic above).
No power is getting to the new transformer until that connection is made, which is actually being ground..


Cold terminal ?

That primary winding of the transformer has a terminal that gets connected to the collector of the TR 1 transistor . . . .which would be called the "hot " terminal.
The other terminal of the transformer supply gets connected to the power supply, which would be called the "cold" terminal.
( Hot and Cold is actually referring to the RF level / presence .)

Also I wanted to confirm that the "floating" metal shell was not just left without its side tabs being grounded.

Related PCB FOIL Layout:

YELLOW is where the jumper is being needed.

TBN8r4g.png





73's de Edd
 
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ATA

May 12, 2016
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May 12, 2016
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I'm not sure what you mean by floating metal shell. It is connected to the pcb by its two legs.
 

73's de Edd

Aug 21, 2015
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You are just fine then.
Since it ended up being slightly above the board a bit more than the other one, I just wanted to be sure that it got its metal outer shell, grounded.
 
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