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Replacing meter in Cobra 29; lost the resistor!

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Replacing meter in Cobra 29; lost the resistor!

Post by jokerls240 » Thursday, 04 March 2010, 19:28 PM

So I was in the process of replacing the signal meter in my 29 WX ST, and while desoldering the wires going to the meter, I dropped the tiny resistor on the nice, tan carpet!

Does anyone know what the value of this resistor is? It isn't listed in the schematic. :pale:
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Re: Replacing meter in Cobra 29; lost the resistor!

Post by Night Crawler » Friday, 05 March 2010, 14:10 PM

jokerls240 wrote:Does anyone know what the value of this resistor is? It isn't listed in the schematic. :pale:
I looked at the schematic and couldn't find any resistor.

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Re: Replacing meter in Cobra 29; lost the resistor!

Post by jokerls240 » Friday, 05 March 2010, 15:32 PM

That's what I said.

I was hoping someone could open up their 29 and see if one is there or not. I know mine had one because I had my pliers on it. The pliers have no teeth so the resistor flew right out as soon as the solder melted.

It was a tiny (maybe 1/6 watt) resistor soldered between the meter's contacts. Now that I think about it, the wires for the lamp had cold joints on the PCB, so maybe the original owner pulled the meter from a different radio and put it in this one.

He/she also did shoddy swing and talkback mods. The swing had a 10 ohm resistor across the (correct) cap, and talkback was done w/ a 470 ohm resistor.

This radio is going to be my tinkering unit. I found a steal on ebay for a 25 LTD w/ the RFX-75 already installed. Got it for the same price as a new, untouched model!
73's - KI4IKT; 240 Joker waving from Music City

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CB: Cobra 29 WX ST w/ RFX-75 (future base) & Cobra 25 LTD Classic w/ RFX-75 (mobile) w/ 102" whip.
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Re: Replacing meter in Cobra 29; lost the resistor!

Post by Night Crawler » Friday, 05 March 2010, 18:13 PM

jokerls240 wrote: maybe the original owner pulled the meter from a different radio and put it in this one.
That's a possibility and that resistor was used as a meter shunt.

If your replacing it with the original meter that resistor shouldn't be needed according to the schematic.

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Re: Replacing meter in Cobra 29; lost the resistor!

Post by jokerls240 » Friday, 05 March 2010, 18:17 PM

Night Crawler wrote:
jokerls240 wrote: maybe the original owner pulled the meter from a different radio and put it in this one.
That's a possibility and that resistor was used as a meter shunt.

If your replacing it with the original meter that resistor shouldn't be needed according to the schematic.
Sounds good to me. The new meter is a direct replacement, so in she goes! Thanks for the help!
73's - KI4IKT; 240 Joker waving from Music City

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CB: Cobra 29 WX ST w/ RFX-75 (future base) & Cobra 25 LTD Classic w/ RFX-75 (mobile) w/ 102" whip.
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Re: Replacing meter in Cobra 29; lost the resistor!

Post by Night Crawler » Friday, 05 March 2010, 18:40 PM

What will happen if that resistor IS NEEDED the meter will be very sensitive.

What will have to be done not knowing the internal resistance of the meter to calculate the amount of shunt resistance required is to experiment with different values of resistance to get it in range.

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Re: Replacing meter in Cobra 29; lost the resistor!

Post by jokerls240 » Friday, 05 March 2010, 18:57 PM

That makes me wonder if it was shunted for the swing kit, which has been reversed in prep for an RFX-75.
73's - KI4IKT; 240 Joker waving from Music City

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Re: Replacing meter in Cobra 29; lost the resistor!

Post by jokerls240 » Friday, 05 March 2010, 20:18 PM

So the new meter is now in - haven't tested yet. It's free swing is much looser than the old one which had a bent needle.

Since I live in an apartment and don't have a dummy load, my only means of testing is to put the radio in the Jeep and on the antenna. I'm going to test the radio before adding the hotter finals. I also don't have access to AC power in the parking lot (need to make friends w/ the neighbors close to the lot) for any solder work. So I would like to take a few resistors and alligator clips outside.

What would be a good starting point for a shunt resistor, if needed?
73's - KI4IKT; 240 Joker waving from Music City

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CB: Cobra 29 WX ST w/ RFX-75 (future base) & Cobra 25 LTD Classic w/ RFX-75 (mobile) w/ 102" whip.
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Re: Replacing meter in Cobra 29; lost the resistor!

Post by longhaireddwb » Friday, 05 March 2010, 21:37 PM

I opened a 29 classic and the resistor is too small for my eyes so I put a meter across it. I got a reading of 350 ohms. Hope this helps you.
Kenwood TS850S, Yaesu 2900, Galaxy 48T, 98VHP, Plus a dozen or so little radios!
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Re: Replacing meter in Cobra 29; lost the resistor!

Post by TheCBDoctor » Friday, 05 March 2010, 22:00 PM

Hi jokerls240

The value of the resistor is approximately 1,000 ohms give or take; the value has changed over the years depending on the radio's meter and the manufacturer. If you measure the resistor while it is in place you will measure the parallel value of the meter's resistance and the resistor.

With out the resistor the meter movement becomes very loose and will not come back as quickly as it should as you speak, it will hang there until you stop talking. By lowering the value of the resistor will make the meter drop back to the carrier level or zero depending on if you are talking SSB or AM.

Good Luck,
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Re: Replacing meter in Cobra 29; lost the resistor!

Post by jokerls240 » Friday, 05 March 2010, 22:35 PM

TheCBDoctor wrote:Hi jokerls240

The value of the resistor is approximately 1,000 ohms give or take; the value has changed over the years depending on the radio's meter and the manufacturer. If you measure the resistor while it is in place you will measure the parallel value of the meter's resistance and the resistor.

With out the resistor the meter movement becomes very loose and will not come back as quickly as it should as you speak, it will hang there until you stop talking. By lowering the value of the resistor will make the meter drop back to the carrier level or zero depending on if you are talking SSB or AM.

Good Luck,
The doctor is in! Nice meeting you and many thanks for the info!

When I was talking about the meter's "swing", I was just referring to the "play" in the needle when the meter is in your hand and you gently shake it side to side.

The radio is still in pieces - I pulled everything apart minus removing the PCB from the chassis. All of the controls and wires are still connected, but dangling. For a first-timer, that was a task to figure out! But I've been taking things apart since I learned the "rightey-tightey, lefty-loosey" rule. :lol:

I decided to take a break (a long one actually) after threading the meter lamp wires through the others and soldering. So I'm calling it a night. Need some rest to help a friend install a cheapo Rat Shack CB and antenna tuning.

73's to the HAM'ers! - KI4IKT, Jeremy
73's - KI4IKT; 240 Joker waving from Music City

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CB: Cobra 29 WX ST w/ RFX-75 (future base) & Cobra 25 LTD Classic w/ RFX-75 (mobile) w/ 102" whip.
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Re: Replacing meter in Cobra 29; lost the resistor!

Post by longhaireddwb » Saturday, 06 March 2010, 7:25 AM

After Doc reply I got a magnifying glass out and read the resistor. It is a brown, black, red. I believe that to be 1000 ohms.

Always listen to what the Doc has to say when he speaks. He's usually right!

HI Doc!
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Re: Replacing meter in Cobra 29; lost the resistor!

Post by jokerls240 » Saturday, 06 March 2010, 8:31 AM

longhaireddwb wrote:After Doc reply I got a magnifying glass out and read the resistor. It is a brown, black, red. I believe that to be 1000 ohms.

Always listen to what the Doc has to say when he speaks. He's usually right!

HI Doc!
You are right about the color code, and I agree to always listen to Doc. His reputation has followed him among many different radio forums. I have plenty of resistors laying around.

I've learned that Cobra's schematics don't always show all of the components, hence this post. If they did show everything, there would be W1-W200 (just a rough number) for the different wire connections. There was another place somewhere that I had read about a 10k ohm resistor on the meter, which gave me another reason to ask before replacing the meter "blind", so to say.

Lesson learned: always make a note of the component you remove before removal! :joker:

Off topic: Nice pic, longhaireddwb! "Justice" is my favorite Metallica album. That is some of the heaviest stuff they put out, if a close second to "Puppets". I've only seen them live once, in '04. They didn't have much of a break before the Nashville show, so James' voice wasn't up to par. I wish I could have gone to last year's show here - Nashville was the first stop in the States after a week break from Europe! I did buy a copy of the show in FLAC format from their website, and it sounded awesome!

Back on topic: Time to get this bargain radio back in service!

Thank you, everyone and enjoy the beautiful weather this weekend! - JokerLS240
73's - KI4IKT; 240 Joker waving from Music City

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CB: Cobra 29 WX ST w/ RFX-75 (future base) & Cobra 25 LTD Classic w/ RFX-75 (mobile) w/ 102" whip.
HAM: Kenwood TM-G707A 2m/70cm (mobile) & TH-G71A 2m/70cm (HT)



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Re: Replacing meter in Cobra 29; lost the resistor!

Post by TheCBDoctor » Saturday, 06 March 2010, 8:36 AM

Hi longhaireddwb
longhaireddwb wrote:After Doc reply I got a magnifying glass out and read the resistor. It is a brown, black, red. I believe that to be 1000 ohms.

Always listen to what the Doc has to say when he speaks. He's usually right!

HI Doc!
Thanks for compliment or is it complement? I am too lazy to go to the dictionary. I have dropped enough of those meter resistors in my early days to forget that value. You will find that if you measure the resistor in the circuit you actually are measuring two resistors in parallel. The resistor in the Cobras are 1k ohm. I thought I was wrong once, but I was mistaken. :icon_e_wink:

Thanks for the plug.

Respectfully,
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Re: Replacing meter in Cobra 29; lost the resistor!

Post by longhaireddwb » Saturday, 06 March 2010, 16:46 PM

It was a compliment for sure Doc!
Your the man when it comes to these radios. I thank you for the help you've given me in the past and for the help I know you will give me in the future!
Kenwood TS850S, Yaesu 2900, Galaxy 48T, 98VHP, Plus a dozen or so little radios!
73 from the shade of the big mountain!
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Re: Replacing meter in Cobra 29; lost the resistor!

Post by jokerls240 » Sunday, 07 March 2010, 0:00 AM

The 1k resistor worked great! I didn't get to test any Tx because I wired the radio's mic connector wrong. :oops:

Just got the majority of the RFX-75 installed, minus replacing R48 & C42 (gotta find a 3300uF cap). Then I may take the radio to the CB tech at a local truck stop, unless it's possible to retune Rx/Tx with just a PDC 600. I also found that the previous owner cut D11. :icon_rolleyes: Going to resolder that tomorrow.
73's - KI4IKT; 240 Joker waving from Music City

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CB: Cobra 29 WX ST w/ RFX-75 (future base) & Cobra 25 LTD Classic w/ RFX-75 (mobile) w/ 102" whip.
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Re: Replacing meter in Cobra 29; lost the resistor!

Post by TheCBDoctor » Sunday, 07 March 2010, 6:53 AM

Hi jokerls240

After you solder D-11 back into place you may need to adjust VR-4 to make sure you are at 100% modulation.

Good luck,
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Rick, The CB Doctor


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Re: Replacing meter in Cobra 29; lost the resistor!

Post by jokerls240 » Sunday, 07 March 2010, 9:46 AM

TheCBDoctor wrote:Hi jokerls240

After you solder D-11 back into place you may need to adjust VR-4 to make sure you are at 100% modulation.

Good luck,
Thanks for the heads up, Doc!

Right now VR4 is at the 1-o'clock position (assuming 12 o'clock is center, facing the rear of the radio).

It's going to take some time to find a 3300uf capacitor as RS doesn't stock them. There are a couple of extra PC power supplies I don't mind opening up and stealing parts from. I really wanted to make a variable DC power supply out of one of them.

Doc, if I completely fubar this radio, would you be willing to make the necessary repairs if I ship it to you w/ prepaid return shipping?
73's - KI4IKT; 240 Joker waving from Music City

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CB: Cobra 29 WX ST w/ RFX-75 (future base) & Cobra 25 LTD Classic w/ RFX-75 (mobile) w/ 102" whip.
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Re: Replacing meter in Cobra 29; lost the resistor!

Post by jokerls240 » Thursday, 25 March 2010, 8:29 AM

So I robbed the resistor from an old component style stereo that had a built-in amp. It was the only 3300uf cap in it. How lucky was that?

After reconnecting D11, the modulation was still spitting over 100%. I'm waiting until I get a dummy load before I tweak VR4. Dynamike still works, so I can just turn it down for now.

Thanks for all the help guys!
73's - KI4IKT; 240 Joker waving from Music City

Gear
CB: Cobra 29 WX ST w/ RFX-75 (future base) & Cobra 25 LTD Classic w/ RFX-75 (mobile) w/ 102" whip.
HAM: Kenwood TM-G707A 2m/70cm (mobile) & TH-G71A 2m/70cm (HT)



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Re: Replacing meter in Cobra 29; lost the resistor!

Post by thumpe » Sunday, 28 March 2010, 16:33 PM

the resistor looks to be 1/8 watt brown/blac/red which would be 10,000 ohm
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Re: Replacing meter in Cobra 29; lost the resistor!

Post by thumpe » Sunday, 28 March 2010, 16:36 PM

i must not know how to add today or something???
brown/black/red= 1,000 ohms
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