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 Impedance of mic for tti TCB-550 (AMC-5011)?

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MadHax
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MadHax

Call Sign : 68-CT-059
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Join date : 2020-07-10
QTH or Location : Jordanstown
Equipment Used : None, researching it.

Impedance of mic for tti TCB-550 (AMC-5011)? Empty
PostSubject: Impedance of mic for tti TCB-550 (AMC-5011)?   Impedance of mic for tti TCB-550 (AMC-5011)? Icon_minitimeWed Jul 22, 2020 7:26 pm

Hey guys, can anyone tell me the impedance of the above mic for the TCB-550.  I have a mic (not the original) but it is about 470R, and can't get any modulation on a rig check, so i think I should be using a mic with a lower impedance, say 50R?  Can anyone confirm or otherwise?

K
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Victor
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Impedance of mic for tti TCB-550 (AMC-5011)? Empty
PostSubject: Re: Impedance of mic for tti TCB-550 (AMC-5011)?   Impedance of mic for tti TCB-550 (AMC-5011)? Icon_minitimeWed Jul 22, 2020 8:49 pm

Hi Keith,

(Public health warning.... I'm gonna waffle, but it may help others as well Smile)


A typical radio microphone is a 'Low Z' device, which basically means low impedance and that equates to 600 ohm or less.
Most CB microphones I've seen run from about 150 ohm up to the 600 ohm mark so would've thought your 470 ohm sits about right.

The service manual is available from ManualLibs and checking the schematic shows a standard affair mic input circuitry, good for most mics including power mics.

Luckily this radio isn't one of the types with 'phantom power', or in most CB radio terms, one that carries 13.8 volts on a pin ready to damage the radio should you inadvertently wire it incorrectly!

A microphone seems the easiest thing to change or 'upgrade' on a CB and is done often, sometimes with disastrous results.

But, that said, the commonest problems I've seen over the years is dropping the damn things Wink

We forget how well these little devices convert moving air pressure from our mouth into an electrical signal for the radio....it's actually quite a delicate device(!) In which case I'd be suss about any second hand microphone, (let alone the fact it's usually full of someone's elses spittle!!! Yuck!)

If dropped at some time it could be completely useless but still measure a typical resistance on the element.


Replacement mics are cheap enough and it's always worth while getting a new one. At least if you drop it, you've only got yourself to blame Smile


Anyway, waffle over....I hope that helps you out somewhat.

All the best,
Victor
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MadHax
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MadHax

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Equipment Used : None, researching it.

Impedance of mic for tti TCB-550 (AMC-5011)? Empty
PostSubject: Re: Impedance of mic for tti TCB-550 (AMC-5011)?   Impedance of mic for tti TCB-550 (AMC-5011)? Icon_minitimeWed Jul 22, 2020 10:09 pm

Thanks Victor, I appreciate the time taken to reply.

Yes, 470R struck me as in the sweet zone, i did wonder if some rigs needed a much lower impedance, hence the question.  I have looked at the inside of the mic and I can see a soldering iron burn on part of the casing, so clearly someones been in there. I checked the magnet with a screwdriver and its field does seem weaker than i would expect;  fear ye not, I'm an electronics engineer. So although the coil and membrane are intact, i think the magnet appears very weak and is delivering a weak voltage output signal. I'll maybe scope it tomorrow and see. 

I have ordered a new mic,... what's the worst that can happen.


Cheers

Keith

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Victor
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Impedance of mic for tti TCB-550 (AMC-5011)? Empty
PostSubject: Re: Impedance of mic for tti TCB-550 (AMC-5011)?   Impedance of mic for tti TCB-550 (AMC-5011)? Icon_minitimeWed Jul 22, 2020 10:27 pm

No worries Keith,

Was nice to waffle as it can often help someone else out being a public forum and all that.
Always a good thing to pass a little knowledge on.

Well, a fellow tech....you'll appreciate how the heat of a soldering iron and the AC field most hobbyist irons produce can completely screw up a microphone magnet! Wink
Ha! Very Happy Seen that one a few times myself with attempts at 'rewiring' or 'repairing'. Then how else do we learn but for our mistakes....if it's not "Eureka" we herald, it's "Woah! That's weird?"


All the best to you Keith and I hope you get it sorted. I'm sure you will.

Victor
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MadHax
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MadHax

Call Sign : 68-CT-059
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Join date : 2020-07-10
QTH or Location : Jordanstown
Equipment Used : None, researching it.

Impedance of mic for tti TCB-550 (AMC-5011)? Empty
PostSubject: Re: Impedance of mic for tti TCB-550 (AMC-5011)?   Impedance of mic for tti TCB-550 (AMC-5011)? Icon_minitimeTue Aug 04, 2020 11:01 pm

Hello Victor, 'twas the mic, bought a new one from ebum, which was the same price as the rig, and wham-bam, i'm modulating; so i'm happy, thanks for the reply.

K

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Victor
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Call Sign : 26-CT-3228
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Join date : 2019-11-10
QTH or Location : Bedford
Equipment Used : Various

Impedance of mic for tti TCB-550 (AMC-5011)? Empty
PostSubject: Re: Impedance of mic for tti TCB-550 (AMC-5011)?   Impedance of mic for tti TCB-550 (AMC-5011)? Icon_minitimeWed Aug 05, 2020 5:59 am

Hiya Keith,

Nice one, glad you got it sorted out and thanks for letting us know. As I said, not just for me but for others who may read this on another day and maybe helps them too.


All the best,
Victor
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