Modifying a Radio for use with a K40 Speech Processor?

[Anon]

I know the principle of modifying a radio to accept a K40 SP mic by removing the TX line and putting a diode inline with it and using a resistor tied to 12v to power the circuit ahead of the diode that's all pretty strait forward but.........................

Once modified can you still use a standard mic or are you tied down to a k40 mic?

Also anybody recommissioning a proper old K40 Mic did you have to replace anything in them?

I am guessing a 30 year old mic the caps would be past their best?

I know the cables go on them but mines still like new apart from the end that needs trimming back a little.

Id be grateful for any tips on getting this up and running

Jodie

[Mitch]

You can still use ordinary mics after the K40 bits are in the rig, it doesn't affect normal use at all. The mic wires do go poorly though, I had to replace two of mine (I still have 3 K40 mics) as they where literally falling to bits, the cable went stiff and cracked when bent, that was an interesting job. I replaced one with a really long wire which was quite usefull, it'll stretch to 10 feet, lol. The other weak point is where they connect to the mic plug, all that bending takes it's toll, the wire is very thin inside. The mic wires are quite thin even on the outside too, so replacing with the wire off a standard mic can be difficult.

As for deterioration you'll find the foam inside the mic head behind the grill will go bitty and flaky so should be replaced or left alone, when touched it turns to dust, not bend or spring back to shape. It's a case of "if it ain't broke...". The capacitors in mine seem to be OK though, the mics charge up on RX and give me a good couple of minutes waffle time even now, but if you suspect a problem then it can't hurt to change them. Do you have originals with the hi/lo switch on the side? I think they're easier to work on because the mic wire is on a connector strip so can be removed for servicing. The newer type have the wires soldered direct to the pcb (copies made by astatic) so fiddling can break them, not so good imho.

Damn good mics though, even now 30 odd years later, I love 'em and always have, but not to everyones taste and don't suit all radios unfortunately, worth a try though

[Anon]

You can still use ordinary mics after the K40 bits are in the rig, it doesn't affect normal use at all. The mic wires do go poorly though, I had to replace two of mine (I still have 3 K40 mics) as they where literally falling to bits, the cable went stiff and cracked when bent, that was an interesting job. I replaced one with a really long wire which was quite usefull, it'll stretch to 10 feet, lol. The other weak point is where they connect to the mic plug, all that bending takes it's toll, the wire is very thin inside. The mic wires are quite thin even on the outside too, so replacing with the wire off a standard mic can be difficult.

As for deterioration you'll find the foam inside the mic head behind the grill will go bitty and flaky so should be replaced or left alone, when touched it turns to dust, not bend or spring back to shape. It's a case of "if it ain't broke...". The capacitors in mine seem to be OK though, the mics charge up on RX and give me a good couple of minutes waffle time even now, but if you suspect a problem then it can't hurt to change them. Do you have originals with the hi/lo switch on the side? I think they're easier to work on because the mic wire is on a connector strip so can be removed for servicing. The newer type have the wires soldered direct to the pcb (copies made by astatic) so fiddling can break them, not so good imho.

Damn good mics though, even now 30 odd years later, I love 'em and always have, but not to everyones taste and don't suit all radios unfortunately, worth a try though

Thanks for all the info,mines an original from 1980,it came with a radio I bought ages ago I wasn't really fussed with and long since sold.

I got my money back on the radio and got the mic for free

Mic lead has never been cut down its still really supple so thinking of feeding it with rubber conditioner?

where it enters the mic its gone a tad iffy and likewise where it enters the original plug but there's plenty of strait cable so ill cut it down to good cable

Its probably the best I have seen short of buying a mint boxed original that you see on ebay for stupid silly money

Its got an old 5 pin Cobra/President plug on at the mo but ill swop that to 4 pin Uniden.

It worked fine on the radio it came on but it wasn't wired internally to power the mic so it just operated as a stock mic

I can get my head round the supply voltage wont ground to tx because of the diode

but cant get my head around why on tx it doesnt send 12v to ground when you press the ptt?

surely there is always 12v then on the tx pin at the pin itself?

You tx that 12v joins to mic ground and ultimately the radio ground as its after the diode

I guess that why i was also worried about using a standard mic

Its one of those rainy day projects

[Mitch]

I'm guessing you're not an electronics person, that's OK, not everyone is. Luckily there's a forum with people that have experience of these things

Thanks for all the info,mines an original from 1980,it came with a radio I bought ages ago I wasn't really fussed with and long since sold.

I got my money back on the radio and got the mic for free
...
Its probably the best I have seen short of buying a mint boxed original that you see on ebay for stupid silly money

Well done! Mine must be worth a small fortune then, I have the boxes for all three

...
Mic lead has never been cut down its still really supple so thinking of feeding it with rubber conditioner?

where it enters the mic its gone a tad iffy and likewise where it enters the original plug but there's plenty of strait cable so ill cut it down to good cable
...

I've never heard of rubber conditioner so can't comment on it's use, I've never used any myself. The two mics I got that were bad were like it when I got them so I just replaced the wires, the mics worked alright afterwards though with no other mods except setting up as per factory specs, there's a service sheet available online (cbtricks?) that details it.
Replacing the wire at the mic end is the most difficult as the wire is glued into the strain relief but can be freed with some care and tiny jewellers screwdrivers to poke it free from the triangle bit and the metal retainer grip thing.

...
It worked fine on the radio it came on but it wasn't wired internally to power the mic so it just operated as a stock mic

I can get my head round the supply voltage wont ground to tx because of the diode

but cant get my head around why on tx it doesnt send 12v to ground when you press the ptt?

surely there is always 12v then on the tx pin at the pin itself?

You tx that 12v joins to mic ground and ultimately the radio ground as its after the diode

I guess that why i was also worried about using a standard mic
...

Radios normally have 9v (or less) on the ptt line which is grounded anyway when the ptt is pressed, the current draw is minimal, but is sometimes just enough to power the K40 circuit, so while it won't be like a power mic it will usually work, just not so loud. The old radios had a relay for the ptt switching and that is what the K40 is designed for originally so would charge quite quickly, there is much more current going to ground on those.

More modern radios use electronic switching, so the voltage is less and the available current can be too little to charge the mic properly, symptoms like loads of delay after unkeying because charging times can be seconds or minutes (with a Ham Int bleep then weeks!) before the radio switches back to rx, plus low tx audio. The extra componants are to get around this and supply the mic with 12v. The diode is to stop the 12v going into the ptt switching circuit, which could cause issues on most radios, and still enable the ptt line to go to ground for switching to tx, the resistor (usually 1k ohm) is to leak some of the supply volts for charging the mic's internal capacitor for the processor circuit, the amount grounded is less than 14mA, so next to nowt really and nothing to worry about (if wired correctly of course). It only charges on receive and will discharge on a long over getting quieter 'til it fades out (been there done that, lol). Ordinary mics won't notice any difference so will be safe to use as well.

[Anon]

I'm guessing you're not an electronics person, that's OK, not everyone is. Luckily there's a forum with people that have experience of these things

Thanks for all the info,mines an original from 1980,it came with a radio I bought ages ago I wasn't really fussed with and long since sold.

I got my money back on the radio and got the mic for free
...
Its probably the best I have seen short of buying a mint boxed original that you see on ebay for stupid silly money

Well done! Mine must be worth a small fortune then, I have the boxes for all three

...
Mic lead has never been cut down its still really supple so thinking of feeding it with rubber conditioner?

where it enters the mic its gone a tad iffy and likewise where it enters the original plug but there's plenty of strait cable so ill cut it down to good cable
...

I've never heard of rubber conditioner so can't comment on it's use, I've never used any myself. The two mics I got that were bad were like it when I got them so I just replaced the wires, the mics worked alright afterwards though with no other mods except setting up as per factory specs, there's a service sheet available online (cbtricks?) that details it.
Replacing the wire at the mic end is the most difficult as the wire is glued into the strain relief but can be freed with some care and tiny jewellers screwdrivers to poke it free from the triangle bit and the metal retainer grip thing.

...
It worked fine on the radio it came on but it wasn't wired internally to power the mic so it just operated as a stock mic

I can get my head round the supply voltage wont ground to tx because of the diode

but cant get my head around why on tx it doesnt send 12v to ground when you press the ptt?

surely there is always 12v then on the tx pin at the pin itself?

You tx that 12v joins to mic ground and ultimately the radio ground as its after the diode

I guess that why i was also worried about using a standard mic
...

Radios normally have 9v (or less) on the ptt line which is grounded anyway when the ptt is pressed, the current draw is minimal, but is sometimes just enough to power the K40 circuit, so while it won't be like a power mic it will usually work, just not so loud. The old radios had a relay for the ptt switching and that is what the K40 is designed for originally so would charge quite quickly, there is much more current going to ground on those.

More modern radios use electronic switching, so the voltage is less and the available current can be too little to charge the mic properly, symptoms like loads of delay after unkeying because charging times can be seconds or minutes (with a Ham Int bleep then weeks!) before the radio switches back to rx, plus low tx audio. The extra componants are to get around this and supply the mic with 12v. The diode is to stop the 12v going into the ptt switching circuit, which could cause issues on most radios, and still enable the ptt line to go to ground for switching to tx, the resistor (usually 1k ohm) is to leak some of the supply volts for charging the mic's internal capacitor for the processor circuit, the amount grounded is less than 14mA, so next to nowt really and nothing to worry about (if wired correctly of course). It only charges on receive and will discharge on a long over getting quieter 'til it fades out (been there done that, lol). Ordinary mics won't notice any difference so will be safe to use as well.

Thanks again

Rubber conditioner is available at most auto stores i use auto glym but just because i already had it

Its to beautify tyres or the put the oil's back into window and door rubbers

Just out of curiosity do the cases match up well on yours?

I had a couple of broken ones back in the day and i remember them being the same as this one ie/the cases don't fit together too well

well i say don't,they do but its like the edges of the back case arnt flush with the front

This one needs the cable cutting back a little and re soldering but the "K40" logo is like new as is the writing on the rear above the screws

Its worth saving

[lambrettadave]

interesting subject i have a k40 but have never ever used it.

It came with a homebase uniden 300 so i would imagine it should be working ok with this set.

so my question is this. How long dose it take to charge the mike up bearing in mind that mine has been out of action for at least 15 years?

[Mitch]

...
Just out of curiosity do the cases match up well on yours?

I had a couple of broken ones back in the day and i remember them being the same as this one ie/the cases don't fit together too well

well i say don't,they do but its like the edges of the back case arnt flush with the front

This one needs the cable cutting back a little and re soldering but the "K40" logo is like new as is the writing on the rear above the screws

Its worth saving

The cases on mine fit together alright but there is a gap around them that I thought was supposed to be there, the writing has faded though, some worse than others but they are old. The magnets are still good too.

They're all worth saving now, being rare an' all, especially if they work

...
How long dose it take to charge the mike up bearing in mind that mine has been out of action for at least 15 years?

They'll usually charge within a few seconds, even from flat, it doesn't matter how long it's been as long as they aren't broken.

[lambrettadave]

Here is a foto of mine

[lambrettadave]

Look inside of mine

[UGLY_BOB]

One way round actually modding the radio, is to get one of those Sadelta echo boxes, and modify that. I added the diode to one I have, and that meant I could use the K40 on virtually any of my radios. You don't need to have the echo turned on for it to work, and it's just one item 'interfered with' rather than any of your rigs. Adaptor cables for Cybernet / Uniden etc. are readily made or bought.

Ugs

[PoisoN]

One way round actually modding the radio, is to get one of those Sadelta echo boxes, and modify that. I added the diode to one I have, and that meant I could use the K40 on virtually any of my radios. You don't need to have the echo turned on for it to work, and it's just one item 'interfered with' rather than any of your rigs. Adaptor cables for Cybernet / Uniden etc. are readily made or bought.

Ugs

Brains aswell as beauty why do they call you Ugly?

[Transwarp]

You only need to use a K40 kit (Diode & Resistor) if you do long overs (cap charge runs out quite quickly)

I've had my brand new K40 for a couple of years (it was bought sealed in it's box never used) and use it on my Cobra 29 LX EU with no mods done to the radio to accept it, great punchy audio (the EU's have low mod)

Back in the 80's I used K40's on Cybernet's & Unidens and found that if the deviation (mod) was set just right on the Cyber's K40's worked superb. On Unidens it depended on your voice pitch (my girlfriend sounded really sexy on her UNi 100 with a K40 as I recall) it was muffled or bassy with some, and flicking to hi mode on the mic sounded too compressed (Unidens had good audio on standard mic anyway)

No sure how compatible a K40 would be with today's modern CB's but it's a good partnership with my Cobby EU. (the old meets the new)

[Dubstar]

Is it possible to wire the diode inside the Mic plug so the K40 can be used on different radios in the same period?

[Mitch]

Is it possible to wire the diode inside the Mic plug so the K40 can be used on different radios in the same period?

You still need 12v to feed the resistor (1k) so the mic'll charge, the diode alone won't work. Just mod all your radios, it's not hard and the bits are pence each, I have.

[LeakyFeeder]

Didnt realise K40 mics were so sought after!

Got two..one requires a new lead and the other is like new.... Never use em tbh...