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Cobra 29 modulation adjustment.

Robb said:
Right.
The second that coil is mashed, it is putting out all of the 54mhz harmonics, and its false watt readings are added to the output watts reading.
Click to expand...
Oh yeah, radios can be heard in the 54Mhz area if you are close enough to a spread slinky coiled radio, oh and on channel 2 with an older tv. :whistle:

The radio does put out more power, just not at the intended frequency.
(When you disable the 54 Mhz trap)
 
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805 california said:
L12 and that's not necessarily true
Click to expand...
I don't believe L12 is mentioned in the factory service manual as an adjustment point.

Other members have mentioned that this coil along with one other and a few capacitors form a "half wave filter", I cannot confirm this because I cannot simply look at the schematic and tell you what is happening there.

I still stand by my statement: It's best to have access to a spectrum analyzer before modifying any pre-made coils inside of a radio.

Many hack sites have (incorrect) information that would create a harmonic splattering distorted "LOUD" radio that will show plenty of swing on a meter, shame is you have to filter through a lot of garbage on the internet to find a few gemstones. (Mods that work in a productive way)

Anytime I find a website with radio mods that mentions spreading coils, I immediately regard the content as untrustworthy, I'm sure in the past there may have been one or two radios created where spreading a coil would actually help, as far as I understand this goes back to the seventies and eighties with Peak n' Tunes back then they didn't care about a dirty signal, and just wanted to show people some impressive swing.

But I don't know what modifying that particular coil will do for you, I would say it's safe to assume any increase in power is ghost watts.
It would take a little testing with a fully equipped bench to get a simple answer, I don't have a spectrum analyzer so I cannot test this. I leave the coils alone as a rule-of-thumb.


Best Regards.
 
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805 california said:
I don't believe L12 is mentioned in the factory service manual as an adjustment point.

Other members have mentioned that this coil along with one other and a few capacitors form a "half wave filter", I cannot confirm this because I cannot simply look at the schematic and tell you what is happening there.

I still stand by my statement: It's best to have access to a spectrum analyzer before modifying any pre-made coils inside of a radio.

Many hack sites have (incorrect) information that would create a harmonic splattering distorted "LOUD" radio that will show plenty of swing on a meter, shame is you have to filter through a lot of garbage on the internet to find a few gemstones. (Mods that work in a productive way)

Anytime I find a website with radio mods that mentions spreading coils, I immediately regard the content as untrustworthy, I'm sure in the past there may have been one or two radios created where spreading a coil would actually help, as far as I understand this goes back to the seventies and eighties with Peak n' Tunes back then people didn't care about a dirty signal, and just wanted to show people some impressive swing.

But I don't know what modifying that particular coil will do for you, I would say it's safe to assume any increase in power is ghost watts.
It would take a little testing with a fully equiped bench to get a simple answer, I don't have a spectrum analyzer so I cannot test this. I leave the coils alone as a rule-of-thumb.


Best Regards.
Click to expand...
Well respected cb shops like screwdriver always tweaks L12.
 
805 california said:
L12 and that's not necessarily true
Click to expand...
View attachment 19972

Hmmm . . . I don't think you are right.
C57 (hard to read on schematic above), L12, C58, and L13 form a filter circuit. Disabling any part of that circuit would nullify its ability to filter any 54mhz signal present. That circuit does not affect 27mhz, so why disable it?

Furthermore, the TR14 final transistor (2SC2078) is rated at only 10w/max. Any more than that can be read as being made up of harmonics and distortion.

Like to hear from Xit13, bob85, or Nomad Radio/others - to weigh in on this - to be certain.

Sorry; don't buy what you are selling here . . .

Capture.PNG
 
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Robb said:
View attachment 19972

Hmmm . . . I don't think you are right.
C57 (hard to read on schematic above), L12, C58, and L13 form a filter circuit. Disabling any part of that circuit would nullify its ability to filter any 54mhz signal present. That circuit does not affect 27mhz, so why disable it?

Furthermore, the TR14 final transistor (2SC2078) is rated at only 10w/max. Any more than that can be read as being made up of harmonics and distortion.

Like to hear from Xit13, bob85,or Nomad radio to weigh in on this - to be certain.

Sorry; don't buy what you are selling here . . .
Click to expand...
I've had nothing but good reports and screwdriver tweaks every single cobra coil he sells. May but not be necessary everytime but it's obviously not hurting the performance of the radio. If it did hurt the radio, make it look bad on a scope and make it sound like hell.... why would screwdriver spread every single cobra radio he sells???
 
'Good reports' don't mean anything.
The harmonics coming out of the radio aren't even on 27mhz. They are 54mhz. Wastes power and heats up the final and helps it go into failure. Will also contribute to a poor SWR out of the radio as well. Not good . . .

I'll admit that I haven't worked on many 29's. But the ones I did work on did far better than 60% modulation and a nasty output w/ NO removed/slinkied components.
 
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805 california said:
I've had nothing but good reports and screwdriver tweaks every single cobra coil he sells. May but not be necessary everytime but it's obviously not hurting the performance of the radio. If it did hurt the radio, make it look bad on a scope and make it sound like hell.... why would screwdriver spread every single cobra radio he sells???
Click to expand...
Keep in mind screwdriver was top of his class with hodors in
Robb said:
'Good reports' don't mean anything.
The harmonics coming out of the radio aren't even on 27mhz. They are 54mhz. Wastes power and heats up the final and helps it go into failure. Will also contribute of a poor SWR out of the radio as well. Not good . . .

I'll admit that I haven't worked on many 29's. But the ones I did work on did far better than 60% modulation and a nasty output w/ NO removed/slinkied components.
Click to expand...
All I'm saying is every radio I've gotten from a "pro" had the coil spread on a cobra. Keep in mind this guy graduated with honors in military communication. So just some food for thought. 73s
 
Robb said:
I'll admit that I haven't worked on many 29's. But the ones I did work on did far better than 60% modulation and a nasty output w/ NO removed/slinkied components.
Click to expand...
I think 805 California means to say he turned the VR to 60% of max, whatever direction that is on his model.

Not that he set it for 60% modulation, and I agree with you it looks like an "LC" filter network @ c57, c58, l12, l13.
 
Quick and dirty test for "ghost watts". Get a low pass filter and a dummy load, take watt meter readings with and without the low pass filter in line and compare. A large difference between the readings would be a sign of a potential problem.
 
The engineers that designed these rigs and write the service manuals have a technical background too...just saying.
 
I've had a used hacked up radio from a tech with the mod limiter removed and other mods and it was loud as heck on the air and all the CB guys thought it sounded badass.

If you put that radio on a scope it would have been all over the place.

Although it sounds good, it won't be putting out a clean signal - but that being said you may not care.

If you want a clean signal that isn't splattering then don't touch the coils - any tech that touches the coils is ruining the filter system and isn't a real tech...they are a CB mod specialist who charges you money to do things to the radio so it sounds "loud" on the air.

If you're happy with the result then no harm, no foul. If you're bleeding through your neighbors speakers and TV then at least you'll know why :)

The best rule of thumb is - don't touch the coils. I've messed with 100's of radios that sounded great on the air and I never touched the coils.
 
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