Shortening L10 on Cobra 25 LTD Classic...

[LeapFrog]

I'm repairing a Cobra 25 LTD that has been modded in the worst kind of way, it's a new-to-me ebay purchase. I am trying to get it respectable and close to it's factory setup once again.
I found the final transistor was blown in half when I took the lids off!

I replaced the following:

*driver (original tested ok but I changed it for one that I had on the bench)
*final (Not w/ a mosfet)
*D9
I then reformed the two spread coils the best I could, L20 is missing a slug at this point so my readings may be way off, going to fix that next. (tonight or way eeearly tomorrow)


The carrier was low (2 watts) so I added a 68pF capacitor across C46 & removed one turn off the L10 coil. This Radio now transmits a 3 watt carrier and under steady modulation it will reach almost 4 watts average power. This radio was made in the Philippines, & after reading some more [on the internet] I see the recommendation is to reduce the number of turns on L10 from 5 to 4 wraps for "better tuning".
(usually done on China models)..

Well after I had removed a turn I came to the realization that I may have only have 3 turns on L10!
[ 4 on one side and 3 on the other, it really depends what angle you look at the radio from.]

*What does shortening the coil on L10 do to the final power amplifier circuit, & will it effect bias current (or bias voltage) for the final transistor?
I have not measured bias current.

Should I locate another coil for L10 and replace the modified one, or do I even stand any risk of blowing the final transistor with this configuration?

*What is considered "Low Output" for an early nineties (factory form) Cobra 25 LTD?
Was a 2 watt carrier acceptable or even common from the factory back in those days while the radio was new? How about today?
I have never bought a new Cobra radio yet, that's why I'm asking what would have been the expected performance level for these back in the day.

* Is an "FCC 4 Watts" 4 peak watts or a 4 watt Average output?

I am going to have to use an SDR to set L20 when I transplant it.
I have a scrap Uniden 68 I am going to pull the whole inductor from instead of just using the slug.
edit #2: I tried putting just a slug in there instead of transplanting the entire inductor it might just be me but it looks like I'm getting a ever so slight increase in power output i'm kind of baffled by that.
Does this particular inductor (L20) always contain a ferrite slug for tuning purposes?

Sorry for the long winded post..

Thank You Gentlemen for any input you may have regarding some of my many questions asked here.
The main question I'm worried about is in red.
-Sam

 

[loosecannon]

ok, L20 is the 54mhz trap and should have a slug in it. It's job is to clean up the RF going out of the radio and reduce any 2nd order harmonics to an acceptable level.

you really need a spectrum analyzer to align this coil, but you can use an old TV tuned to channel 2 and tune for minimum interference.

there is no point in replacing the coil, as the new coil won't be tuned for your radio anyway. you can just add the slug from the 68, but you are still going to have to re-tune it.

DO NOT try to adjust this coil using your wattmeter, thinking that if you peak the power output, you will automatically reduce the 54mhz content.
Wattmeters are dumb, and will add the 27 mhz and 54mhz components together giving you a false reading.

your power output does seem a bit low, but im not sure at this moment what to tell you to try. first thing is to make sure that you are peaking the TX coils using a modulated tone, and not tuning for max deadkey.

as for the turns count on L10, the number of turns is not subjective due to which way you look at it. there is a right way to count turns on an inductor, and you can find that info online.

L10's job is to match the output impedance of the final to the input impedance of the half wave filter.
since each transistor and its associated components are just a bit different in actual value, they make this coil adjustable.

if you still notice the power "peak" at a certain spot, and will dip if you turn past that spot, then you are just fine with the turn you removed.

if you seem to run out of tuning range and the power is still moving up, then you might want to replace that coil.

hope this helps,
LC

 

[LeapFrog]

Excellent LC!

L10's job is to match the output impedance of the final to the input impedance of the half wave filter.

I better make sure that the half-wave filter is in proper shape before I proceed.

I moved the slug from the 68 to L20, I did not "tune" it at all, just "screwed" it in to a similar distance I found in the other 25 LTD I have, and I decided to key the radio and observe any change in output, I was assuming it was going to be much lower with the 54Mhz Trap in semi-operable condition.
I am going to have to find someone with a spectrum analyzer to help me get the radio proper.

I may play with the Ch. 2 trick just for the experience. That's definitely an oldschool technique, I never had the chance learn to adjust a TVI trap, because back in those days I wasn't even thought of yet LOL.

Once again thank you Loose Cannon!

-Sam aka Leap