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Volting a Cobra 148 GTL final?

shockwave i think if my memory serves me right it was this one =look at the image



Shockwave said:
Thanks Willy. That's not the Radio Shack choke I use to install but, it probably works just as well. I agree with keeping the AMC intact since bypassing it can cause every stage from the mic preamp, to the output chip to flat top.

For the customer that was difficult to please without removing the AMC transistor, I found another way to modify the AMC circuit to prevent the AF stages from ever going into distortion while still allowing the full modulators capabilities. It is especially useful if there is real NPC installed.

The idea is to take the input line from the AMC circuit, off of the RF line and connect it to the output of the audio chip. That input to the AMC circuit was off a glass germanium diode but I did forget the number. Now the positive peaks can rise to the full potential of the modulator, regardless of the carrier setting. AMC will only trigger to prevent the modulator from flat topping. If you can prevent the negative peak from hitting cutoff, this will produce louder audio without noticeable distortion.

I'll also mention that when you "volt" the final with only 12 volts and no modulation, you may hit 20 watts of output. However, when you modulate that 12 volts, the peaks can easily hit over 40 watts PEP. When you double the modulated voltage to the final at the same output impedance, output PEP tends to quadruple what it was at 6 volts. The stock 1969 final, will not like this at all.
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dark star which one did you get they had two 270-030 or 270-030A i order one too on the transformer, you will solder 1 wire to the + side of the 1000uf cap and the wire from tp-7 get solder together. then the -- side of that cap goes to pin 9 of the audio chip
then the other wire from the transformer gets solder to the off/on switch to only have voltage when the radio is on i think i also added a diode have to check with shockwave on that part been a long time ago i'll post pics when building it and results by results i mean back in late 80's we would disable r131 but this time i want to try it with r131 intact

Dark Star said:
Well I picked one of these up from fleabay just in case I feel motivated to do this mod down the road....maybe with a Mosfet final to handle the extra.
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1iwilly said:
dark star which one did you get they had two 270-030 or 270-030A i order one too on the transformer, you will solder 1 wire to the + side of the 1000uf cap and the wire from tp-7 get solder together. then the -- side of that cap goes to pin 9 of the audio chip
then the other wire from the transformer gets solder to the off/on switch to only have voltage when the radio is on i think i also added a diode have to check with shockwave on that part been a long time ago i'll post pics when building it and results by results i mean back in late 80's we would disable r131 but this time i want to try it with r131 intact
Click to expand...
I ordered the 270-030 on ebay. Thank you for the info. I am going to attempt to install when it comes in the mail and may need further assistance if I get stuck. Pictures always are the best help. Like they say, a picture is worth a thousand words and that is true for me. Yes the only mod I've done to this radio on the transmit side is removing r131. The 148gtl is one of the only radios that I think benefits from removing the limiter as it does not affect sideband in any way and seems to improve the AM modulation in a positive way.
 
Disabling R131 or leaving it intact in this mod will have no effect either way, as the AMC circuit only reacts to the negative going peaks. The reason it loses it's punch is because it no longer has as fast of a negative going return from the positive peaks, and makes it sound muddy. The high frequency audio components are swamped out by the "fatness" of each syllable, as seen on an Oscilloscope pattern.
 
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modulation transformers from old 29LTD radios will work just fine for this mod and are easily found.

Shockwave, when you introduced me to this mod years ago you had me use a 29LTD transformer, and it worked well, i just can't remember why you told me to use one.

Can you remember?

i saw about 30-35 watts PEP after the mod.
the 1969 gets hot.
sounded great.

now, combine this mod with Exit 13's PPE mod for this chassis and you'll have a freaking monster!

I sure wish there was a bipolar TO-220 transistor that could handle this increased current...
(BCE would be nice too LOL)

really though, at those power levels, i figure you are getting toward the limits of that physical size of transistor and a larger package is required.
LC
 
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Dark Star said:
I did try the mosfet 520 upgrade but didn't see much benefit to it so went back to stock 1969. I'm lucky enough to have two older genuine Motorola 1969s that I've been hoarding for years just in case.
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I meant of course Mitsubishi 1969s lol
 
loose-cannon20210503_185505.jpg 20210503_185522.jpg we always use the radio shack transformer later on shockwave said you can use a cobra 29 transformer but i wasn't happy with its performance like the rs transf mine cam in today will try this weekend to re-test again i can't remember if the diff is in the winding also at times we use a 1000uf instead of the 2200uf


loosecannon said:
modulation transformers from old 29LTD radios will work just fine for this mod and are easily found.

Shockwave, when you introduced me to this mod years ago you had me use a 29LTD transformer, and it worked well, i just can't remember why you told me to use one.

Can you remember?

i saw about 30-35 watts PEP after the mod.
the 1969 gets hot.
sounded great.

now, combine this mod with Exit 13's PPE mod for this chassis and you'll have a freaking monster!

I sure wish there was a bipolar TO-220 transistor that could handle this increased current...
(BCE would be nice too LOL)

really though, at those power levels, i figure you are getting toward the limits of that physical size of transistor and a larger package is required.
LC
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specs for that RS transformer in case people can't find one:

upload_2021-5-4_16-22-59.png

LC


Also, can anyone give me a short explanation of the difference between "grain oriented" and non-oriented" steel as it applies to transformer construction?
 
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Ever play with stainless steel, it's a got a LOT of different alloys - and also when made, annealed - the pressure and heat used to make the sheeting or form, has minor levels of crystalline structure.


So the "grain" refers to the crystalline structure of the alloys working with the carbon, nickel and iron together which forms those crystalline structures they align to form the grain you see as the sheen of polarized reflected light off of the structures and the bonds they form.
 
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Andy i gotta tell you.
that was pretty darn short! LOL
thank you. i'll look into it more.

apparently it's hell to machine.
anyone with a Haas want to go into the transformer making business?
LC
 

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