MOSFET's and the 148GTL

[loosecannon]

L40 is a resistor with a ferrite bead on one lead.

i cant remember where i heard it, but i think i remember someone saying that with a MOSFET, the choke is not needed for some reason.

there is a chance that i am not remembering this correctly.LOL
LC

 

[loosecannon]

well, i cant say that the MOSFET experimentation is going badly, but its not going great either. LOL

i have been playing with the value of the resistance from gate to source in the "companion part", and paralleling different caps across C152.

i am not playing with biasing yet, im only working with the AM mode right now.

the best ive gotten is about 18 watts PEP from a 4 watt carrier.

my findings are showing that when the deadkey is increased, the PEP output suffers, and vice versa.

i sure hope this is not a power supply issue as it would really confuse things.

i feel like pioneer can take this further than i can. i can play with the values and see what happens, but as for the "why" to do something or not; i am not as versed as he or shockwave.

shockwave, i think its time for you to find an old 148 and start experimenting! LOL

i am willing to try anything that is suggested, but as of now, im about out of ideas.
the MOSFET mod works, and will yield the same output as a stock radio, but i am volting the final here using the transformer and i should be seeing much higher numbers.

i think its time for me to step back for a few days and see what other ideas come up.
i am still very interested in this, and will keep up on posts daily.
if anyone has something they want me to try, just post it!

thanks for all the info so far. we need more threads like this!
LC

 

[Robb]

On the MOSFET mod page for the Cobra 29, they removed a choke coil that was in the final circuit. Maybe this needs the same. I remember reading something about the fact that the MOSFET requires current - and a bipolar device doesn't.
??? ??? ???

 

[Shockwave]

I was told the IRF520 was not as good as the ERF2030. It might be a good idea to order the single final kit from CB tricks and start from there. I wish I had a test bench still set up. I closed my shop years ago and design commercial broadcast antennas now. I use to subcontract repairs for Lentini Communications. Sometimes I miss the fun of experimenting with things like this but I never want to try and make a living off repairs again.

Being that I was curious how the FET would work in the 148 I thought I would motivate someone out here to try it by sharing the 12 volt modulator modification. If anything is going to make a 148 take off on AM, the modulator does it. Problem may be the choice of FET being used.

The ERF2030 may be better but I have no way of checking since there doesn't seem to be a datasheet for the damn thing online. My problem with the IRF520 is it's datasheet talks about high speed switching with test data at 1 Mhz and says nothing about RF use. It probably has losses that make it difficult to drive at 27 MHz.

Generally speaking, changing the final to a "better" one doesn't make a big change in power output. Higher output requires more drive or more gain within the stage. Both of these usually mean retuning of the input / output circuits because of the change in impedance. More power means more current and that means a change in impedance.

I wonder how hot the final is getting. If raising the carrier reduces the PEP there may be a thermal issue due to mismatch or running the transistor beyond it's frequency limitations. I use to like the MRF477 because it had several more db gain then any other similar RF transistor. It's also out of production now. Wish I could be of more help in the hands on part of the FET mod but I'm limited to sharing what I know about the circuit for now.

 

[loosecannon]

if you want to see the IRF520 in cb circuits, look at the new galaxy radios.

they have switched over to using the IRF520 in all their new radios.

the galaxy 959 i had, which has a similar output circuit (somewhat) to the 148GTL, i could get about 30-35 watts PEP just by giving the radio a tune up and turning up a few pots.

anyway, i will keep on keeping on, but progress will be slow.
later,
LC

 

[Shockwave]

if you want to see the IRF520 in cb circuits, look at the new galaxy radios.

they have switched over to using the IRF520 in all their new radios.

the galaxy 959 i had, which has a similar output circuit (somewhat) to the 148GTL, i could get about 30-35 watts PEP just by giving the radio a tune up and turning up a few pots.

anyway, i will keep on keeping on, but progress will be slow.
later,
LC

That lends some credibility to using the IRF520 for a CB final then. It does have an impressive power dissipation.

 

[Pioneer621]

Loosecannon,

This will likely continue to be a slow project for me as well. I started playing with the MOSFET in the radio probably two years ago & Shockwaves thread kind of got me playing with it again, but progress will probably continue off & on as I get time. I too will try to keep up on this topic as maybe someone else will give the MOSFET a go in this chassis.

As I play around with mine when I can, I will post any updates as I move forward, but since it’s already been a long project for me, it may be slow. LoL

Shockwave,

I think your right on biasing, & the method I was using is not the greatest & should be improved upon, it is a quick way however to get the SSB a little cleaner. Like you say, modifying the existing biasing circuit would probably yield best results & retain adjustability, probably would not be too difficult to get setup either.

Again, this will likely continue to be a slow mod for me & I will work on it as I get time to do so, & when I do I will probably tinker around with the biasing as well & try modifying the existing bias circuit to accommodate the MOSFET.

I enjoy working on this stuff, so it has already been a lot of fun!

I’ll be curious to see if anyone else gives it a shot.

 

[MrExtreme]

Transformer specs

Old thread, good info. Does anyone know the specs on the transformer you are using for this mod?

Thanks Lee

 

[Chipper]

Has anyone figured a good mod to put mosfet in 148’s as drive and final just for fun I know u can still get the bipolar but just wondered for the old 5 pin mic jack radios

 

[loosecannon]

Yes. search the forum for the keywords in your thread title and search for posts by a member called exit13.
He came up with these mods a few years back.
LC

 

[Chipper]

Ok thank

Yes. search the forum for the keywords in your thread title and search for posts by a member called exit13.
He came up with these mods a few years back.
LC

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[Shadetree Mechanic]

Good read, interesting about the 29 transformer in the 148.

 

[tecnicoloco]

just use one out of a Cobra 29 LTD,GTL and variants

 

[Pioneer621]

I just clicked on this thread to read it as I do most other threads, I forgot I had posted in it! I guess 10 years went by rather fast.

I haven't tinkered with radio stuff in quite some years now, local radio pretty much died off, and I stopped fiddling. I do miss playing with radio gear, had a lot of fun over the years.

I'm in the process of moving and seen my old Grant XL recently, not sure where I left off on it or what was last done to it as I haven't used it in a long time, but I do still have it.

 

[1iwilly]

hello to all trying to revive an old thread i just pick up an old 148 gtl made in Taiwan i thought to give this a tried again i don't have an old 29 transformer so I'm using one from my realistic trc-427=TF129 dead key 2 watts swings bird rms to 9 watts on am ssb just shy of 12 watts look at pics and vid on one of the images it shows the pins to use p3=positive side of 2200uf electrolytic cap p1=green wire going to final from tp-7 add a 1n4003 diode cathode towards final p2=12 volts from off and on switch on the pic showing the connection i had to rearrange
cap to the single tap and red wire to where the cap was otherwise the audio is low hope this helps others who want to experiment