MOSFET's and the 148GTL

[Shockwave]

Back when I repaired CB radios the best set of finals was the 2SC1306 driver and the 2SC1307 final by NEC. Apparently a lot has changed since then with small bipolar RF transistors no longer being manufactured for CB's. I've heard many wild claims about how they make crazy power in the 25LTD and 29LTD and that it doesn't do as much in the 148GTL.

I'm curious what the difference in output power really is compared to the 2SC1969 and 2SC2312 and what mosfet has the best potential? When properly matched this could be a big jump in power. Back in the 1980's I discovered why the 148GTL doesn't make as much power on AM as some other models and will share the cure in the hopes someone will correctly apply it to the mosfets and report back.

In the 148GTL the final and driver are only supplied modulated 6 volts on AM. When you go to SSB it changes to unmodulated 12 volts. The key is you have to get just the final running on modulated 12 volts in the AM mode. The trick is simple. I forget what wire feeds voltage to the final but it plugs into the board right next the one that feeds the driver. I think it was purple. Unplug the final wire from the pin on the board.

Since in AM the audio output from the 5 watt audio chip is only used to drive the series pass modulator, it's output is buffered and allows us to make use of it's output to modulate a new 12 volt line for the final only. There are two ways this can be done, use a modulation transformer or just a choke.

If you use the modulation transformer from a 29LTD you feed the primary right off the output terminal of the TA7222AP audio chip through a DC blocking electrolytic (2200 uf) with the other end of the primary tied to ground. The secondary gets 12 volts on one side and the final wire to the other. If you phase the transformer wrong you're audio will be very low and you just reverse one set of wires. In SSB the radio does not have audio on the audio chip so the modification requires no switching between modes. Basically copy the 29LTD.

The choke method simply apples 12 volts through a DC choke that will decouple the power supply from the audio line. The output of the audio chip goes trough the same DC blocking electrolytic and tied to the output of the 12 volt choke and the final wire. I think the transformer works better as it provided more RF isolation between stages.

I think I've posed this information here before but not in as much detail. Maybe someone will try this and see if it works as well with the new mosfets? If someone is interested I'll be happy to provide more details on the modulated 12 volts final power modification for the 148GTL. I'm confident these radios have more potential then the AM only models with this modification employed.

 

[Robb]

http://www.cbtricks.com/miscellaneous/fet_papers/ekl/graphics/erf2030_148gtl_mod.pdf

 

[Robb]

http://www.cbtricks.com/miscellaneous/fet_papers/ekl/graphics/erf2030_148gtl_mod.pdf

California cb radios plus, Pacific CB Radio Sales, http://www.pacific.us.fm

These mods are for the newer Cobra 148GTL w/4 pin front mic plug radios.

Left: single final ERF 2030 kit @ California CB Radios Plus
Right: Dual final kit from the same

 

[Shockwave]

Thanks Robb. That information should make it a lot easier. Almost wish I kept some of my old 148GTL's to test the idea of combining my modulated 12 volt modification with the new mosfet. Changing the final voltage from 6 to 12 volts in the 148 makes a huge difference in output with the old final. If the new mosfet can perform better that would just be the icing on the cake.

Can I convince anyone here with technical experience to try this on a 148GTL with support? I would leave the driver stage alone and just focus on the final. Stepping it up to modulated 12 volts is going to work the ERF2030 hard enough. All you need is the single final mosfet kit, modulation transformer out of just about any AM CB, and a coupling capacitor between 470 and 2200 uf.

 

[15minigrass]

Hey rob, will those work in a 146GTL??

 

[Robb]

Hey rob, will those work in a 146GTL??

EKL is the mfr of the ERF 2030 MOSFET. EKL is linked to the same company that mfr's 'Magnum' radios.

I don't think these kits will work/install the same in the 146 as it will in the 148 - but then again they might. Loosecannon or some other members might know best the answer to that question.

Maybe loosecannon might be able to better answer your question if he reads and answers this thread.

 

[loosecannon]

Great Thread Shockwave!

this is something i have wanted to discuss for a while, it just seems that no one with the ability to make it work was ever interested.

first of all, thanks so much for posting that mod using a transformer from an AM cb to modulate the final in the old 148gtl type chassis.

i have heard of it before, but never knew how to do it.

one question though, wouldnt you want to run an anti-spiking diode in series with the line to the final transistor?

all the AM radios do this to keep the "kickback" from damaging the windings on the transformer.

second, i would love to try this mod out on my old school 148gtl, but its all taken apart in prep for a major facelift (148gtl inside, galaxy 959 style face)

if no one else steps up to try it, i will try to get it back together enough to do it. it will take me a week or two to get it done though.

as for the MOSFET conversion on the OLD style 148, with the 5 pin mic connector on the side; if you were to work it out, you would be the first one to post it anywhere on the internet as far as i know.

all the MOSFET mods i have seen for the 148GTL refer to the new 4 pin front mounted mic style, and they are complete pieces of crap.
i barely trust one to do 4 watts carrier, much less anything more.

anyway, this has been discussed MANY times, but no one that i know of has sat down and done it for the old style 148's. i would help in any way i could to make that happen.

again, great thread, i cant wait to see where this goes.
thanks.

15minigrass,

the 146GTL is a completely different animal than the 148GTL chassis.
as far as i know, no one has created a MOSFET mod for it, but that doesnt mean it cant be done!


LC

 

[Shockwave]

The diode to the best of my knowledge doesn't protect the modulation transformer here, I think it protects the final. To be honest, I put the 12 volt modulator in well over 100 radios from 1986 to 1996 and left that part out without any problems in the old radios. The mosfet should be even more resistant to voltage spikes. Because of the limited supply of modulation transformers I mostly used the choke method but as I said before the transformer yields better RF isolation.

If the ERF2030 can work in one radio as a 27 MHz power amp, it can work in all CB's. It will just require a little more work if the radio does not have enough range in bias or impedance adjustment to accommodate the mosfet without further modification. That's just changing the value of a resistor for bias and a capacitor or inductor for impedance. Is the final power amplifier that much different from an old 148GTL to the new 4 pin models?

 

[seedkey]

you got my interest Shockwave.
Can you post more information on how this is done?
The modulation transformer has 5 connections ? I think 2 connections go to the speaker in the cobra 29.
can you explain in a bit more detail?

 

[loosecannon]

i doubt they are much different, i just havent been willing to experiment with it on the one working 148 i have.

i would love to see a schematic for the new 4 pin 148's, but as of yet, i have not heard of anyone posting one online.

if anyone out there has one, im sure Bennie at CBtricks would like to have a copy.
or you could just post it here i guess.

shockwave, what kind of PEP numbers were these 148's doing after this mod?

LC

 

[Shockwave]

As it turns out it's been 20 years since I looked at the schematics of the 29LTD and 148GTL and after looking at them I remembered how I came up with the idea to use the choke. The 29LTD does not use the modulation transformer with isolated primary and secondary. The primary does all of the modulation while the secondary just drives the speaker in receive and PA. The primary is being used as a center taped auto transformer.

By applying audio drive to the center tap, it provides a high impedance path to the audio chip from the low impedance DC feed on one end and the RF from the final on the other end. The choke does not isolate RF from getting back to the audio chip and can require more bypassing of the chip to work right in some radios. They typically could swing 40 watts PEP on the Bird 43P but that was dependent on the power supply and believe it or not every radio was a little different. Don't try it on a base or you'll need to upgrade the power supply too.

I can't seem to upload pictures here so I'll take the time to outline the 29LTD transformer mod in the 148GTL as a means of obtaining modulated 12 volts for the final. This is probably better then the choke and uses easily found parts from scrap radios. We'll even include the diode.

Start with pin 9 on the audio chip in the 148GTL. Come off of pin 9 into the negative side of a 470 uf cap. Larger caps will produce more bass response but can distort the audio. 1000 uf is a happy medium if you like bass. From the positive side of this cap, connect to the center tap of a 29LTD (or similar radio) audio transformer. We are only using the primary center tap side of the transformer with 3 wires on it. Nothing connects to the two wire side but make sure the wires don't short.

Take one of the end wires on the transformer and solder it to the 12 volt switched side of the power switch. The other end of the transformer goes into the anode of a 1N4003 diode. The cathode side of the diode is bypassed with a .01 uf capacitor to ground. The cathode is the 12 volt modulated output that will feed the final. Unplug the purple wire from TP7 that feeds the stock modulator voltage to L39. Connect the purple wire to the cathode of the 1N4003 diode.

You have a 50% chance of being done now. Ever put a distributor in a motor backwards? The same thing is possible with the transformer. If your audio and PEP AM power are lower then stock, the transformer is 180 degrees out of phase with the modulation arriving at the driver stage. Simply reverse both end wires on the on the 3 wire side of the transformer to correct this. If you get stuck, look up the schematic for the 29LTD, it's almost a direct copy.

 

[loosecannon]

that rocks!
cant wait to try it.

just to make sure, i thought i saw a typo. which side of the electrolytic cap goes to the audio chip?

instead of the switched side of the power switch, how about upgrading the main filter cap from 1000uF to about 2200uF and drawing power from the positive lead?

also, on the three wire side of the transformer, you are saying to use the two outer wires and not the center one, right?

just want to make sure its clear.
thanks again,
LC

 

[Shockwave]

The negative side of the electrolytic goes to pin 9 and the positive side goes to the center tap on the transformer. This is because the output of the audio chip has about 6 volts on it and the transformer has 12 making it more positive. I did make a typo. You could upgrade the main power cap and draw power off it as you suggested. Might improve the peak output. The final won't draw any current with the radio off and no bias or drive. With respect to the transformer wires, you use all 3 on one side, and neither of the two on the other side. The center wire is where you feed audio in trough the cap. One end is where you apply 12 volts. The other end feeds modulated 12 volts to the final trough the diode.

 

[Pioneer621]

Hello all, definitely an interesting topic!

I don’t have a 148, but I have a GRANT XL I have fiddled with in the past, never really toyed with transformer idea though & it is interesting. In mine I just put a MOSFET final in changed a couple parts etc. & wired the final to run on 12 volts, never had a problem with it & get around 30 watts PEAK (PEP). The audio transformer is an interesting idea as you will be modulating the collector or drain of the final, unlike how mine is wired where the final is only modulated by what the driver sends to its base or gate, however, I always get compliments on that radio, so it works either way, but I do enjoy playing with stuff and trying new things if for no other reason then to experiment!

Shockwave noted to use the pins of the transformer, the 3 on one side. Not saying it is a typo, but it may not apply to all audio transformers.

Example:

The pin-out of the Audio Transformer in the Cobra 29 LTD (since the 29 transformer was already referenced I will use that one) is 2 of the Primary pins on one side (the side with 3) & 1 of the Primary pins on the other (the side with 2).

When hooking up the transformer you feed the audio through the 470uf cap to the center tap of the primary (the left pin when looking at the transformer from the bottom & the side with 3 pins towards you), then the 13.8vDC to the primary pin directly next to that pin (the middle pin), then you connect the 1N4003 diodes anode to the last Primary pin diagonally across from the center tap pin, then the purple TP8 wire to the cathode of the 1N4003.

I will include a rough drawing of how it is wired / connected in the Cobra 29 LTD which is basically how you will be hooking it up in the 148. My drawing is based on a Cobra 29 LTD Audio Transformer as that’s what I pulled it out of to make the diagram (no I did not destroy a perfectly good 29, someone already cannibalized it & took some major parts out).

Here is the diagram of the basic wiring configuration: (I hope I uploaded the picture ok)

 

[Robb]

It would be very interesting to know how you put a MOSFET in the earlier Cobra 148GTL and Grant XL/5 pin radios. I have both of those radios and would be interested in what you did to make the transition from a bipolar to a MOSFET. It is not so much the increased power output and PEP; but another means to restore these fine older radios if need be to the more available MOSFET's. Parts needed list, parts removed list, locations, abd other pertinent notes. There are no places on the web that I have found such info, as it would be a first on this forum to my knowledge.

Please start another thread - or put it on this one if you like.
I appreciate that info; as I am sure others will do also!