MOSFET's and the 148GTL

[Shockwave]

The picture does help. As I recall you are correct and I made another typo. Those audio transformers are often wired with a pin out that does not put the primary on one side and the secondary on the other. Looking at how it's installed in the 29LTD would confirm that.

While the radio will work with unmodulated 12 volts feeding the final on AM it's harder to modulate the radio fully with only the driver being modulated. You'll notice about 25% more peak power on AM with that 12 volts modulated.

 

[Pioneer621]

Robb,

It has been some time now since I installed the MOSFET in the GRANT XL, so I will have to go over it thoroughly to see what all I did, I think it was very similar to the MOSFET mod on the newer 148 or the ones that have the ERF2030 install sheet. I don’t remember off the top of my head exactly how I did mine though, probably very similar. I know I switched just the final though.

The MOSFET installation is actually very similar between radios. I have installed them in a few different radios & have always been able to get at least the same output power of the stock final. They are indeed more readily available & will help restore radios when the older transistors become harder to get & the price skyrockets.

When I get a chance I will get my GRANT XL off the shelf & try to make an install sheet with what all I did; parts list, what needs removed, replaced, etc. I may not get to it right away, but I will definitely put it together incase anyone needs to switch over to a MOSFET final. I am surprised that mod info for those older radios is not on the web somewhere by now.


Shockwave,

There are so many different radios & parts that’s why I mentioned not necessarily a typo, I know I can’t keep track of all of it, & there are probably some used like you described, I just wanted to clarify it for the 29 transformer.

I’ll have to play around with the transformer in my GRANT XL when I get a chance, I’ll be curious to see how it performs once the final is modulated with the 12 volts. Looks like it will work fine & should not be difficult to wire up, especially since I have the transformer already to go pretty much, as well as the other parts needed.

 

[loosecannon]

thanks pioneer and shockwave for sticking to this thread.

i too am surprised that the MOSFET mod for the old style MB8719 chassis is not on the net.

i am going to put my 148gtl back together starting this week so i can follow along and provide input about how it all actually works.

first i will convert the final to a MOSFET, and will measure the current draw with AM modulation, along with a PEP reading on my wattmeter.

then i will add the transformer modulated voltage to the final and do both measurements again.

my radio will make a good test subject for this, as i have replaced all the electrolytic caps with new ones, making it basically a new radio.
it is an old taiwan model, but with new caps, they are my all time favorite era.

since we are still working out bugs and developing the mod; someone should soon make a post with all the corrections and updates in it.
that way when people come across this thread, they will be able to read the correct mod all in one place.

best thread in months IMO,
LC

 

[loosecannon]

ok all,

the 148GTL is back to stock and ready for testing!!!

i have taken out any audio mods that were done to it, just turned up VR7 and thats it.

the driver is a 2166 and the final is a 1969 currently.

the radio deadkeys 4 watts and swings to about 15-16 watts PEP on AM.

the radio is drawing about 1.5 amps of current when modulated.
i am diagnosing a possible problem with my power supply right now, so it might draw just a bit more than that.

so, does anyone have a problem with the way this mod has been layed out?

i am going to try it using a transformer from a 1990's 29LTD, using pioneer's layout.

i do have a couple of questions that i hope someone will answer tonight if at all possible.

does the shell of the transformer have to be mounted to PC board ground?
i was thinking of mounting it to the back wall of the radio as it seems like the only place there is room.

shockwave, maybe you can provide some input on this as you've done the mod before.

next, what should i set the driver and final bias to after doing this mod?

thanks for any guidance, and someone GET TO WORK! on the mosfet mod for the old school 148gtl's.
if you come up with a mod, i will try it.
i will not do the ERF version on cbtricks, as i have read on the net that it doesnt work too well with the old 148's.
LC

 

[loosecannon]

ok, more questions have popped up.
i answered the one about needing to mount it to PC board ground myself. DUH lol. (the answer is NO)

i noticed that in the 29LTD, the negative side of the electrolytic is going to pin 9, but in the 148gtl, the POSITIVE of C175 is going toward pin 9.

is this due to the differences in the circuits?
a little clarity here would be helpful.

sure hope i get to move on with this tomorrow!
LC

 

[Shockwave]

You can mount the transformer to the back wall. I use to mount them on the left wall towards the rear. Grounding the core to the case of the radio is fine.

Bias on the other hand is an interesting question. I never had any problems with leaving the stock bias settings. This modification will not change the bias on SSB but it will bias the final up on AM as if it were on SSB. Some benefits may be seen if the bias could be removed or reduced on AM. It is more difficult to modulate a biased stage then a class C stage.

Since we are leaving the driver stage alone and it is still biased class C on AM, most of the modulation is occurring in the driver stage. Because of this the final can be biased and the modulated 12 volts feeding it will reinforce the modulation and have no problem achieving 100% plus modulation even without removing the bias on AM.

It would be interesting if after the modification you could take PEP AM power measurements and then reduce the final bias potentiometer to reduce the bias. See if modulation peaks are higher with reduced bias on AM. If they are you could wire a relay in to remove the bias on AM. I came up with the mod about 25 years ago and there are probably ways to improve it.

With respect to your last questions you leave C175 intact. It feeds your speaker and the stock modulator that we will continue to use for the driver. Do not connect your speaker to the new transformer. Leave those wires unconnected and insulated on the transformer. The reason why the negative side of the new cap connects to pin 9 is because the positive side is going to have 12 volts applied through the audio transformer. It's what the cap connects to after the audio chip that determines Polarization.

 

[loosecannon]

yeah, i answered the C175 question myself after a glance at the schem, but i guess i didnt edit my post in time. LOL

i just finished doing the transformer mod, and it works!!!

dont have time to take a current reading, but i set the deadkey at 5 watts (could go higher than 10 if i really wanted to. but not for long...)

the PEP swing was over 20 watts.

so, with nothing else changed in the radio, this mod gained about 5-7 watts PEP.

this is only the start and i cant wait to see where this goes.

my 148 is now the official guinea pig for this project!

bring it on!
LC

 

[Shockwave]

Just curious if you retuned L38 for max PEP and what type of meter are you using? You may want to reverse the two end wires on the transformer just to make sure it doesn't do more power phased that way. By end wires I mean the two that are not the center tap.

 

[Robb]

Wonder what this radio would do with this mod you've done - after you put a MOSFET in it?

 

[Shockwave]

Wonder what this radio would do with this mod you've done - after you put a MOSFET in it?

That's the only thing here I was questioning. I already know my 12 volt modulator mod works great in the 148. It usually makes way over 20 watts PEP with the stock final. That's why I'm wondering if he has the transformer phased right, L38 tuned, and a big enough power supply. If the ERF2030 MOSFET is better then the 2SC1969 bipolar, it should shine trough here. One step at a time though, make sure all the bugs in the mod are out with the stock final, then do the MOSFET mod.

 

[Pioneer621]

Sounds like loosecannon is making progress!

I should mention that in my previous diagram (for whoever reads this thread & tries adding the audio transformer in there radio) that the SPEAKER & GROUND pins in my diagram of the audio transformer do not get connected when installing it in the Cobra 148, or Uniden GRANT XL. That was just to show the basic wiring of it in a Cobra 29.

Ok, I went over my Uniden Grant XL & noted down what I did to it to convert it to a single MOSFET final. It looks like I kept it pretty basic & the modification needs to be improved upon, & maybe if a few of us work on it a little bit we can get the MOD perfected. I basically used what parts I had on hand at the time, & by the looks of it I referenced the ERF2030 sheet & did try some of there suggestions & if it did not make any improvements I left it alone. The MOSFET final installs in most radios pretty much the same, slightly different capacitors for tuning / parts values for biasing, but other then that there all very similar. Mine has been modified like this for a while now & it always seems to work well, on AM & SSB, however, the biasing on SSB could be a little better, although the 1N4148 & 470-Ohm resistor do add quite a bit of forward biasing once RF is applied, it is a little slow for SSB & at very low MIC gain levels on sideband you can hear light distortion, although I don’t think most people would hear it on the other end. I tried adding a 1M-Ohm resistor from the TX 8v supply to the GATE of the MOSFET for around 2v biasing & it really helped the sideband performance, however there was a slight power loss on AM. So additional biasing improves SSB, & I recommend playing around with that.

This modification does work, however, the power gain is not really all that great unless you run the final on the 13.8vDC line, which is how mine has always been setup.

I can’t compare the following output numbers to the stock 2SC1969 as I don’t remember for sure what it did before I modified it, but I think it was about the same when ran on the lower voltage AM Power regulator circuit.

Ok, I unhooked the final from 13.8v & put it back on the stock circuit for power testing & it puts out the following with the MOSFET final on stock voltage: Also, the modulation limiter has been cut as well, it has been like that since I got the radio & I never got around to changing the cut transistor as I rarely use this radio these days, but it was worth mentioning as that can add to the output power.

Carrier set at 4 watts:
It swings to 12 to 13 watts PEAK (PEP) on AM
SSB: 18 watts PEAK (PEP)
If I remember right, the 2SC1969 did about the same.

Output with the final ran at 13.8v:
Carrier set to 4 watts:
PEAK (PEP): 28 watts.
SSB: 26 watts PEAK (PEP)

Here are the conversion instructions with what I did to my Grant XL: I will try to post the MOD sheet a made as well, hopefully it posts ok.

IRF520 MOSFET Final Conversion for UNIDEN GRANT XL, Should apply to older Cobra 148 GTL as well

Parts List for MOD:
QTY-1: IRF520
QTY-1: 330K-Ohm resistor
QTY-1: 470-Ohm resistor
QTY-1: 1N4148 Diode
QTY-1: 68PF Ceramic Capacitor
QTY-1:470PF Ceramic Capacitor

Conversion Instructions:
1.) Remove L40, & the 2SC1969 Final Transistor.

2.) Install an IRF520 in place of stock final, use original
insulators & if any are damaged replace them. Use
heat sink compound to ensure good heat transfer.

3.) Add a 470PF Ceramic Capacitor across C152.

4.) Add a 68PF Ceramic Capacitor across C146.

5.) Solder one end of a 330K-Ohm resistor to the GATE-
(PIN-1) of the IRF520, then solder the other end to Circuit-
Ground.

6.) Solder a 470-Ohm resistor in series with the anode lead-
of a 1N4148 diode. Now solder the cathode (banded end)
of the 1N4148 diode to the GATE (PIN-1) of the IRF520.
Solder the free end of the 470-Ohm resistor to Circuit-
Ground. See Diagram-1 for part connection diagram.

Once converted, retune the transmitter.

It is pretty basic really & like I said can be improved upon, & if anybody has any tips or suggestions to better the modification please add your input on what else can be done or tried. I am going to try the audio transformer & see how much of an improvement I get with the final modulated instead of just volted to the 12 volts, should be interesting & of course fun!

 

[Shockwave]

I'm not sure on this but I've been informed that the ERF-2030 shows more gain then the IRF-520 in most CB output stages. I personally haven't worked with either transistor other then using the IRF-520 in switching power supplies. I've tried to find a datasheet for the ERF-2030 and had zero luck. There also may be some improvement if C146 and C152 were replaced with small adjustable caps.

 

[loosecannon]

i am definitely doing this to see what the results are of combining the "modulation transformer" mod with the MOSFET mod.


i am doing this in stages on purpose so that we can see the individual merits of the mods over a stock radio.

yes, i too would love to see what this mod does when the final is changed to a mosfet, but i do not have the know how and experience to make up the MOSFET mod from scratch.

if i did, it would definitely have been posted on this forum already.

so, if pioneer is still with us, i could really use your input on the MOSFET mod right about now.

i have some IRF520's right here, and a pretty good supply of parts.
the wattmeter i am using is a dosy TR-1000.
i dont really care if its perfectly accurate because i just want to see relative improvements.

the power supply i am using is a radio shack 10 amp supply, but im not sure its working properly.
seems not to want to put out much current.
i will check this radio on my 50 amp bench supply when i get home tonight and report back if the output is improved or not.

as for what all i did to this radio; i did align the transmit section for max peak wattage before i did the transformer mod.
i did not re-align L38 afterwards.
this radio is stock with only VR7 turned up.

today i will align the transmit section again, and reset the bias of the final and driver.
i will try turning down the bias to the final also to see if it yields better PEP output.

as for modulation "swing" mods, i could remove R131 and/or add a rectifier diode to R228.

any opinions on what i should try first?

i will also try switching the wires on the transformer to see if it helps or not.

please, anyone with input on this project; now is the time to speak up.

anyone else want to try it on their personal 148GTL and see if it does better than mine?
LC

 

[Pioneer621]

Ok, just a little update as I was playing around with the transformer in my GRANT XL.

Well, I put the transformer in as if I was putting it in a Cobra 29 as my diagram shows, starting out with a 470uf capacitor.

With it wired like my diagram shows & how it is wired in a 29 it was distorted or garbled / choppy / like it was being canceled out as it was out of phase, so I switch the output & 13.8vDC lines on the transformer & it sounded much better, so you do have to make sure you have it wired in phase.

However, on high modulation peaks it still wanted to break up a bit, but a different type of breaking up, so I switched the 470uf cap to a 1000uf cap & it seemed to clear that problem up & for the most part now it sounds totally normal.

The output power did indeed increase with the transformer installed, however, the power is wanting to fade back with a steady audio tone into the MIC, I noticed the back of the radio, driver, final, is running pretty hot after a bit of testing, so is the audio chip. I tried aiming some fans on it to help keep the back heat-sink cool as well as the audio chip & it did seem to help with the power & it did not fade back quite as fast, after about 30 seconds of audio it started to drop back. Before the fans it was around 15 seconds or so. I may be pushing it a bit hard though, & if you were to run it at that power output I think the addition of a heat-sink on the back would be wise & maybe some better cooling of the audio chip. For I guess what would be considered normal talking, you probably would not have a problem with the power dropping back. If I re-enable the AMC circuit & set it properly, that would probably help as well.

Here is the output power I am getting with the audio transformer hooked up:

To try to keep the carrier close to 25% of the PEAK power I set the carrier to 9 watts (plus it sounded cleaner) (I usually always run it at 9 to 10 watts in this radio anyhow).
Peak (PEP) power: 39 to 40 watts (but does want to drop back when modulating after a bit)
I kept the MIC level the same for all my recent testing, this post & my last post.

Well, I think some fine tuning of this overall modification at this point will get this working at peak performance, adding the transformer with the MOSFET does indeed seem to increase output to that of an AM radio like a Cobra 29.

Anyone have any ideas on the power dropping back?

Shockwave,
Since you have some experience with wiring up these transformers in these radios, do you have any other tips or suggestions?

Well, either way this has been fun, I really enjoy working on / testing radios & is probably my favorite part of the hobby! I will keep working on this modification & will try to post back if I have any updates. I’m curious as to what loosecannon figures out as well.

 

[loosecannon]

thanks for the post pioneer!!!

i guess i didnt see it when i made my last post.

i should have all those parts on hand to do the MOSFET conversion tonight.

it looks like you did almost the same procedure as the EKL notes on cbtricks list, with a couple of differences.
im curious if you tried any of the other steps and then put them back or what.

like, did you jumper R181?

what made you add 68pF to C146?

did you remove C149?

did you put a 33K ohm resistor from the Gate leg to R180?

i noticed you changed the parts values in the "companion part" you made.
i will also be making my own.
did you do this because the ones you chose were more standard values and you had them lying around or was it because they improved the output?

hopefully i will have time tonight to do all this and report back.
if not, i fairly certain i will have it all done by thurs.

cant wait to see what happens!
LC