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Cobra 148 GTL mod

Both the Bill's where.

Bill Good REALLY knew how to make a two way radio perform. AM, FM, SSB, didn't matter. We'd send radios up to him from San Diego, as SOON as we'd get them back, I'd start rippin them apart to make the mod sheets up LOL. THEN came the schematics, drawn by hand (no computers or CAD in the mid and early 80s)..... to compare with the originals, that ALWAYS had that crappy fold down the middle....

Bill Cheek, Dr. Rigormortis is worthy of a read, for those that DON'T know him. He is the man behind the Pro series scanners mods, the Pro 2004 BIBLE, the data slicer (which allowed us to 'decode' mdt's in the 80s), etc. As WELL as the man who was the editor and publisher of the Eleven Meter Times and Journal... How many here have heard of them? :)


Anywho, yup. Thems the d00dz I followed to learn what I know today. And amplifiers, none other than the wizard himself, Mr. Paul Williams.

--Toll_Free
 
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I have noticed that there are others that are interested in the unlocking of the clarifier. As TOLL FREE has advised me on my 148 GTL side mike radio. I have hopefully made the modification easier by moving the RX voltage wire to a constant 8V supply and clipping and jumping 3 components.

This modification was done on PCB board EPT014811Z & 13Z motherboards
This board was made by Ranger and to my knowledge there were only 2 board productions used. The clarifier ckt is identical in both boards.

I have attached some pictures of my radio modifications.
steps for mod:

1. clip lead of D52
2. jump D51 (solder jump on other side of PCB)
3. jump R174 (solder jump on other side of PCB
4. follow green wire (RX 8.0V) to the other end and clip off.
5. Take green wire and attach to the + end of C84 (8.0V)

Your done...

Thanks go out to TOLL FREE for instructions and sticking it out on my modification

Good luck.

Larry
 

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The only thing I was going to say to change was, DON'T jump D51 or R174 if you want to maintain STOCK slide range, BUT want the TX and RX to track together. If you do the minimal range mod, you SHOULDN'T need to realign the radio, unless your voltage tap points where different (ie, you went FROM a 8.0 volt source to a 8.65 volt source... That half volt will make a BIG difference in slide range, believe it or not... The varactors are TOUCHY at each end of their range, hence the reason clarifiers are so touchy at the end of their range when expanded max). Try to keep the two voltages as close as possible (sometimes easier said than done).

If you want MAXIMUM range, then DO jump those two, as well as REMOVE R44. This method DEFINATELY requires you to realign the three oscillator inductors.

Hadn't gotten to reply to the email yet. :) Maybe make up a pix of each way, that way people can get it either.

The Cobra 2000 is a different animal, as some want the 'fine' to be rx only, and some want rx and tx to track on both, and some just totally screw the ENTIRE mod up. :)


Thanks to Larry for being a guinea pig as well as making it legible. :)

--Toll_Free
 
larryb said:
I have noticed that there are others that are interested in the unlocking of the clarifier. As TOLL FREE has advised me on my 148 GTL side mike radio. I have hopefully made the modification easier by moving the RX voltage wire to a constant 8V supply and clipping and jumping 3 components.

This modification was done on PCB board EPT014811Z & 13Z motherboards
This board was made by Ranger and to my knowledge there were only 2 board productions used. The clarifier ckt is identical in both boards.

I have attached some pictures of my radio modifications.
steps for mod:

1. clip lead of D52
2. jump D51 (solder jump on other side of PCB)
3. jump R174 (solder jump on other side of PCB
4. follow green wire (RX 8.0V) to the other end and clip off.
5. Take green wire and attach to the + end of C84 (8.0V)

Your done...

Thanks go out to TOLL FREE for instructions and sticking it out on my modification

Good luck.

Larry
Click to expand...

No disrespect Larry, but you might want to consider buying yourself a solder sucker, apart from being much neater than clipping parts, it also allows you to remove the parts intact so that if you ever want to return the radio to spec its a simple job and you already have all the parts.

its a simple task to replace resistors/diodes with a bit of jumper wire than to short them out,and much neater.

clipping parts is such a cowboy way of doing things.
 
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I do have a solder sucker, but I have found in my experience that just clipping the lead and leaving the component in place will allow you to undo the modifications by simply soldering the clipped lead back together. You don't loose or damage the part and it is a simple unsolder any jumpers and soldering the clipped wire back together and your back to stock.

Besides that when you put the cover back on no one but you knows what is in there.

No disrespect taken

Giddy up.............;)

Larry
 
Sooooo . . . . for a STOCK unlocked clarifier range, just remove the wire from the board that gets receive voltage - and hook it up to C84?

Isn't removing D52 also necessary?

Never mind . . . I re-read this page . . .
 
Last edited:
Auh yes ! those sliders from hell !! thank God there at least on the older Grants and 148 5 pin models (the best) Why ? cause they were very stable on SSB and had very little drift after a slight warming (some a little longer then others) Yes those big ass sliders in the motor car can be a real pain in the ass ! (and dangerous for ones health behind the wheel)

Would always suggest a small slider in the motor car , just makes life that much easier on sideband. But hell !! these great radios can have 10 up and down Khz's without a sweat from center slot ! Touchy ? Yes , but stable all the same. I find these big dog sliders best suited for base station operations rather then the motor car. (and a 10 turn verner dial rocks and rolls on these bigger sliders !)

Of course as mentioned in this thread , there's at least half a dozen ways to open for RX and TX on these chassies , this mod I find to be perfect for motor car operations , about 1.5 khz down and up from center slot , not touchy at all and locks in very nicely.


Getting up to speed....we're looking at clarifier/voicelock mod for less slide...

148GTLGrantClarifier.gif


Actually - went home [while I still have one] and photo'd the old Cobra 148 GTL that I still have.

For those that just want a simple open clarifier do the following.

Orient radio 's front face towards you - speaker/component side up [assuming you've taken the covers off].

Look in centerline of radio - separate right and left - locate voicelock/clarifier [Grant had same board] and it's color code [e.g. Red - Blue - Orange]. Locate VR 3 - by meter dispaly right corner closest to you. Note white wires here. There are two.

VR3location.gif


Using a soldering iron, remove white wire CLOSEST to VR3. leave other white wire there - see above photo.

Locate PLL by channel selector BCD switch. Note ribbon cable - locate R147 next to it. To ribbon cables' right.
R147location.gif

At R147, to it's left, you will see an empty hole - hole closest to PLL right next to R147 - the white wire is to be soldered into this hole [constant 8V] BUT NOT YET! In this photo - white wire is installed there - DON'T PANIC

Locate D52 and D75 - these steer and compensate TX 8V line. See photo - they are by Xtal that sits right next to the clarifier wires. See photos'. Clip or remove one of these diodes - You may want to unsolder it instead - there's more below.

D75andD52location.gif

NOTE:
If you want less drift - place diode you unsoldered - on the end of the white wire going to hole located by R147. Solder BANDED end to White wire lead - UNBANDED side of the diode goes in hole.

IF YOU DON"T REALLY WORRY ABOUT DRIFT skip diode and wire directly:
Solder in - and double check your work for blobs, loose wire/frayed ends - or and potential problems on soldered joints and check for shorts.

This white wire is the 8V line from the RX side - you now have put in with the same power supply feed that goes to the PLL. The PLL and the white wire get their voltage from the same source and can use the same filter network - see photos.

(and thank you H.A. for your time)
 
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Has anyone done the clarifier mod on the newer China made 148 GTL ? I would like to open mine up to slide on TX. Can anyone help me with this mod ? Any help here will be greatly appreciated. the 8 volt line on the clarifier is BLACK, with 2 RED wires on the other terminals. thanks for reading this post, hope to hear from someone on this, 73.
Dave
kc8pcl@yahoo.com
 
psycho said:
Unless you are using a freq counter, don't unlock the clarifier. The clarifier will zero at different locations on the dial when the ambient temperature changes.
As far as R131 goes, that'll only open up AM....there is a variable pot on the board to open up the ssb. Cobra 148s sound great with the limiter clipped.....most exports don't.
Click to expand...

Who keeps bringing these old posts back to life??
 
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Toll_Free said:
Cutting out D52 disables the radios internal VR that controls TX frequency. You CANNOT have it in, and have an unlocked clarifier.

The simple fact that you can slide a complete channel means something is fubar.

You do NOT want to ground one side of the clarifier control. THAT is going to get you the massive amount of slide, or screwing with the voltage divider around the clarifier... Which is what has happened here. We need to get the voltage divider set back up correctly, and then you'll be back 'stock', but with tx tracking the same as Rx.

I'm gonna GUESSTIMATE that the 800 ohm resistor is still tied to RX only voltage. One end should go to the junction I pointed out earlier, then the OTHER end should go to the full time 8 volts. If it's still on part time, so to speak, then the voltage divider is NOT going to function correctly.

Shorting out R174 will ground nothing. That's the resistor in series with the potentiometer, at least on the two schematics I'm looking at now. Do NOT SHORT THIS RESISTOR OUT!!!!!

It also sounds like you either lifted the wrong end VR402.

One side of R44 is tied to 8 volts, BUT ONLY IN RX!!!. This needs to be changed to the TX/RX voltage. Depending on HOW you performed the mod to change the voltage, you may or may not have removed that from the switched voltage. R44 is SUPPOSED to be 8200 ohms, btw.... At least on the schematic I have, it's marked 8k2. On coding method of resistances in the form of 4k7, 8k2 or 560R?, they give that as 8200 ohms, which is how I've been interpreting it for years.... BUT, we'll go on.

If you have a schematic, and can read it (I was assuming you couldn't), then I can give you another way to perform the mod..... You can work it out by looking at the schematic and the mod, then apply it 'in effect' on the radio.

The junction of RV104 and R44 is supposed to go to +8 volts ALL THE TIME.

Then remove D52. You're done.

If you're an entire channel off and get no slide (or voltage change), it sounds like you screwed up and pulled the WRONG side of the RV104... If BOTH sides are at 8 volts, it's going to read 8 volts no matter what you do..... One side has to be lower (closer to ground) than the other.

Jumpering or changing ANY of the following is going to SCREW your clarifier up, and make it NOT track like stock.

R175
R174
R44

Kill d52. Take the red wire from the clarifier and put it on a CONSTANT 8 volts. Go to R44. One side will join R174 and D51. Pull the OPPOSITE side and connect it to constant 8 volts (the SAME connection!!!!, preferably, connect it to the hot side of the RV104).

You're doneski. If you change ANY of the resistors above, you screw up the voltage divider in the radio.

I know I've repeated myself, but I tried to explain it a couple different ways. Grab a decent schematic you can blow up online, and I think it will make sense... You seem to have a basic grasp of electronics, so I'll give ya an idea of how it works.

The clarifier changes voltage on a diode that changes capacitance with a change in voltage. It is NOT a linear change, meaning at the high and low ends of the voltage range, things change VERY quickly!!!! Leading to the problems with clarifiers being WAY too touchy!!

They limit this effect by operating the diode in it's 'linear range', meaning if 1 volt = 1 pf of change from 3 to 6 volts, THATS where they are going to design the clarifier control to operate.... Even though it's connected to 8 volts on one side, and ground on the other. They form a resistive divider that allows your actual clarifier control to ONLY change from the 3 to 6 volts (and these are numbers pulled outta you know where.... I'm not positive what the range on the clarifier control is, and not wanting to do the math right now lol).

Hope this makes sense. I'm GLAD to see you actually did it, and then are willing to see if you can't make it work. BUT, I think you have R44 wrongfully placed (which I didn't make it clear before that you needed to make sure it was on the now constant voltage) or you pulled the WRONG wire from the clarifier. Just make SURE it's the red wire that goes to 8 volts now, and that you lift the 8 volt switched side of R44 and put it to the same connection as the red wire. Then clip D52 and you're done, center should be center, and you'll have stock slide.

--Toll_Free
Click to expand...

hi r44 shows voltage drop on both sides when mic is keyed
 
One side of R44 is tied to 8 volts, BUT ONLY IN RX!!!. This needs to be changed to the TX/RX voltage. Depending on HOW you performed the mod to change the voltage, you may or may not have removed that from the switched voltage. R44 is SUPPOSED to be 8200 ohms, btw.... At least on the schematic I have, it's marked 8k2. On coding method of resistances in the form of 4k7, 8k2 or 560R?, they give that as 8200 ohms, which is how I've been interpreting it for years.... BUT, we'll go on.



in a stock clarifier the 8 volts that the pot varies is only available in RX, thats why you see the voltage drop off in TX.

the point of the mod is to lift the RX only 8 volts and provide a constant 8 volts to the clarifier.
pin 1 of the MB3756 chip is a good place to get this from.
LC
 
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