Cobra 148 GTL mod

[larryb]

I am getting back into the CB world again. Back in the 70's I did alot of playing around with a GE AM/SSB unit with a 50w Polamar amp.
The GE radio is gone but I still have the amp. I now have a Cobra 148 GTL sside 5 pin mike.

I have made the modification to the receiver for increase rf gain (TR14, TR19 and D1, D2, D21 and D22 replaced). I have also cut R131.

On my GE, I made a modification on the clarifier to slide the freq slightly to help move my carrier out of the center of the channel (16 back then). This greatly helped me get out of the middle of the band and was able to really get some good DX'n.

I have the modification to unlock the clairfier of the 148 but no one can tell me what the results will be.

I do not want the extra channel modifications.

Can anyone tell me what advantage the unlocking of the clarifier will do on the 148?

Larry

 

[ExitThirteen]

Hey there!

Unlocking the clarifier on a 148 GTL is really going to make it slide. I've unlocked a BUNCH of 148's and they all consistently slide more than -10kc, or 1 full channel back. I'll unlock it, but then I modify the circuit so it doesn't slide as much. I don't like a clarifier that's so touchy that looking at it funny makes it slide half a kc... LOL

It's worth it to unlock the clarifier, though... nice to be able to just "dial up" someone, rather than to have to give the other person a count to have them dial to you, then have them give you the count to lock them in. You know the drill, I'm sure. Good luck and have fun with the 148!

 

[wcsd106]

When you unlock the clarifier, you'll also need to center the clarifier in all modes. A good benchtop frequency counter capable of reading at least 40MHz will be needed to do this.

I've got the clarifier mod and the alignment documented on the computer in my repair room. I'll post it tomorrow if you'd like.

 

[radiohobby]

If you don't want to recenter it or don't have the gear just make a mark on the face of the radio where the new center is after you have received some ssb signals. That way you will have the location on the dial for normal operation. I did my own and that is how I did it anyway since that way I got a little more upward slide because it really goes down but not so much up and since it is my personal radio I just wanted it that way.

 

[larryb]

Do both the rec and transmit freq's slide or just the rec?

Larry

 

[psycho]

Do both the rec and transmit freq's slide or just the rec?

Larry

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.

 

[Toll_Free]

The BEST method to rework the clarifier in the 148 is this:

Remove the 8 volt receive wire on the clarifier. This color depends on year / vintage / country, so I can't help you. BUT, it will go to a wire that will have 7.9 or more volts on it IN RECEIVE ONLY!!!! Find a spot on the board that has 8 volts ALL THE TIME.... There is usually a spot near the PLL that is unmasked, but has a hole in it, and just has to have the solder sucked off the mask on the pc board side. Don't go much farther. ONE other thing MUST be done to FINISH the clarifier mod on this chassis:


Clip D52. This disables the TX Freq Set potentiometer (VR5).

Check to make sure "center slot" is still straight up and down. On some chassis IT IS NOT. DON'T WORRY, WE ARE GOING TO FIX IT NOW.

Get an 800 ohm resistor. Solder it from the +8 volts ALL THE TIME to the junction of R174 & D51. This should put your clarifier back at the stock position.

DON'T DO ANYTHING ELSE. YOU'RE DONE, IF YOU WANT THE STOCK RANGE AND FEEL OF THE CLARIFIER. Anyone that clips this or that, or jumps out all these resistors and diodes is affecting the temperature compensation and the limits of the range of voltage presented to the varactor (which is the 'special' diode that causes the frequency to change).

NOW, if you have a freq counter or want more slide (typically I can get 12 kC down and 6 up. My clarifiers still present 12 o clock as being 'center slot' for the channel shown. USB / LSB / AM have a SLIGHT different amount of slide, but it's really insignificant, you'll be able to hit 5Kc jumps and drops as well as 10kC channels (RCs). I'm capable of fat finger operation of it, mobile, you are to.

Remove R44
Remove D52
Remove the resistor placed above (the 800 ohm one), if you had to do it. OTHERWISE
Jump R174
Remove R175
Jump D51


There ya go, that's the hackshack version of a clarifier mod for the 148 chassis. LOTSA slide, but differing amounts in temperature, drift, etc. caused. You can play with the values of the resistors above to center, get the amount of positive and negative offset you want, etc.

Take a look at the schematic and compare it with these notes. You can now unlock ANY clarifier. Hell, even a 29LTD will slide down 10


--Toll_Free

 

[larryb]

Thanks Toll Free for the info.

One of the modifications you mentioned confuses me. Your adding a 800 Ohm resistor from +8V to the junction of R44, R174 and D51. The other end of R44 is connected to VR402 (clarifier) The wire coming from the junction of VR402 and R44 is also going to a +8.0V.

The same results can be done by replacing R44 (8.2K) with a 800 ohm resistor.

Am I missing something???


Larry

 

[Toll_Free]

Thanks Toll Free for the info.

One of the modifications you mentioned confuses me. Your adding a 800 Ohm resistor from +8V to the junction of R44, R174 and D51. The other end of R44 is connected to VR402 (clarifier) The wire coming from the junction of VR402 and R44 is also going to a +8.0V.

The same results can be done by replacing R44 (8.2K) with a 800 ohm resistor.

Am I missing something???


Larry

R44, if present, is 800 ohms in my chassis.... HOWEVER, I'm NOT on a 'current' rev chassis. It may be changed to an 8.2k today......

If you HAVE R44 installed, LEAVE IT ALONE, it's where it's supposed to be. Some of the variants I've seen had a jumper. Hence me pointing out if it's 'screwy', just put that resistor in. That should restore the proper 'spread' to the voicelock (clarifier) control.


Glad to be of service


--Toll_Free

 

[larryb]

So the way I read it is to do the following:

* replace R44 with a 800 ohm
* cut D52
* jump R174
* jump D51
* remove R175
* tie one side of vr402 to 8.0V (IC1 pin 9, or C84 + side)

Simple....

Again, thanks

By the way, what are the best SSB channels???

Back in the day it was only 16.

Larry

 

[radiohobby]

These days the national call channel is L38. Some areas have locals on L36,U37,L39, it just depends on what part of the country you are in I guess. Around here there are no locals, it seems like unless I have dx coming in I don't have anyone to talk to except trucker on 19.
Have fun with the 148, I am still talking on the one I bought when they were first sold.

 

[Toll_Free]

So the way I read it is to do the following:

* replace R44 with a 800 ohm
* cut D52
* jump R174
* jump D51
* remove R175
* tie one side of vr402 to 8.0V (IC1 pin 9, or C84 + side)

Simple....

Again, thanks

By the way, what are the best SSB channels???

Back in the day it was only 16.

Larry

People typically run 38 as the call channel, but as someone else said, it does change regionally. Haven't heard anyone on 16 since I moved out of the high desert the first time in the 80s....

But, yeah, you pretty much got the mod down.

--Toll_Free

 

[larryb]

Ok, TOLL FREE, I made the mods and found that I slid back one full channel (18 is now 19).

Also the voltage swing on the end of R44 (800 ohm) is nil when I adjust VR402 (8.34V).

Looking at some of the other variations of modifications suggest shorting out R174 (grounding one side of VR402).

So, I put R175 back in the ckt, removed R44 (800 ohm) and centered the clarifier. Now 19 is 19 and I can swing the voltage from .8V to 8.3V. and get best reception on 19 at center swing (4.0V).

I am going to assume that I will be sliding the freq due to cutting out D52.

I dont have a freq counter or another SSB unit to check it out.

I know there are different versions of the 148, I do have one made in Maylasa
and ckt board number EPT014813Z.

Looking at the schematic and PCB I found that one side of R44 was tied to 8.3V and by changing the R44 to 800 Ohms fixed the voltage to 7.6V and then VR402 would not change the voltage until I got to the end of the travel and it would then go to the 8.0V.

You may have a different version of PCB and the 800 Ohm did the trick, but not on mine.

I will give this a try for awhile. The problem I really have is trying to find someone to talk to on SSB or wait until the skip rolls in again.

Thanks for the input and ideas.

Larry

 

[Toll_Free]

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

 

[Toll_Free]

I just went back and reread everything we had posted.

R44 is supposed to be 8200 ohms. 8.2K , however ya wanna put it. Brainfart.

BUT, the end that went to the 8 volt supply NEEDS to go to the 8 volt supply that you moved the red wire to, NOT the original spot where it was.

Sorry for the confusion, was trying to go by memory, I just reread the entire thread...

Make SURE you lifted the red wire and that it's the one going to 8 volts now. Then check R44, change it to the 8200 ohms (original), and you should be good to go.


--Toll_Free