Cobra 148 Clarifier Mod Problems

[RedEye]

I am opening the clarifier on a 148 Maylasia Board # EPT014811Z
When I clip d52 the TX drops way out of sync with RX and I can't center the two. Reconnect it and I can align RX & TX just fine, undo D52 again and TX drops (EX Chan 19 RX 27.1850 - TX 27.1607) and VR5 does not work with D52 removed...

Any Ideas? Thanks in advance.

What was done:

Clarifier Modification
1. Remove D52.
2. Lift the hot (orange) Clarifier wire from the board, and connect to pin 3 of IC4 (MB3756).
3. Slide range can be increased by jumpering or lowering the value of R174 or R175, or both.

[Rick]

This is the way I have done this in the past.
1 Remove D52
2 Remove R44
3 Remove R174 and solder a jumper in its place.
4 Unsolder the red wire that comes from the Voice Lock pot from the PCB and solder in to pin 1 IC4 MB3756.
5 Change R175 to a 1K

Rick.

[RedEye]

Thanks a TON Rick! I redid it to your specs and it works great. R44 I guess was the missing link. when I cut it both TX and RX tracked together.
I didn't change R175 yet though, does that just expand the range?

Channel 19 for example will now go from 27.1775 to 27.1929 (27.1850 being center)

You DA Man!!!!!!!

[Min]

Rick, what is the difference from pin 1 & 3
Pin 1 is Vout-0= voltage output
Pin 3 is Vref... what is Vref???

I`m looking at the pdf file for the MB3756 and I dont see anything about Vref- pin 3.

[Rick]

Pin 3 is a voltage reference to keep the current mirror, which is an internal function of the chip running at constant load, in most circuits, which use this chip you will see a cap from pin 3 to ground this is want stops the chip from hunting when you change from RX to TX hence keeping a constant current. Pin 1 is always at 8.volts and this is controlled by the voltage reference at pin 3 which also controls pins 6 RX function and 8 TX function.

You should never hook any think up to Pin 3 as this is how you blow the chip up.

See my diagram it may help see how the MB3756 voltage reg works in the Cobra 148 GTL

http://www.cbtricks.com/radios/cobra/14 ... redraw.pdf

[Min]

Rick, are you able to redraw full radio schmatics??? If yes, I`d love for you to do some for me. Your redraws comes up so nice and clear and easy to read.

I was just asking cos I have seen a few 148`s come through with them wired to pin 3 and a lot of them to pin 1. Just wasn`t sure what the difference was.
Plus the fact that i`ve got a 148 on the way, should be here any day now.

[nomadradio]

Wiring anything to pin 3 on the MB3756 is a mistake.

Use pin 1. It's intended to supply some current. Pin 3 is NOT. Hooking to pin #3 is the kind of misinformation that circulates the web forever, since most folks who pass it on have no idea what is inside that chip.

Hey Rick, that's a righteous redraw there, dude! Rock on!

73

[Chevin]

Rick knows is stuff all right its just amazing how much he knows. some of you may know that he is also active on 5 more forums. I think he genuinely likes helping people it is a rare to find some one like this in the modern world. He also plays a mean guitar. Keep on rocking Rick.

[Min]

Rick, with the clarifier mod your way. You use just the red wire and not the orange. Why do all the other mods use the orange 1 as well.

[Min]

Rick or nomadradio, I will be hopfully unlocking my clarifier later today. The radio is a Cobra 148GTL #PC-412AD.

What is the best method for the most slide and for it to sit at 12 o`clock???

[Jamie]

Rick,
I've seen this version on the net. It promises to be centered with +/- 6 kHz of slide and perfect tracking of Tx with Rx. Is it too good to be true? Could you just use pin 1 instead of pin 3 for this version?
Jamie


<<If you do the changes below you will get -6kHz to +6kHz with 0 right in the middle.
The Tx will track exactly with the Rx (confirmed on recently calibrated IFR1200S).

1. jumper R175
2. jumper D51
3. remove R149
4. remove R188
5. remove R44
6. remove the wire from the high side (+ or full cw pos) of the clarifier
7. connect pin3 of IC4 to the high side of the clarifier
8. replace R174 with a 3k3 value
9. attach a 12k resistor from the jumper of D51 to pin3 of IC4

[Min]

Rick already said to use pin 1 instead of pin 3. Pin 1 runs a constant 8volts, so the answer to your question is YES. Use pin 1.

[Jamie]

Rick already said to use pin 1 instead of pin 3. Pin 1 runs a constant 8volts, so the answer to your question is YES. Use pin 1.

So of all the ways I've seen to unlock the clarifier on a PC412 148gtl is this then the preferred way and will it give the best performance?
Revised.
1. jumper R175
2. jumper D51
3. remove R149
4. remove R188
5. remove R44
6. remove the wire from the high side (+ or full cw pos) of the clarifier
7. connect pin1 of IC4 to the high side of the clarifier
8. replace R174 with a 3k3 value
9. attach a 12k resistor from the jumper of D51 to pin1 of IC4

[Rick]

The method I have used as been used for many years with no trouble and as found to be very stable method, if you require more shift with this method I am using just replace the varicap D35 with one which has a ratio of 1.7 to 2.2, the varicap I use is SVC251 or BB909 this will give you a slide of about 7 KHz down and about 6 KHz up.

As for method, which Jamie as found looks ok but for one think when you remove R149 the varicap will become unstable, this may lead to frequency drift in use.

There are lot on method, which have been tried, but you use the one, which you are happy with.

[Jamie]

The method I have used as been used for many years with no trouble and as found to be very stable method, if you require more shift with this method I am using just replace the varicap D35 with one which has a ratio of 1.7 to 2.2, the varicap I use is SVC251 or BB909 this will give you a slide of about 7 KHz down and about 6 KHz up.

As for method, which Jamie as found looks ok but for one thing when you remove R188 the varicap will become unstable, this may lead to frequency drift in use.

There are lot of methods, which have been tried, but you use the one, which you are happy with.

Rick,
Thank you for your insight and knowledge. It is always appreciated! And I mean that. Regarding your comment on R188. Don't you mean R149 instead of R188 for making D35 unstable? I think you are right about the unstability if you meant R149.

I find the clarifier circuit to be an interesting switched circuit. I have the descrption from the manual and it reads as:

<<"Clarifier Circuit
In the receive operation, the control voltage varied by the clarifier control VR402 is phased through the clarifier gate diode D51 and is applied to a variable capacitance diode D35 which is connected to the off-set frequency oscillator crystals.

The above circuit may vary the off-set frequency by means of rotating the clarifier control VR402 at the range of +/-1 kHz. In the transmit operation, clarifier gate diode D51 is reverse biased and that separates the variable capacitance diode D35 from the clarifier control.">>

In looking at the circuit (A big thank you for the schematic!) I see that there is a wire (white) going from R44 to the RX voltage bus supplied by pin 6 of IC4. This goes to zero on transmit and unbiases D51. Thus turning off the clarifier on transmit. Then the TX frequency pre set circuit VR5, R187, R188, D75, and D52 is turned on by the TX voltage bus. This pre set circuit looks alot like the clarifier circuit but with different values. All the unlock methods seem to disable this circuit usually by removing D52 or R188. Then a constant 8 volts is applied to the clarifier high side and the RX voltage is removed either by removing R44 and the orange wire or by just removing the white wire behind R44. The values of R44, R174 and R175 seem to determine both the amount of slide and the centering of the clarifier control on frequency. Interesting!

[Rick]

Yes it should have been R149.
I do know about wire colours from the pot VR402 as this changes from model to model.

[Jamie]

Yes it should have been R149.
I do know about wire colours from the pot VR402 as this changes from model to model.

Sometimes I think thats done just to confuse people working on the radios. Its the factories little joke. After this great thoughtful discussion I like the way you do it best of all. How much slide do you get? +/- 5kHz?

[Jamie]

See my diagram it may help see how the MB3756 voltage reg works in the Cobra 148 GTL

http://www.cbtricks.com/radios/cobra/14 ... redraw.pdf

Rick, Your schematic is the best! Its clear and straightforward! Thanks! I think I may have found an omission on it for your next revision. R77 3k3 is connected to the anodes of D59 and D60 and the RX bus from pin 6 of IC4. Also the TX/RX indicator circuit could be added for people who want to play with that LED.

[Rick]

My diagram is for the original 148 GTL PCB PC412.

Next time you look in the original 148 GTL PCB PC412 you will find that the R is not fitted.

Most of the original 148 GTL diagrams had lots of errors, that why I have re-draw It.

The R you are talking about is part of potential divider R77 and R259 this is across the RX + line and feeds D59/60 and his only need on later radio like 148 NW ST which suffers from bad voltage regulation.