Cobra 142gtl component location

[Handy Andy]

In some radios - to limit the "Slide" the R187 was a 5.6K - not on the schematic as per value but still same function.

The main premise was of handle the different Varactors' used in the years that chassis was made. When they got a different supplier or had to upgrade the part and it's support, that Varactor had a different characteristic for slide - so to keep it linear as possible -the Varactor being different - they changed the support around it to fit that need.

 

[Nowurtalkin222]

In some radios - to limit the "Slide" the R187 was a 5.6K - not on the schematic as per value but still same function.

The main premise was of handle the different Varactors' used in the years that chassis was made. When they got a different supplier or had to upgrade the part and it's support, that Varactor had a different characteristic for slide - so to keep it linear as possible -the Varactor being different - they changed the support around it to fit that need.

Ah, I see, said the blind man. That's what I meant when I suggested maybe it was used to change (or limit (the bandwidth (of the clarifier). Altho I'm surprised that a far lower resistance would somehow cause the varacter to offer less slide... Would've thought a higher resistance was needed to limit the slide. I know it takes more inductance to increase the slide. In any event, since I initially couldn't find the darn r187, it was a good "catch" for me to notice your resistor was different than mine. Let's see... a varactor works to change its value by way of the voltage it's fed, so less voltage due to a lower value resistor, makes the diode vary less?

 

[Handy Andy]

What it does is as you follow along on your schematic, the line that feeds the Clarifier WORKS alongside the RX from the MB3756 (RX switch) Pin 6...


It is a pull-up resistor - works with Voice Lock control - PARALLEL'ED with it. so the total resistance range of tuning is reduced.

The Smaller the value of Resistor used in R187 place, the smaller the slide - ability due to ohms laws ACROSS power feed to Center Wiper element output - and for those playing along with their Galaxy radios - this means less Slide, but also less drift...

 

[Nowurtalkin222]

What it does is as you follow along on your schematic, the line that feeds the Clarifier WORKS alongside the RX from the MB3756 (RX switch) Pin 6...
View attachment 37171

It is a pull-up resistor - works with Voice Lock control - PARALLEL'ED with it. so the total resistance range of tuning is reduced.

The Smaller the value of Resistor used in R187 place, the smaller the slide - ability due to ohms laws ACROSS power feed to Center Wiper element output - and for those playing along with their Galaxy radios - this means less Slide, but also less drift...

Thats what I thought I said, ie, that the lower the resistance, the less the slide. I thought (perhaps mistakenly) that you had said you had a smaller resistor to increase slide. I must've read it wrong. Maybe you said (or meant) to make a more stable slide (drift).

What it does is as you follow along on your schematic, the line that feeds the Clarifier WORKS alongside the RX from the MB3756 (RX switch) Pin 6...
View attachment 37171

It is a pull-up resistor - works with Voice Lock control - PARALLEL'ED with it. so the total resistance range of tuning is reduced.

The Smaller the value of Resistor used in R187 place, the smaller the slide - ability due to ohms laws ACROSS power feed to Center Wiper element output - and for those playing along with their Galaxy radios - this means less Slide, but also less drift...

Haha, I caught your "drift" as to the galaxy line of radios. Prolly the older ones, no? I think the newer models are more freq stable.

 

[Nowurtalkin222]

Thats what I thought I said, ie, that the lower the resistance, the less the slide. I thought (perhaps mistakenly) that you had said you had a smaller resistor to increase slide. I must've read it wrong. Maybe you said (or meant) to make a more stable slide (drift).

Haha, I caught your "drift" as to the galaxy line of radios. Prolly the older ones, no? I think the newer models are more freq stable.

If I'm not mistaken, isn't

Thats what I thought I said, ie, that the lower the resistance, the less the slide. I thought (perhaps mistakenly) that you had said you had a smaller resistor to increase slide. I must've read it wrong. Maybe you said (or meant) to make a more stable slide (drift).

Haha, I caught your "drift" as to the galaxy line of radios. Prolly the older ones, no? I think the newer models are more freq stable.

Thats what I thought I said, ie, that the lower the resistance, the less the slide. I thought (perhaps mistakenly) that you had said you had a smaller resistor to increase slide. I must've read it wrong. Maybe you said (or meant) to make a more stable slide (drift).

Haha, I caught your "drift" as to the galaxy line of radios. Prolly the older ones, no? I think the newer models are more freq stable.

If I'm not mistaken, isn't that mb3756 a dual op amp that does both the tx/rx switching and acts as a regulator as well?

 

[Handy Andy]

If I'm not mistaken, isn't that mb3756 a dual op amp that does both the tx/rx switching and acts as a regulator as well?

Well, it's really a 3-terminal OUTPUT -Dual Switched - regulator - one terminal for 8V constant and the other two serve as a functional toggle switched output. The control line CONTROL toggles the other two outputs...

​

PDF attached for MB3756

 

[Handy Andy]

Thats what I thought I said, ie, that the lower the resistance, the less the slide. I thought (perhaps mistakenly) that you had said you had a smaller resistor to increase slide. I must've read it wrong. Maybe you said (or meant) to make a more stable slide (drift)

Yes, that's what "lessens" the slide - the working value of the pull up resistor - keeps applying a constant voltage across the variable output line - so what that does is acts like a pull up resistor keeping the Varactor voltage higher throughout the tuning range of the Voice Lock control. This works as it does because of ohms law of parallel resistance and how the potentiometer - being a voltage divider - the hot-side of the element and it's wiper arm is working against the fixed resistance and it's voltage - which in turn affects the Varactor - what it sees in voltage and current - which affects it's capacitance in the circuit it's biased for.

Haha, I caught your "drift" as to the galaxy line of radios. Prolly the older ones, no? I think the newer models are more freq stable.

A lot of people have differing viewpoints about Galaxy, mine though is more positive - if you get my drift.

They are a popular radio and many "gadgets" for radios - are really made for installing in these radios - it's a crazy separate market for them in noise toys - echo's, beeps - processors - you name it - it's been tried...

 

[Nowurtalkin222]

Well, it's really a 3-terminal OUTPUT -Dual Switched - regulator - one terminal for 8V constant and the other two serve as a functional toggle switched output. The control line CONTROL toggles the other two outputs...

View attachment 37177​

PDF attached for MB3756

Yes, that's what "lessens" the slide - the working value of the pull up resistor - keeps applying a constant voltage across the variable output line - so what that does is acts like a pull up resistor keeping the Varactor voltage higher throughout the tuning range of the Voice Lock control. This works as it does because of ohms law of parallel resistance and how the potentiometer - being a voltage divider - the hot-side of the element and it's wiper arm is working against the fixed resistance and it's voltage - which in turn affects the Varactor - what it sees in voltage and current - which affects it's capacitance in the circuit it's biased for.



A lot of people have differing viewpoints about Galaxy, mine though is more positive - if you get my drift.

They are a popular radio and many "gadgets" for radios - are really made for installing in these radios - it's a crazy separate market for them in noise toys - echo's, beeps - processors - you name it - it's been tried...

Great info & explanations of how these devices work. I have that info sheet somewhere for the 3756. I think I was trying to investigate why it is a bad thing to use pin 3 for the 8v source, as so many do, due to all the bad mods being copied for years. I think I was also checking to he sure which pins were 1st & last. I could've sworn I read info at the time that it was a dual op amp.. Apparently not, but still a good use if it as a dual function regulator. I know they did away w/it in the crappy insult to the 5 pin 148s, ie, the Chinese made ones. Ironically, the best 1st ones were also Chinese made in Taiwan. You know, now I'm thinking that when I once ordered or outright bought an NTE version (1250 or something close to that..) I thought it said dual op amp on the plastic bag it came in. Maybe not..

I love the galaxy radios, both for all the repairs needed it has provided me with and the great performs & sound (when it works) and great functionality. I have always loved the 959 as a great replacement for the long gone goid 148s with very good function & fashion. Plus those guys in Escondido make great amps too and they are very good at standing behind their products.

 

[Handy Andy]

 

[Nowurtalkin222]

Sorry, now I figured out what I was talking about. I was conflating 2 different devices in my mind. . I was thinking about the 4558 dual op amp common to so many radios.

I thought it was an nte 1250 but it wasn't. I just remembered it was an nte 1271 which is listed as a vcr voltage control regulator. Not positive if the exact same thing as 3756, but I think that's the cross and I'm fairly certain that I once used it as a replacement in an 8719 radio. And the nte 291 was, a replacement for the regulator/switcher used in the Chinese 148s. Prolly alot cheaper, like the rest of the radio. Apparently, it doesn't regulate ssb very well.

 

[Handy Andy]

That's ok, I've been accused of calling a TA7256 a TA7222P - so I've had my share of fun...

 

[Nowurtalkin222]

That's ok, I've been accused of calling a TA7256 a TA7222P - so I've had my share of fun...

No really... The nte 1271 is a vcr control thingy and replaces the mb3756. In fact, I think I remember having a 1271 that was marked as a horizontal control chip which I used to replace a 3756. But I can't remember my dreams tho.

 

[Handy Andy]

Don't worry, you are correct...

PDF attached​

 

[Nowurtalkin222]

Don't worry, you are correct...

View attachment 37209
PDF attached​

Don't worry, you are correct...

View attachment 37209
PDF attached​

Must be years ago I needed a 3756 but didn't have any & did a cross reference to find the nte version. I doubt very much they make the mb3756 anymore(?) and maybe some fixers out there didn't know about the 1271.i remember after getting the 1271,being surprised at what it said on the plastic pkg it came in...something about a TV or vcr and I wondered if I had the right thing but it worked. Those 3756 switches don't go bad that often in my experience, but I certainly have had to change out a few.

Anyway, today's cb trivia question: why is the Texas Ranger 296dx called that?

 

[Nowurtalkin222]

Must be years ago I needed a 3756 but didn't have any & did a cross reference to find the nte version. I doubt very much they make the mb3756 anymore(?) and maybe some fixers out there didn't know about the 1271.i remember after getting the 1271,being surprised at what it said on the plastic pkg it came in...something about a TV or vcr and I wondered if I had the right thing but it worked. Those 3756 switches don't go bad that often in my experience, but I certainly have had to change out a few.

Anyway, today's cb trivia question: why is the Texas Ranger 296dx called that?

I just thought of a question about that resistor change in your 142.. I think it was a lower resistor & you explained it is to lessen the slide bandwidth to make clarif more stable. What occurred to me... isn't the slide bandwidth only about 5kc or less w/the original 22k resistor? Maybe I'm recalling that wrongly, but usually w/o adding chokes & such, basic unlocks don't slide all that much... I mean plenty enough to get the desired control, but I'm thinking often around 5kc, give or take. As far as I'm concerned, around 5kc is about all that's really needed, especially if you can get it balanced w/the center slot. And should still be pretty stable. The ability to go up/down 5kc, pretty much also covers the space in between channels. So if I have this right, my question is why would there be a need to lessen the slide bandwidth any more than that. 5kc or so, isn't all that too touchy. I think sometimes an unlock might slide close to 10kc in one direction, but once you achieve a balance up/down, it's often closer to 5kc. I'm thinking about 8719's in general. In a little while, this aft, I plan on finishing up the clarif job on the 142 I have & I'll make note of the total slide I get. I'm guessing it won't be much more than 10kc total. Getting it balanced is often tricky. I'm guessing it's not too tricky for you, but for me it has been. Sometimes, after setting the center slot where it should be, leaves one of the side bands out if reach of the inductors adjustment, to get that band where it needs to be. When that happens, I discovered a cheap trick to get the inductance to go further in the needed direction for the one rogue upper or lower band (I think it's usually upper, but I can't remember). I won't say what the trick is that I discovered myself, but a hint is that when there isn't enough adj left, when raising or lowering the slug reaches the same dead end, cutting the slug in half can help get a little bit more inductance in the right direction (if I remember correctly) but still, often not enough. The dirty trick can get you there at that point. I'm reluctant to reveal the trick, because when I did it, I didn't have a spectrum analyser to see what it did or didn't do to the signal..