internal swr adjustment

[1iwilly]

handy andy there was a post where I ask about if any adjustment on a cobra 2000 gtl &148 gtl side mic
for internal SWR calibration, I can't locate it meaning when on a dummy load and set the radio to see SWR instead of 1.0 or 1.1
they both are showing 1.3 to 1.4 you reference out to 2 resistors I believe r176&175 along those lines been the radio up there in age perhaps the resistor is down in value or the caps that are associated in that circle
I have a few days off and want to give it a try to fix it.here is a pic of the 2000 gtl

 

[Handy Andy]

Oh - you mean 148GTL - I thought you were asking about a Cobra 29 - so my appologies...

Yes, if you remember this post...
https://www.worldwidedx.com/threads/vr7-on-148-gtl-help.259212/#post-738643

Well, the 148 has something similar...

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Man sorry for that confusion, though you were talking about the 29 Comp you once mentioned earlier...

But yes, they are R176 - 330 ohm , Reflected, R177 470 ohm FORWARD - BUT they also have SERIES resistance too, meaning they provide current limiting in the meter movement itself...

In Series they use R235 470 ohm for REFLECTED, R234 - 560 ohm for FORWARD (CAL position).

So "forward power" is higher in REFERENCE than Reflected. but in doing this, they have to limit the current power flowing into the meter - else it will burn out - so they use 560 ohms in SERIES to reduce current.

That is why there is such a discrepancy.

They want to provide the best sensitivity, not necessarily the best TRUE IMPEDANCE measurement - just the RATIO of Forward to Reflected.

 

[1iwilly]

thanks andy i still do have that 29 and that one reads fine on point but yes the 2000 and 148 is what i was referring too i know it won't be perfect but atleast 1.1 would make happy and know that at least it's close when doing a quick reference like in the mobile

 

[Handy Andy]

I got to get a better schematic...So I went and located the stock version of the 2000s'

Came up with this...still the same Resistor numbering but - again, like I had said in the previous thread - lots of times these resistor values change...

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So you've been warned...

 

[1iwilly]

thanks, andy i have the sams for it, on r176 sams shows 2 possibilities either a 560 or 680 look at pics. i bought this radio this year i knew it needed work having time off in between i started working on it and got it back working putting it back to factory stock i'll tinker with them resistor if work fine if not no lost

 

[Handy Andy]

Ok, thanks for understanding...

Wanted to pass this along, because in some ways, this might seem that they "Fly By The Seat Of Their Pants" but in reality, it's not.

It deals with Meter the movement in it, how much ohmic in winds are in the coil, and how much "movement" the d'Arsonval coil movement itself with have to operate in and work somewhat linearly...

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What seems odd to many is the selection of those resistors used in the SWR metering circuit, doesn't necessarily equate to "accurate" impedance response, no - it is more for the Meters power consumption for it to reach full scale.

So the meter in a typical 148 (READ this as SIDE-VIEW metering) the scale and movement is different than the typical meter full-face full swing metering for your 2000 - let alone the scale or swing the needle uses to display the effects of current passing thru it. So you have differences between the typical 90 degree and (maybe) 75 degree swing with much of the power accuracy being "scrunched up" near the top of both meters - which is why the "S/RF side" meter face writing seems to be packed in together all on one side with a BIG expanse of real estate on the other.

Welcome to the world of Log versus Linear scaling - and the mechanical problems of the current analog metering system versus the digital realm. .

You know they use a spring inside that meter movement; as its' pivot point is a "jewel" designed to offer the least amount of friction but the spring is designed to apply tension to allow for some accuracy (a process of work) - in a lose term - but it will still have limits in displaying things properly using such a small face for you to glance and adjust your antenna SWR by.

So as you work between these two schematics, keep in mind that the meters used - have a different response to the amount of current - so you think you have it right, you can wind up burning out the meter movement because you placed too much current thru the coil and overheated it and damage the delicate aspects of the pivot and spring. So the Resistors and values used, are dependent on the meters power and current consumption - not so much effort to the accuracy to Bridge Impedance measurements - they only want you to see the power levels and too much reflection can destroy your radio.

• There is enough accuracy in the internal meter for a RATIO which then can be verified by an EXTERNAL SWR meter more dedicated to properly show impedance and SWR - don't sweat details - use the radio for it's purposes intended.
• It's why I mentioned the "Fly By The Seat Of Their Pants" because they knew they would have problems trying to squeeze a good dedicated metering system into the radio for detection and proper display - so they instead use a small known bridge reference for this power flow and it's direction and developed their own version of scaling using it.

So if you wanted to know why there is such a discrepancy, they placed more emphasis on the meters scale and face than keeping the meter and it's measurements true to faith of impedance measurement across a bridge.

The effort they did in keeping the current flow adequate enough to show full scale and measure it properly relied more on the meters face and needle position than the real actual impedance measurement.

So they selected resistors that act more conservatively in performing the task of SWR calibrate and then the reflection - measured across that circuit. They did the general calculations for you, they want you to pay attention to the RATIO of signal (OUT) to reflected (IN). THAT is what is more important, to keep the radio from having it's Finals blown.

 

[1iwilly]

thanks andy well i put a trimmer in r176 it didn't help r177 check good so i'll leave an old dog alone and make a note 1.3 on radio is 1.0 on the diamond sx200 meter

 

[Handy Andy]

Oh you're not out of the woods yet.

The problem may lie in the caps they used to "tidy-up" their Pi filtering and 52MHz trapping back there before the SWR meter.

This isn't any true fault except to show a potential problem as a reflected power you may not always see getting sent out - from an over peaked or mis-tuned section or two...

Not all the power going out is in the CB Band, you can have reflections of the 2nd harmonic arriving back to , and thru the meter - which it will sense and does show this as reflected power as the effort of sensitivity to scale - again, a not true -held faith of display, to "True impedance Measurements Across A Known Reference".

Just be aware of this when you peak and tune radios to meet any amp with it's own input reflection problems in matching them together. It's simply a power flow of current the needle in your radio picks up because of the sensitivity - it's designed this way to be more sensitive to reflected power than Forward power - yet use the Forward power in which to Calibrate it's sensing circuit.