Cobra 148 GTL 455 filter.

[Low_Boy]

I have a Cobra 148 GTL older Malaysia version it receives and transmits very high-pitched on upper side band and lower-pitched to normal on Lower Side Band. I checked this also with the antenna unplugged going from upper to lower like Nomad said to do for another radio and I still get the same sound very tinny on upper very Bassy on Lower Side Band. Would the 455 filter cause this?

 

[kaos513]

I have a Cobra 148 GTL older Malaysia version it receives and transmits very high-pitched on upper side band and lower-pitched to normal on Lower Side Band. I checked this also with the antenna unplugged going from upper to lower like Nomad said to do for another radio and I still get the same sound very tinny on upper very Bassy on Lower Side Band. Would the 455 filter cause this?

ill start off by saying is the radio recapped?

 

[bob85]

Yes you can end up with darth vader vs pinky& perky on ssb if the ssb filter (not 455 IF filter ) center frequency is not close to what it should be & you set the oscillators to service manual specs, i have come across faulty filters but id look elsewhere to start with,

set the oscillators to spec & check voltages to make sure you don't have other components pulling voltage supply down,

a faulty filter would be low on my list of likely candidates but it can happen, especially on hand built filters like early 148 uses.

 

[sonoma]

Hi Low Boy, have you aligned the receive portion of the radio and have you checked by turning the clarifier if the pitch changes on each as it should. also is it TXing properly to another radio? does AM work as it should.

 

[Low_Boy]

The radio has been aligned. It's a Malaysia version so all caps are rated 16 volts or above. I did not listen to the pitch by moving the clarifier. Also this different sound is also on the receiving end tinny sounding on upper and Basie sounding on Lower.

 

[sonoma]

if they are stock caps you may still have one or 2 going bad just from age. the esr rating may have gone up from age. I have a grant LT that was having noise on LSB receive and I changed all the caps in the RX area and it went away.. just some thoughts.

maybe Nomad will chime in on this one also.

 

[kopcicle]

The radio has been aligned. It's a Malaysia version so all caps are rated 16 volts or above. I did not listen to the pitch by moving the clarifier. Also this different sound is also on the receiving end tinny sounding on upper and Basie sounding on Lower.

The radio has not been properly aligned. If the carrier oscillators were equally distant from the filter center there would be no difference in USB to LSB "tone".
The issue is the tune up procedure lists the ideal case where the filter is expected to be just this wide and centered right here. In practice they are not.
A real tune up will pick the sideband that most sounds natural and measure it's distance from filter center by either plotting the filter with syn/gen and calibrated attenuators or sweeping it with a spectrum analyzer generator.
I usually find carrier oscillators in the 1.4KHz offset to have more bass , 1.8KHz to have more treble. If you don't know what the filter passband looks like and exactly where it is you don't have a chance. Again the numbers 7.7975 and 7.8025 are only typical and not ideal set in stone numbers.

 

[Shadetree Mechanic]

The radio has not been properly aligned. If the carrier oscillators were equally distant from the filter center there would be no difference in USB to LSB "tone".
The issue is the tune up procedure lists the ideal case where the filter is expected to be just this wide and centered right here. In practice they are not.
A real tune up will pick the sideband that most sounds natural and measure it's distance from filter center by either plotting the filter with syn/gen and calibrated attenuators or sweeping it with a spectrum analyzer generator.
I usually find carrier oscillators in the 1.4KHz offset to have more bass , 1.8KHz to have more treble. If you don't know what the filter passband looks like and exactly where it is you don't have a chance. Again the numbers 7.7975 and 7.8025 are only typical and not ideal set in stone numbers.

Would it be possible to adjust it by ear so it sounds normal? Sorry for the noob question. I really have no idea.

 

[Low_Boy]

Kopcicle, I just double checked my work. USB, CT2,, 7.8015MHz +5Hz, -0Hz set right on the money at 7.8015MHz

LSB L30,, 7.7985MHz, +0Hz, -5.Hz Right on the money 7.7985MHz.
This part is a bit above my knowledge. A real tune up will pick the sideband that most sounds natural and measure it's distance from filter center by either plotting the filter with syn/gen and calibrated attenuators or sweeping it with a spectrum analyzer generator.

Bob85, I am not sure what voltages would be giving me trouble?

 

[bob85]

voltage that feeds the oscillators,
if the fiter is not too far away from center you can use your ears to get in the ball park tone wise & adjust the opposite sideband for similar sound as Nomad told you,

if you don't have radio ears you can do it the old way with a watt meter & audio sig gen,
pull up an old yaesu service manual on the www for instructions on how to do it with a watt meter,

very rarely does aligning ssb cb's to the service manual oscillator settings give the best sound or align the radio as it should be with reasonably equal frequency response on both sidebands,

that would be like giving you a capacitance value setting for a crystal trimmer & expecting you will be on frequency.

 

[loosecannon]

Are your VCO cans also on freq?

remember that what you hear out of the speaker is the 34mhz VCO signal, mixed with the 7.8mhz signal.
so, if one of the two settings is off, the output freq will be off also.

also, are you sure that your freq counter didn't load down the IF circuit?

that signal is at a pretty low level, and can be tricky to sample.
LC

 

[Low_Boy]

Bob ..thinks I will try that later or tomorrow never did it that way before.
Loosecannon... I've tuned up many 148 probably not as many as some but I have never had this problem with the one of them I've been tuning them up the same way using the same equipment never had one like this before. I did buy this one for parts not working so who knows what else could be wrong with it besides a couple things I did fix it does looks like new LOL. And it also has not been touched no mods whatsoever big plus.

 

[loosecannon]

That's what i figured Low Boy, i know you know your way around this chassis.

Just to try something, disconnect the bias wires from the driver and final, turn the mic gain up, radio on LSB or USB, channel 20, clarifier centered.

Transmit a known accurate 1K tone from the mic, and measure the output freq at the antenna jack.
What freqs do you get on each sideband?
LC

 

[kopcicle]

http://www.w7ekb.com/glowbugs/ModsEtc/Optimizing%20the%20HW101.pdf

Pg 7 starts "I.F. Filter Passband Improvements:"

Rather than reproduce it here and change the IF and carrier oscillator frequencies to match I'll leave the math to you.

In short, yes you can do this by ear, for the initial diagnosis.
Then you need to place the opposite carrier oscillator where it belongs.

It's not rocket surgery but it is brain science. Read and understand what the process is (ignoring the frequencies) before you start and you may not have to do it multiple times (like me) to get it right. I tend to miss a sign, decimal place, transpose numbers or something equally brain dead. I know what I want as a result so I eventually get it right. Every once in a while I get it the first time (when it's not my radio) and feel like I'm cheating the customer (until I remember all the times it took me hours to get it right).

Just the one rule of thumb with a caveat. Get the carrier too far away and it gets not only buried in the hiss but is annoying to the point of migraines . Get it too close and the signal gets muddied not only by the bass response but the noise blanker and the AGC loop acting on cycles below 180Hz that do nothing for intelligibility but hammer the AGC.

Most any SO42P/AN612 8719/2824C chassis can benefit from the 1N5817 mod but if the radio is aligned as in the above it will be hard to tell. The radio's internal noise floor will be a bit lower and the detected signal will be a bit higher allowing you to avoid some of the inherent distortion of the amp-on-chip audio mess that accompanies and cripples all these radios. In the end it's a wash. My equipment can tell the difference but in practice my ears can't .

Thought I'd include some personal truth to the process so I look like the hack I am, so it isn't so daunting a task to a novice, as an apology to all those I charged real dollars, and possibly make someone smile.

Many Thanks to Mark, WB8JKR for the compiled knowledge that I've used so often (credit where credit is due ya know)

Bored yet ?

 

[Low_Boy]

Loosecannon I am just firing up the radio's been on for a few hours so it's nice and warm.
USB 27.2059 MHz
LAB 27.2038 MHz
I am imagining SUB should be 27.2056 KHz and LSB 27.2044 MHz