Cobra 148 GTL-DX MK1 Help Please.

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Retrodog
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Cobra 148 GTL-DX MK1 Help Please.

Post by Retrodog »

I wonder if someone could advise me please on an issue I’m having with my MK1 Cobra 148 GTL-DX.
On FM the RX is a little odd. I’m aware of the low “S” meter reading and the info about FM tx audio being poor but this is a little different.

When I tune the rig to any desired FM signal the receive audio is distorted and the S meter moves around lots whilst the station I’m listening to is talking.
Now you may assume that I have not got the Coarse tune set right but it is, when I tune slightly off frequency the signal strength drops right off but the FM receive audio sounds great.

So to recap, if I tune the rig to an FM carrier and it peaks say at S8 on the meter, once that signal starts to modulate it distorts wildly and the S meter moves around wildly, if I tune slightly off frequency the signal strength drops but stays constant and the rx audio is good.

Thing is the signal sounds better at 1.5khz away from where it should be and I can’t understand why.
I checked the FM discriminator and can make no improvement; in fact anything I do degrades the rx audio quality. I have 3 MK 2’s and I am use to using the Coarse tune to get the best sounding FM signal which usually corresponds with the highest signal strength.
This MK1 does not behave this way at all. Are all MK1’s like this or has mine got a problem.

Thanks,
Dave.
wa10
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Re: Cobra 148 GTL-DX MK1 Help Please.

Post by wa10 »

if i remember correctly the s-meter on the mk1 meters all modes through the ssb filter same as the stalker9dx/fdx,
you get 3 signal peaks, none of them are on frequency, when you adjust for correct frequency the meter drops off the peak and bounces as if the guy at the other end is over deviating,

mk2's fixed that and other issues.
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Retrodog
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Re: Cobra 148 GTL-DX MK1 Help Please.

Post by Retrodog »

wa10 wrote:if i remember correctly the s-meter on the mk1 meters all modes through the ssb filter same as the stalker9dx/fdx,
you get 3 signal peaks, none of them are on frequency, when you adjust for correct frequency the meter drops off the peak and bounces as if the guy at the other end is over deviating,

mk2's fixed that and other issues.
Thanks,
The meter behaves all as you say but also on the correct frequency the signal sounds seriously distorted too,
Almost as if the deviation on the signal is way over the top. I had a MK1 many years ago but don't remember it being like this.

Thanks for your help, I didn't buy it for FM use but having tried the thing it does seem very odd.
I need to listen to another one alongside I guess just to see if it is a characteristic or mine has something else going on.
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andymuza
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Re: Cobra 148 GTL-DX MK1 Help Please.

Post by andymuza »

Have you connected it to a frequency counter to make sure it is actually on the correct frequency?
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Re: Cobra 148 GTL-DX MK1 Help Please.

Post by Retrodog »

andymuza wrote:Have you connected it to a frequency counter to make sure it is actually on the correct frequency?
Hi Andy,
Yes I have, and on RX/TX it is where it should be, AM and SSB are 100% ok BTW.
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Re: Cobra 148 GTL-DX MK1 Help Please.

Post by wa10 »

retrodog,
if you are sure the quadrature is set correctly its worth making sure nobody has replaced the ceramic filter with a crystal filter and not impedance matched it with two resistors,
terminating filters with a low impedance causes the filter to be too sharp and peaky,

good luck.
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Re: Cobra 148 GTL-DX MK1 Help Please.

Post by Retrodog »

wa10 wrote:retrodog,
if you are sure the quadrature is set correctly its worth making sure nobody has replaced the ceramic filter with a crystal filter and not impedance matched it with two resistors,
terminating filters with a low impedance causes the filter to be too sharp and peaky,

good luck.
Thanks, I had a look last night, and can confirm it's the original filter,
I spent some time on air with it last night too, it is possible to find a happy medium and get decent RX just slightly off where it should be. On TX the station I was talking to said it sounded ok set that way so I will see how it goes.

Thank you all for offering some great advice.

Cheers,
Dave.
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Re: Cobra 148 GTL-DX MK1 Help Please.

Post by Mudslinger »

Something isn't right with it for sure.

The Mk1's I have had always sounded really good on FM RX, although they were slightly deaf on that mode.

It is probably something simple like a filed capacitor or something, the trouble will be finding which one.

Maybe a trip to Black Spirit is in order? ;)
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wizzer2k
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Re: Cobra 148 GTL-DX MK1 Help Please.

Post by wizzer2k »

Although an old thread i had one in the shack for repair this week with what is described here and it turned out to be the 455khz filter had gone faulty replaced it with one out of a scrap audioline and job was done just thought i would post for reference in case anyone else experience this fault
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Re: Cobra 148 GTL-DX MK1 Help Please.

Post by cb4ever104 »

Thanks for the info . It'll help people in the future .

Cheers
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Re: Cobra 148 GTL-DX MK1 Help Please.

Post by wizzer2k »

i have also found that if the 10.240 crystal is off it will create this fault outlined above as the 10.240 is mixed in with the fm receive circuit but not on the other modes the pll circuit seems unaffected by the discrepency hence the other modes working fine
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