Cobra 148 GTL-DX MK1 Help Please.

A forum for the discussion of matters of a technical nature. All such activities are undertaken at the readers discretion and own risk. If you don't know what you are doing, don't blame us if it all goes wrong!
Post Reply
User avatar
Retrodog
Super Member
Super Member
Posts: 401
Joined: 19 Jan 2014, 17:15
Call Sign: 26SW07

Cobra 148 GTL-DX MK1 Help Please.

Post by Retrodog » 20 Jan 2015, 11:40

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
Top Poster
Top Poster
Posts: 1168
Joined: 19 Feb 2008, 00:37

Re: Cobra 148 GTL-DX MK1 Help Please.

Post by wa10 » 20 Jan 2015, 12:35

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.

User avatar
Retrodog
Super Member
Super Member
Posts: 401
Joined: 19 Jan 2014, 17:15
Call Sign: 26SW07

Re: Cobra 148 GTL-DX MK1 Help Please.

Post by Retrodog » 20 Jan 2015, 14:39

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.

User avatar
andymuza
Top Poster
Top Poster
Posts: 2071
Joined: 05 Oct 2014, 22:19

Re: Cobra 148 GTL-DX MK1 Help Please.

Post by andymuza » 20 Jan 2015, 15:13

Have you connected it to a frequency counter to make sure it is actually on the correct frequency?
I will not be commanded, I will not be controlled.
n+1 the formula for working out how many radios you need with n being the current amount.

User avatar
Retrodog
Super Member
Super Member
Posts: 401
Joined: 19 Jan 2014, 17:15
Call Sign: 26SW07

Re: Cobra 148 GTL-DX MK1 Help Please.

Post by Retrodog » 20 Jan 2015, 15:49

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.

wa10
Top Poster
Top Poster
Posts: 1168
Joined: 19 Feb 2008, 00:37

Re: Cobra 148 GTL-DX MK1 Help Please.

Post by wa10 » 20 Jan 2015, 21:32

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.

User avatar
Retrodog
Super Member
Super Member
Posts: 401
Joined: 19 Jan 2014, 17:15
Call Sign: 26SW07

Re: Cobra 148 GTL-DX MK1 Help Please.

Post by Retrodog » 21 Jan 2015, 07:57

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.

User avatar
Mudslinger
Legend
Legend
Posts: 6744
Joined: 28 Jul 2010, 07:11
Location: Oxfordshire

Re: Cobra 148 GTL-DX MK1 Help Please.

Post by Mudslinger » 21 Jan 2015, 08:12

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? ;)
Mudslinger - Simon
26TM274
26CT612

There is no such thing as a Fidelity 1000 which is too good to scrap.

wizzer2k
Regular
Regular
Posts: 60
Joined: 24 Jul 2010, 21:03

Re: Cobra 148 GTL-DX MK1 Help Please.

Post by wizzer2k » 08 Nov 2017, 15:23

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

User avatar
cb4ever104
Veteran
Veteran
Posts: 4465
Joined: 28 Jul 2011, 21:26
Location: Alicante , España

Re: Cobra 148 GTL-DX MK1 Help Please.

Post by cb4ever104 » 12 Nov 2017, 15:38

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

Cheers
30TM060
Paul aka "Sammy". (108) PY60 Old Paisley DX Group (RIP Rab Markie)
Ex GM0 , now EA5 .
Handle "Fortune Hunter" back in "the day"
Radios ....? Anything that says "TAIWAN R.O.C" on the back.

wizzer2k
Regular
Regular
Posts: 60
Joined: 24 Jul 2010, 21:03

Re: Cobra 148 GTL-DX MK1 Help Please.

Post by wizzer2k » 29 Dec 2017, 20:16

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

Post Reply

Who is online

Users browsing this forum: No registered users and 3 guests