Cobra 148 gtl DX same power on all modes

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
mutley
Regular
Regular
Posts: 77
Joined: 11 Jun 2010, 14:32

Cobra 148 gtl DX same power on all modes

Post by mutley »

Morning all,

Wondering if anyone has any advice on an issue I have with an old 148 gtl dx.

Symptom is SSB and AM/FM are all roughly the same power output.

The ALC control does work, as does everything else really, just can't get more than whatever I set AM as for SSB.

The radio was a auction site scrappy special, parts or not working, and when it arrived, the AM/FM was cranked to max (maybe this fault was already present), final blown, and regulator not too happy either.. so replaced them and now AM can be turned up to 9 watts or more, so think all that stuff is healthy.

I did read about a fault with a burned out track that can cause this symptom after a reverse polarity, but I don't think this radio has had that and I can't see any burned tracks or components.

Is anyone aware of this fault and have any areas to take a look at?

Cheers
User avatar
cb4ever104
Legend
Legend
Posts: 6551
Joined: 28 Jul 2011, 21:26
Location: Alicante , España

Re: Cobra 148 gtl DX same power on all modes

Post by cb4ever104 »

What's the voltage on the collector of the driver/final on SSB ? You should have about 5V on AM/FM and 13.8 on SSB .
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.
mutley
Regular
Regular
Posts: 77
Joined: 11 Jun 2010, 14:32

Re: Cobra 148 gtl DX same power on all modes

Post by mutley »

cb4ever104 wrote: 10 Jun 2022, 07:20 What's the voltage on the collector of the driver/final on SSB ? You should have about 5V on AM/FM and 13.8 on SSB .
Thanks for the reply

About 5.5V on both AM and FM so guess that's the reason for the AM/FM level power output on SSB, just not sure why and I don't think I've had it before where AM power control affects SSB before.

Cheers
mutley
Regular
Regular
Posts: 77
Joined: 11 Jun 2010, 14:32

Re: Cobra 148 gtl DX same power on all modes

Post by mutley »

whoops, I just measured again, properly.

5.5V AM, 9V SSB. Still not full voltage though....

Cheers

Nick
User avatar
cb4ever104
Legend
Legend
Posts: 6551
Joined: 28 Jul 2011, 21:26
Location: Alicante , España

Re: Cobra 148 gtl DX same power on all modes

Post by cb4ever104 »

You should be getting the full 13.8V on SSB on the collectors . I presume it's a mk 2 ? If so , then check TR52 first , to make sure that it's switching as it should , and then TR51 . There isn't really much more to that circuit .
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.
mutley
Regular
Regular
Posts: 77
Joined: 11 Jun 2010, 14:32

Re: Cobra 148 gtl DX same power on all modes

Post by mutley »

Thanks Sammy,

Will check in the morning and post back.

Cheers

Nick
User avatar
ch25
Top Poster
Top Poster
Posts: 1673
Joined: 03 Dec 2016, 11:07
Call Sign: Lemmy
Location: Poland

Re: Cobra 148 gtl DX same power on all modes

Post by ch25 »

cb4ever104 wrote: 10 Jun 2022, 08:17You should be getting the full 13.8V on SSB on the collectors
Wrong. It should be PS voltage minus V drop (0.7V) on C-E junction.
I bet there is a drop on fuse and lousy DC connector too.
Chris
WE ARE MOTÖRHEAD, AND WE PLAY ROCK N' ROLL
You can't have too many antennas...
User avatar
cb4ever104
Legend
Legend
Posts: 6551
Joined: 28 Jul 2011, 21:26
Location: Alicante , España

Re: Cobra 148 gtl DX same power on all modes

Post by cb4ever104 »

ch25 wrote: 10 Jun 2022, 19:28
cb4ever104 wrote: 10 Jun 2022, 08:17You should be getting the full 13.8V on SSB on the collectors
Wrong. It should be PS voltage minus V drop (0.7V) on C-E junction.
I bet there is a drop on fuse and lousy DC connector too.
Chris
My calculations were made assuming that his PSU was outputting 14.5V , so my statement is correct.
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.
User avatar
ch25
Top Poster
Top Poster
Posts: 1673
Joined: 03 Dec 2016, 11:07
Call Sign: Lemmy
Location: Poland

Re: Cobra 148 gtl DX same power on all modes

Post by ch25 »

:thumbup: :thumbup:
WE ARE MOTÖRHEAD, AND WE PLAY ROCK N' ROLL
You can't have too many antennas...
mutley
Regular
Regular
Posts: 77
Joined: 11 Jun 2010, 14:32

Re: Cobra 148 gtl DX same power on all modes

Post by mutley »

Thanks guys. All sorted.

someone had stuck a different transistor in TR52 and it was dead. Replaced and putting out what it should.

Cheers!
User avatar
cb4ever104
Legend
Legend
Posts: 6551
Joined: 28 Jul 2011, 21:26
Location: Alicante , España

Re: Cobra 148 gtl DX same power on all modes

Post by cb4ever104 »

mutley wrote: 11 Jun 2022, 11:06 Thanks guys. All sorted.

someone had stuck a different transistor in TR52 and it was dead. Replaced and putting out what it should.

Cheers!
:thumbup:
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.
Post Reply