Cobra 148F GTL DX+

Animal · Post by **Animal** » Saturday 17th May 2008, 7:55

Hi

I'm looking to improve the RX section of a EPT690010Z PCB inside the Cobra 148F GTL DX+

I'm not too bothered about the AM/FM modes but I'd like to increase sensitivity of SSB without increasing the noise floor.

Would replacing the stock AM detection diodes (1N60P) with ECG583 Shottky devices improve things or just affect the AM reception???:?:

Many thanks

ATB

Animal · Post by **Animal** » Sunday 18th May 2008, 21:05

Well, I've managed to find some info about improving the RX on this board.

Change TR(Q)17 from 2SC1674 to 2SC2999

Change D1, D2, D30 & D31 to 1N6263 Shottky diodes

This should improve signal to noise ratio & reduce the 'hash' noise

I'm also replacing TR(Q)54 (the stock part is a 2SB754, which is a 60-watt, 7A power amp device) with the 2SB817 to stabilize the voltage on the board.

ATB

angusuk · Post by **angusuk** » Sunday 1st Jun 2008, 11:40

hi m8,
did u try it if so what did u think to the results.
regards angus

Animal · Post by **Animal** » Sunday 1st Jun 2008, 17:44

Yes, I did modify it & whilst there was an improvement it wasn't dramatic & was more noticeable when trying to copy a very weak station.

If it means the diffence between making that elusive contact and missing it then I'd say it's worth it given the relatively small cost & time involved.

Let me know if you need the components - I've got some spare.

ATB

Steve

Exit_Thirteen · Post by **Exit_Thirteen** » Monday 2nd Jun 2008, 0:11

Just out of curiousity, were you using this radio as a base or as a mobile? I've done the Schottky diode/2SC2999 transistor swap on a few radios, and while it didn't make a real big difference on a base setup, the results were quite impressive for me in my mobile radio, which is a silver faced President Jackson (PB042 board). Cheers!

-Exit

Animal · Post by **Animal** » Monday 2nd Jun 2008, 8:48

Well, I noticed that when I ran the rig mobile I got a virtually flat needle anyway on SSB but at home I get S4 - S5 of hash (probably from next door's plasma screen or wireless router!)

When I'd done the mod, I noticed the hash level dropped to S2 - S3 at home & made it possible to hear weaker stations.

Not sure if I needed to re-peak the receive once I've done the mod???

Exit_Thirteen · Post by **Exit_Thirteen** » Monday 2nd Jun 2008, 9:08

In some cases you do need to do a small re-alignment of the RX, I never did one on my Jackson, and it works fantastic. If you had the time and the tools, it wouldn't hurt to do one, but that is something that is optional.

-Exit

speedracer6g · Post by **speedracer6g** » Monday 2nd Jun 2008, 10:21

You should go through an allignment to get the full benifit of this mod. I have seen good improovemnts with this mod. I always see better results after the allignment though. Cheers

MIKE

angusuk · Post by **angusuk** » Monday 2nd Jun 2008, 10:44

hmm i did this mod today on a ss3900 but doesent seem to do much to me.
btw for superstar 3900 its tr17 d1 d2 d23 d24 that need changing.

speedracer6g · Post by **speedracer6g** » Wednesday 11th Jun 2008, 12:05

You may not want to use the 817 it has lower hfe gain. I would use the 827 in place of the 754. If you take a look at the specs you will see what i mean I would only use the 817 if you are doing alot on fm mode.

MIKE