| Author |
Message |
    
Adshar64
| | Posted on Sunday, July 06, 2003 - 4:59 am: | 
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13.5 Volt mod cobra2000 pre-driver procedure developed by adshar64
Recommended if you cant achieve more than 18watts output, due to
broadbanding etc. This mod greatly helped power out over wide
frequency range and keeps a good drive to the driver. Modulation
tests proved good and looked great on scope. Modulation response is
greatly improved with higher average modulation achieved at same peak
levels.
See Photo's of mod in photo's section.
http://groups.yahoo.com/group/Cobra_2000/
Procedure;
Find L44 near TR39 pre-driver at back left side pcb. JP35 link is
just to the right of this. This wire link feeds the TX 8v to 2SC1973
collector via L44.
Cut this link in centre and lift left side up and solder wire from
here (left side of link) to positive radio supply, 14v at D55 or
other good source. I used feed to final as has had npc-volt mod (14v
feed) already done in this section. I put a 22ohm 5w resistor in
series with this wire as a safeguard and limiter as I run this radio
at 15v anyhow. This gave me 13.5 volts to pre-driver collector. Leave
the R185 8v switched supply to pre-driver base circuit as is.
All done, quite easy this mod, and is done from top of pcb. On-air
modulation reports are the best I've had 5/5 with dynamic gain set at
2 o'clock.
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Kc0gxz
| | Posted on Tuesday, July 08, 2003 - 2:12 pm: |
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Adshar64
Are you saying that the NPC mod needs to be done before your pre-driver will work?
Your mod really sounds interesting. I have a couple of old Grants here that I use as experimental radios and one of them I did the NPC mod to with good results.
Question....
Do you know what the results of your Pre-driver mod will be on a radio that has NOT had the NPC mod as compared to one that has?
And how much life can be expected from the 1973 after the mod is performed?
I'd love to try this mod. Thank you for sharing it with us.
73s
Jeff, kc0gxz. |
Adshar64
| | Posted on Tuesday, July 08, 2003 - 11:00 pm: |
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OK the 2SC1973 stands up well to this mod and have tested it riguously.The npc mod is recommended to make the most out of this mod on am but will improve performance anyhow. the 2sc1973 runs well under rating at 8v. At 12v the difference is significant. The peak out will increase by ~20% or final saturation level.The average modulation will increase by ~50% and you will really see big improvement when driving a amp.Check modulation on a scope and the waveform is great.You can overmodulate as always if you set mike gain too high or gain on a power mic, so set that up and you are ok.22 - 20 ohm 5w inline resistor recommended for safety and performance.With collector over 13.5 volts the performance drops off. Depends on where your power supply runs, mine is at 15v.Just check voltage on collector side of resistor during transmit to check.You will notice very responsive swing where it may have been lazy before. you will note a big output improvement on distant frequencies.
photo of mod before wiring tidy up at
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Kc0gxz
| | Posted on Wednesday, July 09, 2003 - 12:14 am: |
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Adshar64
Another question if I may.
Can you think of any reason why your pre-drive mod wouldn't work on any other sideband radio after the NPC has been done?
Thank you for your time. I really appreciate this.
Jeff, kc0gxz. |
Adshar64
| | Posted on Wednesday, July 09, 2003 - 2:58 pm: |
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Not really, depends on the existing volt supply to pre-driver collector in those circuits and the device used. This is a mod I picked up on after looking hard at the schematic to improve gain in the rf chain. Let me know your results when you do the mod
cheers |
Adshar64
| | Posted on Thursday, July 17, 2003 - 7:37 am: |
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How did it go? |
Kc0gxz
| | Posted on Friday, July 18, 2003 - 2:07 pm: |
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Adshar64
I have two Grants here that I like to experiment with. I performed your mod on both of them.
The one without the PNC mod only showed a little higher wattage. Nothing to really get excited about since the Grant is very limited in power anyways. Audio wise, it seemed to let the radio over-modulate easier since the limiter is already cut.
The one with the NPC mod showed a improvement in power on the band edges in both SSB and AM mode. As far as audio goes, I didn't see much of a difference on the scope when in the AM mode since the NPC mod seems to already bring the best out of the radio WITHOUT the limiter being cut.
I really find your predrive mod interesting and I want to play with it more when I can devote more time to it. I am very curious to see what it will do (if anything) in a dual final radio. Or have you already tried that?
LOL. Who would ever think of volting the predrive other than you? I thank you for peaking my curiousity. I needed that. And I'm looking forward to "playing" with it some more.
Jeff, kc0gxz. |
Adshar64
| | Posted on Friday, July 18, 2003 - 6:00 pm: |
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That's great, yes the main advantage was a responsive higher average modulation and better endband operation, It's so easy to do , so worth it in my opinion.I think you need to do the so42p broadband mod and an612 mod to bring out the best with it.I dont know if you joined the 2000 group,
but all the info is there in files section.
cheers
|
Sdc1342
New member
Username: Sdc1342
Post Number: 2
Registered: 12-2003
| | Posted on Thursday, March 11, 2004 - 2:19 pm: |
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hello adshar64 im new to the board and just did your pre driver mod to my robyn sb-520d with nice results. my driver is 2312 and my final also 2312 on the bench i see 4wt dk swing to 8/9 on a dummy load thats average i dont beleive in pep. i also own a magnum 257 with 2312 driver and matched pair 2312 do you know if this mod can be done to my mag or any other npc mods help please! thanx |
