Cobra 29 High Power mod with 2sc1969 transistor

[AudioShockwav]

399 , were are you getting the 25 watts peak figure from?

73
Jeff

 

[kopcicle]

...yeah but 4 times 12 is 48, right ?

 

[kopcicle]

399 , were are you getting the 25 watts peak figure from?

73
Jeff

My guess would be experience. About the same as mine. I've seen 12w CW out of a 2SC1969, easy. That would lead one to believe that 4 times that carrier via 100% amplitude modulation would be possible. It isn't. I could work out a plausible reason from the limited white paper documentation available ( might even anyway) but a fools assumption (mine) is that when you add up total dissipation it goes pop.

 

[AudioShockwav]

I don't agree that data sheets are rock hard limits to transistor output figures.
A very good example of this is the Toshiba 2sc2879.
The data sheets show a max output of 120 watts and we all know that this has been proven wrong many times over.
Data sheet test jigs are not optimized to run the devices at maximum output.
I agree with Nomad and Tim that in a AM only radio the 1969 is capable of more than 25 watts peak.
I wish Tim had posted the bird 43P readings and spectrum analyzer results that were part of the original mod.

73
Jeff

 

[AudioShockwav]

From Tom
W8JI
A respected and well known engineer.

It is common for people to think data sheet numbers are all hard limits, but a large amount of the data in sheets is NOT a hard limit.

A Zener diode would be a hard limit for voltage, but not for current or dissipation. The part might actually handle two or three times the rated current or dissipation before failing.

Resistors are the same. The resistance is absolute, but the dissipation or voltage rating (yes, they have voltage limits) is not. A 2 watt metal resistor might stand 5 watts for months or years before failing. A 2 watt resistor, like a carbon comp, might fail in a few thousand hours at 75% percent of rated dissipation. This is why standard resistors make about the worse fuse in the world, but some people think they are fuses that pop open with a modest overload.

Tubes and transistors are no exception. If you look at all the data sheets for the MRF150 you won't find it outputs 600 watts per transistor anywhere, but in pulse service it can. I built a medical device that ran over a kilowatt on two MRF-150's and they last years and years in the field.

A single 3-500Z can output several kW in low duty cycle operation, and can run 150% of rated anode current for years without accelerated failures, but the data sheet doesn't say it can do that.

73 Tom

73
Jeff

 

[711]

I've tested 2SC1969, 2SC1307, 2SC2312 to destruction .
I have never seen any useful power over 18W for any significant period of time with anything close to acceptable IMD or even approaching a linear amplification characteristic.

If you want more from a 29* Talk to Nomad about a peel and stick carrier control and buy, build, bowwow, steal a broadband 2 device BJT amplifier. A 2SC2290/2SC455 pair will accept 4W a 2SC454 pair a bit more and the 2SC2879 still more but do read the data sheet.

As for RM- Spaghetti and their fustercluck of plastic cased FET's of dubious origin you are on your own. In the time it took to write this 4.5 more of them died an instantaneous death due to force feeding their input with a steroid saturated radio.

There is one easy way to get a few watts out of many radios if you actually understand RF I/O .

View attachment 28460

Yes it's a HR-2510. So what ? It's driven with a 2SC2166 as well. It does have somewhat more heatsink area than a '29 but if you're doing these hotrod mods you already installed some sort of additional heatsink. (you did , didn't you? )

Oh ? You can't find 2290's, 455's, 454's 2879's or even the 2SC1969 anymore? (at least not reliably and economically)
Welcome to the new century. You have about 20 years of catching up to do.

TANSTAAFL

MosFets, LdMos, and a correctly aligned dilithium matrix are the new millennium boys and girls. Get on board or get left behind....

I just bought some new 455,1969 ,2312 and 2879 all made by HG. All are same as original and sold by a few different sources now. Just an FYI. I'm gonna build the same Lincoln board for my bad final 2510's I have laying around

 

[543_Dallas]

The data sheet is based on the device being used in a linear amplifier. The cobra 29 uses high level modulation. The final is ran in class C and the collector is modulated. Modulator power is where those impossible watts come from.

Look at what the 6146 is rated for when used in an ab biased rf amp. 20 watts CCS and 25 watts ICAS. This is the final used in the Johnson Viking Ranger. That rig makes about 40 watts of AM carrier and a little over 150 pep. How'd they get that from a 25 watt tube? How have all of those 6146s survived the old buzzard transmissions?

 

[Shadetree Mechanic]

Bump. Good info on the 29. I have one that I am goofing around with. The final is still good so I won't be replacing it, this is still an interesting read.

 

[Coolhand Luke]

Regarding the question #44. I looked all over this session on the Cobra 29 mod and can't find the answer to this. What side of the diode "D11" do you put the 1K resistor? Anyone know???

 

[Handy Andy]

Either side, Banded end or unbanded.

It simply reduces current thru the Diode - forcing the AMC to "lag behind"

Just has to be at D11. Any other holes or spots is not where that mod goes.

​

 

[Coolhand Luke]

Handy Andy, Thanks for the fast reply. Got all the parts and will try this mod. There are many mods for the Cobra 29 and this one looks to be the best. Hope it works. I will post with the results when I get it done. Again, Thanks........

 

[Jason Ekstrom]

Although the mod sheet provided points out the use of the 1K ohm resistor with D11, it doesn't say if the resistor goes to the right of the diode banding. That is, looking from the front of the radio twards the back. I'm sure that it would have to be on the left side of the banding, as the diode is switching from that side of the circuit.

If I'm wrong, please correct me before I start with that mod. My plan is to do one mod at a time. I'm going to do the MOSFET final mod first, then the TX/RX audio enhancement with the .2 monolythic/33K ohm, then the 3300mf cap/variable pot removal/1K ohm resistor replacement.
But-
Isn't it redundant to put a 1K ohm resistor in place of the variable pot AND the 1K resistor in series with D11 diode?
I thought the variable pot WAS feeding D11 - or is that wrong?

Q: Why wouldn't the addition of the 2SC2314 work? Doesn't the MOSFET final get fed from the stock bipolar driver as well?
So, if the MOSFET gets a bigger boost from a better driver; wouldn't that potentially drive the MOSFET final harder?
Or - would it be too much? Especially since I am using a IRF520N as Exit 13 suggests? But I can use either the ERF2030 or the IRF520N; as I have both. And either MOSFET can be driven with the same EN1230 - rrrright? My point in saying this is purely speculative and needs correction - that is why I saying it here to be examined and considered.
What's right/wrong with these ideas?
Please comment!

BTW - Does anyone know when CB Tricks will be fully operational again? They still don't have everything available like they used to. That info provided from that site is indispensible and essential. It's too bad someone wrecked it for everybody else...

Does anyone prefer a 1969 over mosfet vice versa? I have an original used 69 or two laying around but wonder what's best... Thanks

 

[Robb]

As you've read, the 69 isn't exactly a drop-in replacement.
Neither is the mosfet.
But I would put in a mosfet - and save the 69 for a radio that will need it, since they are hard to find now.

 

[Chipper]

Has anyone tried the latest 2030+ yet besides me it must need a different bias setup from the 520 mosfet with a 470 a diode and a 3.3K to run it cause it only did 5 watts pep pulled it out and dropped a 520 in and 35 watts output then

 

[Coolhand Luke]

Regarding the Final & Driver - Cobra 29. I see awhile back someone asked where they are, but no answer. I believe that looking from the front at the alum tri heat sink - back left - the Final is in the middle facing the front. Is that the driver on the left corner facing the back ? They don't even show them on all of the prints of the board. By the way, this one is China W/ 5 coils & the board is # KEPC 1106-K.