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Cobra 29 High Power mod with 2sc1969 transistor

What I did to make it work, was put all of the parts back in. With the exception of the driver; I put in a 2SC1307 that I had.

Now, should I put in a different bias resistor at R56?
I know this final is capable of getting 5-25 swing. But with the current bias resistor in it - it is doing no more than the stock 4-8 watt swing. Just as anemic as the stock final.
Back to square one - or is it???
 
Rob,

I just thought of something last night while I was working on one of these 29's.

On the tuning coil, did you take a turn off or just pull out the slug? If you removed a turn, don't forget that the coil wire has a coating on it. You have to scrape it off to re-solder it to the pad. If you didn't do that, then the tuning coil isn't working for you.

The other thing is those .2uf monolythic caps...make sure you got the right one, or put the stock ones back in for testing. I bought some .2's a while back, used a couple of 'em and it screwed up the modulation on one radio. When I looked at the values on the caps, I found that the package was mislabeled :mad:
 
moleculo said:
Rob,

I just thought of something last night while I was working on one of these 29's.

On the tuning coil, did you take a turn off or just pull out the slug? If you removed a turn, don't forget that the coil wire has a coating on it. You have to scrape it off to re-solder it to the pad. If you didn't do that, then the tuning coil isn't working for you.

The other thing is those .2uf monolythic caps...make sure you got the right one, or put the stock ones back in for testing. I bought some .2's a while back, used a couple of 'em and it screwed up the modulation on one radio. When I looked at the values on the caps, I found that the package was mislabeled :mad:
Click to expand...

When I looked into this early 29, it didn't have the 5-turn coil. So, no problem there.

I took the tuning slug out as per instruction, and it did worse. So, I put it back in and re-tuned it for peak output.

As far as the caps go - that is another issue. I didn't put them in. Are the .2 monolithic caps supposed to be as big as a match-head in size? The ones I would be taking out were much bigger, so I thought that didn't look right to me. I know we are talking about two different kind of cap construction; but it didn't make sense to me that the difference in size should be so great.

I'm going to have to find a 1969 and a 2314 transistor to do this right. I had some ERF2030's and some IRF520N's that arrived DOA in the mail. Seems that they cannot handle all of the static they get exposed to in the mail. Where is the best place to buy this stuff online? Anyone on this site sell these parts - I will buy some! I can get all kinds of caps, switches, resistors, etc, out here in Silicon Valley; but I haven't found a single place that sells RF finals and drivers. I need some 1969'a, some 2314's, and some 2091/2092's.

I'm also looking to replace the finals in a Uniden Honest Abe. Someone posted a mod here on another thread I started to find mods for that gem I found. I'd put the 2091/2092 diver/final in it as suggested; but where can I get them w/o having to buy more than I need? Same problem with the 29 too. Is there any mod - similar to the Cobra 29 C42 cap/resistor change to improve audio output on the Honest Abe? The mods for the Honest Abe are the same as the Cobra 29 XLR. I need a swing mod for it - too.

I need the parts to work on stuff - who has them for a REASONABLE price? If they have them and can sell me what I need - I will continue to do business with them!
 
I have a few 1969 finals I could part with for cheap. I know for a fact that one works and the other 2-3 should work as they are in radios that did work before I started pulling parts out of them and they turned into parts radios.
 
I ordered 6/2SC1969's and 6/2SC2314's for $13 online on eBay. They are manufactured by Fairchild, so I am not too sure of the quality. Especially considering the price. But I will give them a shot.

What I cannot understand about this mod as published, is how can a single 1969 transistor put out 50 watts? It is only rated for 25 watts - so how does that work? Is the 2314 acting more as a final than a driver - and thereby paralelling the output for the added power? Like I said, I'm a newbie to this - so don't be too harsh with the criticism!

If people are getting 50 watts from this mod. so be it. When I get the parts, I will put them in and give a report here. The eBay source may play out and be a good, inexpensive source for the hard-to-get 1969's. I'll be using a Dosy meter for the numbers I publish, so keep that in mind...
 
I ordered a bunch of 1969's and 2314's off ebay from a supplier in Hong Kong. The first combination of transistors I tried was bad. I hope you don't have the same problem.

Here's why you can get that much power out of a 1969 on an AM radio. This was posted by NomadRadio a while back. He is one good tech...I hope he comes back one day:

an AM-only radio uses a voltage step-up transformer to modulate the final and driver. This roughly doubles the peak voltage FED INTO the final and driver. If you fed steady direct current into this radio's power stages, 28 Volts DC would get you a 45 to 50-Watt carrier. But not for long. A steady carrier that large would be very brief. Since every (28-Volt) voice peak is followed by a (zero-Volts) voice VALLEY, the stress on the final is averaged out, and it will deliver 40-plus peak Watts. Since this remains only 10 to 12 Watts AVERAGE power, the final will take it just fine. Modern SSB radios do NOT contain that step-up transformer, and will only give you about half as much PEP per transistor.
Click to expand...
 
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Were ALL of the Hong Kong transistors bad - or just a few? If I can expect a few bad ones, and the rest are good - that is still a good deal. Right?

The one thing you can do with a bipolar transistor that you can't with a MOSFET - is test them with an ohm meter. The MOSFET's need a minimum voltage before they turn on, so it takes a bit more effort to test them. I had a bad experience buying ERF2030's and IRF520N's on ebay - all were bad...
 
I have too many of them to test all at once.

I have a multimeter with a diode tester that you can use to do a basic trasnsitor test. Both transistors that I had tested good on that, but apparently had no gain. That tells me that these transistor may really be good, but may be something else that they just silk-screened the part number on :( I won't be able to tell until I have a reason to try a few more.
 
I was looking for alternatives in bipolar transistors in the TO-220 package.
The most important specification is the "gain bandwidth product", and must be far above 27MHz to be useful. The others are the power dissipation rating and max. collector current.

One item that comes up is the On-semi MJF44H11. Its worth looking at because its gain-bandwith-product is at 50MHz, and is an 80v part with 10A current rating, 36 watts.

Similarly from fairchild, there is the KSE44H. 50MHz for unity gain, 50 watt @ 25C, 10 amps.

I'm sure theres more, just takes time to look for them.
 
Me again- go to Grandata Online -they have 14 available as of 30/04/2009 20:40 GMT...At the fantastic price of £1.60...which I think is around $2.30 ? + post & packing....does'nt mention posting to the states...if you have any trouble getting these RF Output transistors either over there or here in the UK let me know and I could send for them here and post them on to you Guys over there in the good old US of A...Noooo problem, just give me a nod...Dave ( Near Manchester,England)
 
switch 2sc1969 with 2sc2312

Can i get more power if i switch 2sc1969 with 2sc2312? 2sc2312 have Pmax 25w. Also the Pin is 1.5 watt. 2sc1969 have the Pin 1watt. Will be enaugh to use 2sc2314(1.8watt) as a driver for 2sc2312?
 

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