need some advice on replacing a transistor

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dalbert02
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need some advice on replacing a transistor

Post by dalbert02 » Wed Sep 20, 2006 8:26 am

I have a R/C transmitter that uses a 2SC2314 transistor. According to the ARRL handbook, I am allowed to transmitt with up to one watt of power on 6M. Would anyone have any advice on what transistor I should replace my current output transistor with? Here are the specs on the current transistor:
http://members.tripod.com/Malzev/comp/2sc2314.htm
Thank you kindly for any and all advice,
-Dave
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jwax
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Post by jwax » Wed Sep 20, 2006 5:02 pm

Why would you want to replace a 1 watt device with a different 1 watt device? Am I missing something?

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Bob Scott
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Post by Bob Scott » Wed Sep 20, 2006 7:45 pm

Dalbert,

Your question is very vague. I'm just curious: Is this the 49Mhz CB band?

I have to assume a few things. You are thinking that by changing a transistor in your transmitter that you will be able to increase the transmitted power. It is not that simple. The output power depends on the values and characteristics of every part in the whole circuit, and the entire circuit is tuned like a formula car.

As another analogy, think of replacing the carburetor on your Briggs & Stratton lawn mower with a 750 CFM Holley 4 barrel double pumper. It won't cut grass any faster. It probably won't even start. If it did start and you did manage to double its RPM, the connecting rod would probably shatter.

Good Luck,
Bob :cool:

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Post by dalbert02 » Thu Sep 21, 2006 3:50 am

This is in the 6m band, 50.860Mhz to be precise.

The current output is less then 100mW, I want to use the full 1W I am legally entitled to.

The car engine analogy works for me. I didn't realize the 'system' approach to it, but I understand now, thanks! I guess it is a bit more involved then I imagined.

So, is it best I look for schematics on building an rf amp?

Any suggestions?

Thanks again,
-dave
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haklesup
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Post by haklesup » Thu Sep 21, 2006 4:37 am

The first step is to reverse engineer the board to derive an as built schematic. From there you will need to figure out how to change the bias on the output transistor. This will involve changing at least a few resistors. Hope its a 2 sided board or the reverse eng will require a lot of probing for continuity.

If its a small circuit, get the free version of electronics workbench and simulate it. This makes it easier to experiment with new values and allows you to see if your amplified signal is clipped or distorted. You only need to simulate the output stage.

A less intrusive approach is to build an output amplifier stage (RF buffer). It might be as simple as connecting the output (to antenna) to the base of another output transistor with its own higher current power source and appropriate bias resistors. I've seen this done to boost the power of an IR remote. The original IR LED was replaced by three brighter ones and they were driven by a similar second stage transistor circuit (similar to what we use to interface a relay to an IC output. The same can probably also be done with an Op amp with appropriate freq and power specs. Care must be taken to match the output impedance of the amplifier to the inpedance of the antenna or much of the energy will be reflected and wasted as heat. This is important if you are also changing the antenna to a more efficient one. This is the kind of stuff that makes RF design a challenge.

Naturally increasing the output by 10X will suck batteries dry, so you'll want a P/S or a bigger battery. Might even need a higher voltage which will require additional modification so as not to overbias the original Ckt.

Anyway, if you can obtain a schematic, repost and we'll have a chance of actually giving useful advice.

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Post by dyarker » Thu Sep 21, 2006 6:10 am

"The current output is less then 100mW, I want to use the full 1W I am legally entitled to."

Are you entitled? Just because Hams can use 1W in 6M band, doesn't mean an off the shelf unlicenced remote control transmiter can. Bands are divided into different services with different rules.

Any Ham operator want to pick this up and explain better?

Cheers,
Dale Y

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Chris Smith
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Post by Chris Smith » Thu Sep 21, 2006 9:01 am

Look into the newer RF transistors. [round with X shape leads]

Four wire operation, one is ground, the other is positive, while 3 & 4 are DC in and RF out.

Saves a lot of work.

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Post by dalbert02 » Thu Sep 21, 2006 2:09 pm

dyarker wrote:"The current output is less then 100mW, I want to use the full 1W I am legally entitled to."

Are you entitled? Just because Hams can use 1W in 6M band, doesn't mean an off the shelf unlicenced remote control transmiter can. Bands are divided into different services with different rules.

Any Ham operator want to pick this up and explain better?

Cheers,
So the ARRL (American Radio Relay League), the voice of amateur radio, is incorrect when they say licensed operators may modify and operate r/c devices with up to one watt of power? (The ARRL Handbook for Radio Communications, pg 9.23 & 9.24)?

I am a licensed operator, FCC license KI4HVT

-dave
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jwax
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Post by jwax » Thu Sep 21, 2006 2:50 pm

If that device is only putting out 100 mw, you could use that as a driver for a similar stage using the same type device, since it is rated for a watt output.

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Post by Robert Reed » Thu Sep 21, 2006 5:35 pm

The advice you are getting so far on using your transmitter to drive a power ampl. is probably your easiest bet. 1 to 5 watt RF transistors in the 6 meter frequency range are easy to come by, just Google up most Semi. Manufacturers and weed them out by category for info. Did you say that this device has a rating of 100mw? if so that will be the DC input power to its final amplifier. Maybe 50 milliwatts out to the antenna. Most RF power transistors are rated in DB of gain (RF pwr. in / RF power out). If in fact your now driving source (the transmitter in the subject of this post) is 50 milliwatts and you desire 1 watt of out put, then you need a minimum gain of 13 DB, and this should be easy to accomplish at 6 meters. However you will need to match its output (50 milliwatt) to the input of the new power transistor which will also need a harmonic suppression network and matching network to your antenna. This can be as smple as the coil in the collectors tank circuit adequately tapped down to match the the antennas impedance and then followed by a simple lo-pass harmonic filter on its way out. If you are AM modulating, this new ampl. will have to be of the linear variety. If you are FM or Phase modulating. then class 'C' operation (much more efficient) is the way to go. Some of your circuit design will take some experimenting, but this is what makes the project fun. As to DC power required, you can certainly get by with less than 12 volts if desired. Since you have your "tickets" and probably the knowledge that goes with them, I hope I haven't oversimplified things here. Also the ARRL manual will be loaded with power amps and theory for this subject.

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Post by dalbert02 » Thu Sep 21, 2006 6:14 pm

Thank you Robert, et. al,
The device is FM modulated. It is a Futaba 9CAP transmitter and a buddy with a coaxial dynamics watt meter measured the output at a little less then 100mW. Although I am fairly confident in my electrical skills for building kits and building/repairing power supplies, the RF world is new to me. I bought a few of the ARRL books, and although very good, they don't exactly provide the background necessary to design an amp. As you can see, I though it might have been as easy as a transistor swap, which is quite clearly is not. I appreciate your input, but it looks like I need to learn a lot more before tackling this type of project. Any more thoughts or suggestions on books I should be reading or web pages I should be studying would also be appreciated. From what I can tell, it may also be necessary to have a spectrum analyser to tune the circuit and those don't come too cheap.
Thanks,
-dave

No 2SC1957 - it is a 2SC2314
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Post by dalbert02 » Thu Sep 21, 2006 6:17 pm

Chris Smith wrote:Look into the newer RF transistors. [round with X shape leads]

Four wire operation, one is ground, the other is positive, while 3 & 4 are DC in and RF out.

Saves a lot of work.
Would you happen to have any more info on these so that I may research them?
Thanks!
-dave

PS Is it like this? http://rfparts.com/pdf_docs/57704-68776/57735.pdf
"one carefully controlled experiment is often times worth more than a thousand expert opinions"

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Chris Smith
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Post by Chris Smith » Thu Sep 21, 2006 7:03 pm

Mine go back a ways but now they package them a little different now for cell phones etc.

Looking in the old book they have several type of numbers. This should be a start? ECG345 marine 160 mhz,...ECG339 27 to 50 mhz, ECG360 ammature.

The old list is several pages long. Try searching Global Specs, or just RF transistors. [billions of them!]

These are just a few of the old types, the newer ones are DC in as apposed to some very specific NPN or PNP types. Some also have a huge band width.

They still continue with these devices as Wireless hand sets often boost their wattage using these devices, and even one experiment in N&V mentioned their ease. [5 years back?]

Four terminal RF devices all built in and ready to go.

I had a set of 4 x 100 watt units I was going to pirate some CB gear with.

It sure beat the station tube amps and their heat, even with the top down in winter.

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Post by Robert Reed » Thu Sep 21, 2006 10:03 pm

In looking at your circuit, it may be possible to accomplish what you want as it stands, albeit a few modifications. Q1's input is cut out of the picture so I can't tell whats going on prior to this. If you increased drive from prior stages ,this would boost your output. Also with out knowing the value of L3, I cant tell if it is an RF choke or part of the final's tuned circuit, but in any case, R1 is putting a hefty strain across that resonant circuit. Is this intended to be a broadband device? Also. is it's operating frequency now in the 50 Mhz range?With class 'C' operation , youshould be able to push that transistor close to your output needs. May require some heat sinking if it isn't already. You dont need a spectrum analyser to tune this (although it would be nice), as you can tune it with your friends wattmeter and also check antenna SWR at the same time. Even a simple Rf probe made up along the lines of the Q2 input circuitry would allow you to peak the Tx for max. output (R2 would be reduced to 1K) by monitoring with a DMM at the junction of R2,C2 for highest voltage. As to books and info, there are so many out there, I wouldnt know where to start, Look for something described as 'basic'. The web has a wealth of info in googleland but it can be patchy. Some of the electronic websites have radio circuits that may work for you. The last ARRL manual I bought was 15 years ago, but I found it to be one of the better sources of circuits and general theory. Have they changed that much over the years?

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Post by dyarker » Thu Sep 21, 2006 10:23 pm

Many pardons please, I don't remember seeing the call sign in your posts before, and in the first two it didn't seem like the kind of question a licensed operator would ask. I didn't say the ARRL was wrong, I suspected your interpretation due to what looked like a weakness in technical expertise (it took Bob Scott's explanation that a simple transplant wouldn't work).

You've got good suggestions form other posters, good luck with the project.
Dale Y

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