Just got two radios done and forgot to mention the mosfet deal to the tech. I see radios on youtube all the time doing 40-ish watts out the back with them but wondered, Are they really worth it? Is it still clean? How long do they really last? And other than "more power" are there any benefits to using them?
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Mosfet upgrade? Worth doing or waste of money?
- Thread starter Cruiseomatic
- Start date
if the radio is working and you can not find a stock final then the mosfet is the next step. from 20 to over 30 watts the only person that is going to know it puts out 30 watts is you. the receiving person will not see any difference unless the mod was done wrong and the audio is choppy.
Thats what I was thinking but wanted to make sure. Now I know if you did somehow get an actual 40 watts out basically doubling your output, Then the receiving station might notice or help you get over a obstacle but then its, Is it really worth modding it and spending the extra money?
That doubling of power might.....I say MIGHT result in your signal increasing by one half an S-unit on the other end. It will also strain other components in theradio and power supply and generate more heat while doing it. The heatsink is already barely adequate for stock power.
True and the incoming line on most of these radios is barely enough to handle what load is put on it. But strangly the negative is more than enough... Thanks.
Converting a CB radio from the factory-original bipolar transistor mainly accomplishes two things that I can see.
1) Slightly more power, but never enough to discern on the S-meter at the other end.
2) A much higher sensitivity to fail from high SWR, whether from a poorly-built/installed antenna or from coax-connector trouble or lack of proper ground at the antenna's mount point. The bright side of having a MOSFET final fail from a modestly-high SWR is that it's a lot cheaper to buy the replacement. Problems arise after the third or fourth time, when the foil pads begin to lift from the circuit board. The advantage of leaving the factory-stock bipolar RF transistor in the radio is that it tends to last longer.
One incidental bonus is the higher power consumption. The switchmode MOSFET transistors like ERF2030, IRF520, FQP13N10 consume more power from your DC supply to get the same RF wattage. Shortens the life of modulator transistors, modulation transformers/audio chips and base-station power supplies.
The older bipolar transistors built to use as RF amplifiers like the 2SC2166,2078,2312,1969 were designed to tolerate some SWR. Pushing them past the 'stock' rating reduces this a little, but at least they're designed with this in mind.
The switchmode MOSFETs were not. Engineers at International Rectifier were quite surprised to find that these parts would even work as RF amplifiers. They started getting calls asking for the specs you use for this type of circuit ten or fifteen years ago, and didn't believe that a IRF520 would do this at all. Apparently they had not tried it.
And if you make a living replacing blown finals, the switchmode MOSFETs are the best thing since sliced bread.
Just resist the temptation to increase the size of the radio's fuse beyond reason when it starts to trip the fuse you're using.
That's what this guy did.
Oh, and leave the "Top Gun" switch "OFF" if the radio has one.
73
1) Slightly more power, but never enough to discern on the S-meter at the other end.
2) A much higher sensitivity to fail from high SWR, whether from a poorly-built/installed antenna or from coax-connector trouble or lack of proper ground at the antenna's mount point. The bright side of having a MOSFET final fail from a modestly-high SWR is that it's a lot cheaper to buy the replacement. Problems arise after the third or fourth time, when the foil pads begin to lift from the circuit board. The advantage of leaving the factory-stock bipolar RF transistor in the radio is that it tends to last longer.
One incidental bonus is the higher power consumption. The switchmode MOSFET transistors like ERF2030, IRF520, FQP13N10 consume more power from your DC supply to get the same RF wattage. Shortens the life of modulator transistors, modulation transformers/audio chips and base-station power supplies.
The older bipolar transistors built to use as RF amplifiers like the 2SC2166,2078,2312,1969 were designed to tolerate some SWR. Pushing them past the 'stock' rating reduces this a little, but at least they're designed with this in mind.
The switchmode MOSFETs were not. Engineers at International Rectifier were quite surprised to find that these parts would even work as RF amplifiers. They started getting calls asking for the specs you use for this type of circuit ten or fifteen years ago, and didn't believe that a IRF520 would do this at all. Apparently they had not tried it.
And if you make a living replacing blown finals, the switchmode MOSFETs are the best thing since sliced bread.
Just resist the temptation to increase the size of the radio's fuse beyond reason when it starts to trip the fuse you're using.
That's what this guy did.
Oh, and leave the "Top Gun" switch "OFF" if the radio has one.
73
Mosfets= less gain requiring more stages resulting in less fidelity and more heat.
Bipolars= more gain requiring less stages resulting in more fidelity and less heat.
Sounds like a no brainer to me.
Bipolars= more gain requiring less stages resulting in more fidelity and less heat.
Sounds like a no brainer to me.
Fidelity?
Fidelity isn't an issue. As far as linearity goes, they seem to be doing just fine; but this may depend on the bias setting. Other than that, they work just fine 'n dandy. Never had an issue with them yet. Still waiting.
Fidelity isn't an issue. As far as linearity goes, they seem to be doing just fine; but this may depend on the bias setting. Other than that, they work just fine 'n dandy. Never had an issue with them yet. Still waiting.
Maybe distortion would be a better word. Same concept though.
More stages = more distortion
Less stages and less distortion would be more desirable in my book.
Well fidelity is a measure of how well a certain frequency can be transmitted while distortion is a measure of how accurate the output waveform compares to the input waveform. More stages does not necessarily mean more distortion if things are done right.
Not increasing any fuse sizes, My two radios I just sent off came with IRF520s factory from what I'm told. I think my 68Elite has one aswell but it doesn't say anything about that on it and is only doing 12-ish out the back. I'm asking because on the never lying youtube, You see cobras and such doing 30-40 out the back and they all said the same thing, Mosfet finals. I was very skeptical about it and thinking the ONLY way to get that is severely volting them on the bias adjustments or cracking the RFI trap wide open. But it never hurts to ask I suppose. I have some much older radios that have original bi-polar finals that are weak and not performing as they should and looking for either newer or better replacements instead of trying to find a actual 1969 replacement which I believe they all use.
Money is better spent on a small amplifier. You will have a cleaner and stronger signal than trying to squeeze a lot of power out of the radio.
Those little rm Italy jobs are cheap. Buy a 200 watt amp and run it at 100. Just my 2 cents.
Those little rm Italy jobs are cheap. Buy a 200 watt amp and run it at 100. Just my 2 cents.
I'm pretty much done with these cheap $3 MOSFETs used in CB/Export radios today. They just do not hold up in a base station environment. The ones used in ham radios are a different beast and seem to hold up okay so far.
I'm keeping the tried and true Bipolar transistor radios: 2sc1969, SD 1446, Toshiba 2sc2290, 2sc2879 all of which still work well for me. I've had multiple cheap MOSFET rigs blow. Literally...
Now if someone comes up with a reliable plug and play MOSFET final replacement; even if they cost $25-$50 each, let me know before I kick these $3 MOSFET rigs to the curb. Let me tell you how I really feel
I'm keeping the tried and true Bipolar transistor radios: 2sc1969, SD 1446, Toshiba 2sc2290, 2sc2879 all of which still work well for me. I've had multiple cheap MOSFET rigs blow. Literally...
Now if someone comes up with a reliable plug and play MOSFET final replacement; even if they cost $25-$50 each, let me know before I kick these $3 MOSFET rigs to the curb. Let me tell you how I really feel
I've done a lot of mosfet conversions in old CB radios, and so far, I haven't had a single one blow up in normal service. My original mosfet Washington and my neighbor's mosfet Cobra 139XLR are still in service today, 6+ years after I installed them. From a cost standpoint, if you're wanting a cheap replacement, they are viable.
But overall, they aren't worth it, since there still are bi-polar transistors available. If your radio still has a working bi-polar final in it, keep it in there. Changing over to a mosfet is NOT worth it. The gain in output isn't worth the hassle.
When I install mosfets, its because the customer wanted a low cost alternative to a bi-polar. I bias them more conservative and I do not run the AMC/ALC wide open. I also tell the customer to make sure the SWR on their setup is less than 1.5:1.
They have their plusses and minuses, but overall, it's not worth a swap unless the bi-polar transistor supply has dried up and are no longer an option.
~Cheers~
But overall, they aren't worth it, since there still are bi-polar transistors available. If your radio still has a working bi-polar final in it, keep it in there. Changing over to a mosfet is NOT worth it. The gain in output isn't worth the hassle.
When I install mosfets, its because the customer wanted a low cost alternative to a bi-polar. I bias them more conservative and I do not run the AMC/ALC wide open. I also tell the customer to make sure the SWR on their setup is less than 1.5:1.
They have their plusses and minuses, but overall, it's not worth a swap unless the bi-polar transistor supply has dried up and are no longer an option.
~Cheers~
I'm pretty much done with these cheap $3 MOSFETs used in CB/Export radios today. They just do not hold up in a base station environment. The ones used in ham radios are a different beast and seem to hold up okay so far.
I'm keeping the tried and true Bipolar transistor radios: 2sc1969, SD 1446, Toshiba 2sc2290, 2sc2879 all of which still work well for me. I've had multiple cheap MOSFET rigs blow. Literally...
Now if someone comes up with a reliable plug and play MOSFET final replacement; even if they cost $25-$50 each, let me know before I kick these $3 MOSFET rigs to the curb. Let me tell you how I really feel
Exactly. The MOSFET technology is not new by any means and is not the problem. The selection of the wrong device for the job is the problem. MOSFETs were used in commercial gear since the 80's. I field tested the first Nautel 1 Kw AMPFET-1 broadcast transmitter back in 1982/83 and the first Nautel FM-7 solid state FM broadcast transmitter at 7Kw back in the early/mid 1990's.Back in the 80's they used eight MOSFETS per module and each module produced 200 watts of RF power for the AM tx. The later FM unit used the MRF-151G 300 w device in quads to make 1200 watt modules. Those transmitters are still in use today. Yes there have been a few MOSFET failures with the AM tx but they were always related to a direct lightning strike. The devices used today are much more rugged and produce much more power output per device. Ham gear has and continues to use MOSFETS in the finals for at least 20 years or more with very few issues. The key is to select the proper device for the job and run them within spec. Do this and the devices should last as long as the rest of the radio.
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