RF final output transistor substitutions

[needairtime]

Have anyone done some substitutions of RF final power transistors (without hacking the circuit or board -- meaning assuming the transistors are the same form factor)?

Just wonder what the success rate of doing such, or will these transistors succumb badly to impedance mismatch of the circuit board design?

Curious as to what past experience is -- what frequency transmitter, original and replacement transistor?

I suspect if form factor (TO-220->TO-263), mode of operation (BJT->MOSFET), or massive current capability change (20W transistor ->100W transistor) would require significant rework, but assume the form factor is same and wattage/voltage rating is about the same.

I also wonder, as mentioned somewhere else I forget, a lot of the transistors actually mention VSWR 20:1 no-damage guarantees. Some don't mention it. If there's no mention, is there no guarantee (as assumed) or do most transistors go through rigorous testing to reduce RMAs?

 

[prcguy]

I've had to do this on occasion starting in the CB days of the 1970s upgrading AM rigs with bigger transistors from SSB rigs for more output. Some replacements were not a good substitute and some worked fine with a little more gain and more output. The 2SC1307 was good for this and I still have some originals laying around somewhere.

I recently had a Mirage 100w VHF amp fail and had a more rugged ST Micro transistor with more gain on hand that worked great and gave more power out.

Have anyone done some substitutions of RF final power transistors (without hacking the circuit or board -- meaning assuming the transistors are the same form factor)?

Just wonder what the success rate of doing such, or will these transistors succumb badly to impedance mismatch of the circuit board design?

Curious as to what past experience is -- what frequency transmitter, original and replacement transistor?

I suspect if form factor (TO-220->TO-263), mode of operation (BJT->MOSFET), or massive current capability change (20W transistor ->100W transistor) would require significant rework, but assume the form factor is same and wattage/voltage rating is about the same.

I also wonder, as mentioned somewhere else I forget, a lot of the transistors actually mention VSWR 20:1 no-damage guarantees. Some don't mention it. If there's no mention, is there no guarantee (as assumed) or do most transistors go through rigorous testing to reduce RMAs?

 

[needairtime]

Interesting... so you pretty much did a trial and error direct replacement? Perhaps it's okay at HF, maybe even VHF, but I'm on my own at UHF?

So I was looking into what it would cost to get a replacement for my U400's final, I was wondering about possibly swapping for a cheaper nonidentical transistor, maybe even a pull. I was a bit concerned when I was looking at several spec sheets for matching, while most UHF transistors seem inductive (versus capacitive) at frequency, some were actually more capacitive, which seems like it would interfere with whatever's on the circuit board. Granted the impedances were all fairly small, which is kind of expected for semiconductors compared to tubes, which made me wonder if perhaps there's a bit of slush here.

Just don't want a new transistor (or especially its predriver, since it typically does not have protection on the board albeit the amount of power we're dealing with there is much lower) to blow up simply because the mismatch on the board caused problems. The external antenna mismatch might actually be a larger portion that needs to be properly matched. While I could try to hack out board level matching, I just don't have a big pile of small inductors and capacitors to try.

Or perhaps I'm being more paranoid than I need to be?

 

[prcguy]

Yes you will find some similar spec transistors that will not match well as a replacement and many newer high power transistors have internal matching replacing some external components. Years ago I was fortunate to have almost unlimited supplies of CB, VHF and UHF power transistors for experimenting at no cost, so it was only my time wasted when a replacement didn't match up. My supplies are dwindling now so I have to be ready to remove a device carefully for reuse if it didn't work out where years ago I would just toss it.

I remember now the last one I substituted a 2SC something final in a Mirage B-310-G amp with a STMicro SD-1477 I had on hand and it worked out great. You can usually tell when tuning it up and the SD-1477 put out about 120W and there was a nice dip in current near maximum output and it runs fairly cool. I even transmitted full power for quite a time with no antenna connected and it didn't seem to care, so I now have faith in this amp where the original transistor died for no good reason.

What part # are you trying to substitute and with what part #?

Interesting... so you pretty much did a trial and error direct replacement? Perhaps it's okay at HF, maybe even VHF, but I'm on my own at UHF?

So I was looking into what it would cost to get a replacement for my U400's final, I was wondering about possibly swapping for a cheaper nonidentical transistor, maybe even a pull. I was a bit concerned when I was looking at several spec sheets for matching, while most UHF transistors seem inductive (versus capacitive) at frequency, some were actually more capacitive, which seems like it would interfere with whatever's on the circuit board. Granted the impedances were all fairly small, which is kind of expected for semiconductors compared to tubes, which made me wonder if perhaps there's a bit of slush here.

Just don't want a new transistor (or especially its predriver, since it typically does not have protection on the board albeit the amount of power we're dealing with there is much lower) to blow up simply because the mismatch on the board caused problems. The external antenna mismatch might actually be a larger portion that needs to be properly matched. While I could try to hack out board level matching, I just don't have a big pile of small inductors and capacitors to try.

Or perhaps I'm being more paranoid than I need to be?

 

[needairtime]

After a bit of bad experience with greedy sellers, I'll keep the exact part numbers under wraps, though the CTC CM45-12 is one of the transistors that are typical. Of the 5 possible transistors I found data for so far, the OEM device has the highest impedance input and output percentagewise, though the magnitude is not high. I'd suspect magnitude is more important than percentage however, because impedances tend to add and subract. hFE, leakage, gain, collector efficiencies, and collector capacitance of all the transistors are reasonably close. I should have also tabulated Miller capacitance but didn't (oops) and whether additional gain killing is needed to prevent the transistor from oscillating in another mode.

I could not find data for the CTC CM45-12 so it's out of the running because I don't gain anything from using this transistor, learning wise from comparing datasheets - though it too will fit in the form factor.

Then again I have not totally ruled out the predriver as faulty. I need to somehow ensure this is not the case before making a purchase, though I'm not quite sure how to test it without removing something - which will upset impedances on board. I'd have to attach a dummy load to the predriver if I pulled the final to test said predriver (YOW!!!)?.

 

[prcguy]

You can use an oscilloscope that will cover the frequency range of the amplifier then probe the base and collector of the driver and final. You should see an appropriate and larger voltage at the collector compared to the base and larger voltages on the final compared to the driver if the impedances are similar. You can also make a very small insulated loop at the end of some coax fed to a spectrum analyzer and sniff base and collectors of RF transistors for activity. Just don't let the bare center conductor of the coax touch anything hot or it will be an expensive fix for the analyzer.

I used to have a home made RF probe made from a small panel meter that had a short probe with a couple pf capacitor coupled to a pair of diodes and another cap for filtering that I could probe RF transistors with. The coupling cap was chosen where a 30 watt radio transistor collector would give near full scale on the meter and I could probe around and tell in a few seconds where a problem might be in a VHF or UHF transmitter. If I still did radio work I would make another one.

After a bit of bad experience with greedy sellers, I'll keep the exact part numbers under wraps, though the CTC CM45-12 is one of the transistors that are typical. Of the 5 possible transistors I found data for so far, the OEM device has the highest impedance input and output percentagewise, though the magnitude is not high. I'd suspect magnitude is more important than percentage however, because impedances tend to add and subract. hFE, leakage, gain, collector efficiencies, and collector capacitance of all the transistors are reasonably close. I should have also tabulated Miller capacitance but didn't (oops) and whether additional gain killing is needed to prevent the transistor from oscillating in another mode.

I could not find data for the CTC CM45-12 so it's out of the running because I don't gain anything from using this transistor, learning wise from comparing datasheets - though it too will fit in the form factor.

Then again I have not totally ruled out the predriver as faulty. I need to somehow ensure this is not the case before making a purchase, though I'm not quite sure how to test it without removing something - which will upset impedances on board. I'd have to attach a dummy load to the predriver if I pulled the final to test said predriver (YOW!!!)?.

 

[needairtime]

Yeah, if I had access to tools... My 'scope is a 300MHz 3db which will just "love" UHF frequencies (honestly, I didn't even try - perhaps if I have the gain up all the way it would still show something at 450MHz). I don't have a spectrum analyzer, though if I could make a 1M:1 probe or something like that and keep an HT completely shielded from other RF other than from this probe ...

My RF probe is pretty much just that, a homemade unit with a capacitor and a diode. Not sure if its bandwidth is high enough for UHF but it seems to do something with it (VHF works perfectly however.)

Hmm.

 

[prcguy]

Obviously the goal is to identify if an RF transistor is defective before removing it, especially high power planar types with big flat leads that can be surrounded by lots of chip caps, etc. Then when you get them out they are difficult to test without a transistor checker that can test at RF frequencies appropriate for the device. The high power types usually have several dies inside with tiny wire bonds between them and one or two can blow out where it will draw normal current but the overall gain is way down. These can test ok with an ohmmeter or simple transistor checker but they are otherwise toast.

Having another similar radio to compare with and probing base/collectors with an RF probe should give a good idea if a device is defective before going through a lot of work removing parts. When I did two way radio repairs a long time ago I usually had a warehouse full of same model radios I could use for documenting correct RF levels at points inside the transmitters.

Yeah, if I had access to tools... My 'scope is a 300MHz 3db which will just "love" UHF frequencies (honestly, I didn't even try - perhaps if I have the gain up all the way it would still show something at 450MHz). I don't have a spectrum analyzer, though if I could make a 1M:1 probe or something like that and keep an HT completely shielded from other RF other than from this probe ...

My RF probe is pretty much just that, a homemade unit with a capacitor and a diode. Not sure if its bandwidth is high enough for UHF but it seems to do something with it (VHF works perfectly however.)

Hmm.

Yeah, if I had access to tools... My 'scope is a 300MHz 3db which will just "love" UHF frequencies (honestly, I didn't even try - perhaps if I have the gain up all the way it would still show something at 450MHz). I don't have a spectrum analyzer, though if I could make a 1M:1 probe or something like that and keep an HT completely shielded from other RF other than from this probe ...

My RF probe is pretty much just that, a homemade unit with a capacitor and a diode. Not sure if its bandwidth is high enough for UHF but it seems to do something with it (VHF works perfectly however.)

Hmm.

 

[needairtime]

Obviously the goal is to identify if an RF transistor is defective before removing it, especially high power planar types with big flat leads that can be surrounded by lots of chip caps, etc. Then when you get them out they are difficult to test without a transistor checker that can test at RF frequencies appropriate for the device. The high power types usually have several dies inside with tiny wire bonds between them and one or two can blow out where it will draw normal current but the overall gain is way down. These can test ok with an ohmmeter or simple transistor checker but they are otherwise toast.

This is a key bit of information, I do see something to the extent that the radio is drawing considerable current - but supposedly the gain is horrid because I don't see power being transmitted. I don't quite understand this however, because I don't know where the power is being dissipated. I suspect the preamp is drawing some of the current, but if the output has no gain and is eating power, but where... if the collector bond blows, it can't draw current; if the die shorts, it will always draw current. This isn't exactly what I'm seeing as I see only 600mA or so receive, and 6+ amperes when transmitting... which seems like the transistor is working as it should, I just don't see RF output.

Having another similar radio to compare with and probing base/collectors with an RF probe should give a good idea if a device is defective before going through a lot of work removing parts. When I did two way radio repairs a long time ago I usually had a warehouse full of same model radios I could use for documenting correct RF levels at points inside the transmitters.

Yeah I only have this one radio, nothing to compare to, so that avenue is dead

 

[prcguy]

There could be other things in the transmitter that have failed limiting power output. I've seen PIN diode T/R switching fail and other things in the path that have opened or shorted and the transistors are still fine. Probing the base/collectors with an RF probe of some kind then throughout the transmitter path will usually find the problem.

I've not done much bench work in recent years and hopefully there are some current radio techs out there that can comment. Or maybe loan you some equipment!

This is a key bit of information, I do see something to the extent that the radio is drawing considerable current - but supposedly the gain is horrid because I don't see power being transmitted. I don't quite understand this however, because I don't know where the power is being dissipated. I suspect the preamp is drawing some of the current, but if the output has no gain and is eating power, but where... if the collector bond blows, it can't draw current; if the die shorts, it will always draw current. This isn't exactly what I'm seeing as I see only 600mA or so receive, and 6+ amperes when transmitting... which seems like the transistor is working as it should, I just don't see RF output.

Yeah I only have this one radio, nothing to compare to, so that avenue is dead

 

[needairtime]

That's also weird, yes, working with a 30+ year old radio. Do newer radios still use relays to separate T/R paths? For low power radios like HTs, I was kind of surprised not to see relays after even seeing CBs using relays.

I need newer radios to study...

 

[RFI-EMI-GUY]

Somewhere in my junk box, I have a small low voltage panel lamp with a short piece of wire on it. I hold it with my fingers and touch it to the collector and though capacitivly grounded by my fingers through the glass, it will glow. You can get a rough idea where the signal stops if a coupling cap or PIN diode is toasted.

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[RFI-EMI-GUY]

That's also weird, yes, working with a 30+ year old radio. Do newer radios still use relays to separate T/R paths? For low power radios like HTs, I was kind of surprised not to see relays after even seeing CBs using relays.

I need newer radios to study...

I have not seen TR relays in commercial LMR radios for decades. In external power amps, yes.
.

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