Testing RF power transistors

[Raff]

G'day everyone,

I have been given an old General Electric UHF 70 watt power amplifier (PA) module to try and repair.

The fault is very low output power, the thing should draw around 15 amps, but barely draws 2, and puts out about 2 watts for 1 watt in. It looks like the finals, and drivers are all faulty, or at least damaged and now have low gain.

It is full of GE branded transistors but Ive found a list of replacements that should work.

My question is, can a RF power transistor like say a Motorola MRF646 go low gain? That is fail without just going short circuit like say audio power transistors?

My background is in audio type equipment, I haven't done much RF stuff let alone high powered UHF goodies.

Thanks guys,
Raff

[Zero999]

I would've thought any BJT can go low gain, from over.reverse voltage.

[ElectroIrradiator]

Some/most RF power transistors are comprised of a number of individual transistors run in parallel on a single chip die. Each transistor is complete with its own on-die emitter resistor, to ensure the various transistors share the total collector current.

It is quite possible to blow only part of the transistors, giving the symptoms you describe.

Another thing to check, is if it is a linear amplifier. If it is, then it should have a bias control, which may be broken or set incorrectly. That could also result in similar symptoms.

Note that it isn't trivially easy to replace power transistors similar to these. The gold plated copper base is remarkably soft, and you need to actually torque down the mounting screws by the proper amount, and sometimes in the right order. Too little mounting torque, and the transistor will not be cooled properly, causing it to fail prematurely. Too much torque, and you run a risk of deforming the copper, resulting in either reduced cooling due to a warped base plate, or a cracked die.

They can also be quite sensitive to static electricity.

[Raff]

Thanks for your input guys.

I didn't realise that transistors such as these had multiple paralleled cores inside, fascinating.

And thanks for the heads up on replacing the little sods, yes it looks like it might be quite the task!

Thanks again,
Raff

[KJDS]

It's entirely possible that the low gain is due to a problem with the matching circuit. High power RF transistors have horrible input and poor output impedances and matching these is an art. 

I'm not that familiar with old RF transistors, almost everything I've done has been with recent devices, but I've never seen a device half fail.

It could be that you've lost one out of a pair in the output stage, that would unbalance things enough to drop the gain substantially. Checking the bias conditions without an RF input could indicate which one has gone.

[vk6zgo]

I had this reinforced one day years ago,when I built a small RF linear amp to increase the output of my Yaesu FT290 Ham radio from 2.5watts to around 15watts.

I had the board all done except for soldering in the transistor.
At work while everybody was at afternoon tea,I grabbed the RF sweeper,quickly soldered in the transistor,applied power & realised I had connected it the wrong way round --it was one of the "helicopter" packages.

In great trepidation,I refitted it & was delighted when it seemed to have gain.
A quick tune,& it looked like what I had expected.---around 8dB gain,nice & flat over the band,good input match.

I took it home,& was talking to another Ham--my ex-boss,in fact.
I said,"Hold a on sec,I'll connect up the amp!

"OK,how does that look!"

He came back "Don't slash your wrists------------"

It appeared that one section of the transistor was,indeed, still intact,giving me  the gain figures at the low level from the RF sweeper,but not when it was hit with the 2.5watts,when it looked more like an attenuator!!

[Raff]

Thanks for the input once again guys.

I asked the owner what the failure process was, he described using it, smoke coming out, kept using it and power reported to drop off eventually to next to nothing.

I initially found one of the 100 ohm 2 watt resistors in the power divider on the output of the PA transistors as the owner of the now escaped smoke.

I then found a cracked solder joint on the heavy lead that supplies power to one of the 2 PA power transistors, that probably explains the resistor getting cranky.

The pre drive transistor was removed and checked, it had a gain of 2 with my tester, I thinks its cactus.

I found a substitute transistor out of another old radio and checked its gain, 150, under the same conditions as the now damaged transistor.

Replacing that pre driver didn't help much so that's why I suspect the driver and probably the final PA pair.


Your advice taken on board, thanks guys.
Regards,
Raff

[Zero999]

Some/most RF power transistors are comprised of a number of individual transistors run in parallel on a single chip die. Each transistor is complete with its own on-die emitter resistor, to ensure the various transistors share the total collector current.

It is quite possible to blow only part of the transistors, giving the symptoms you describe.

Thanks I didn't know that either.

I've seen normal, low power/frequency transistors go low gain before too though.

[KJDS]

This is a useful primer to RF power transistors.

http://www.nxp.com/documents/handbook/RF_Fundamentals.pdf

[lewis]

This is a useful primer to RF power transistors.

http://www.nxp.com/documents/handbook/RF_Fundamentals.pdf

Any deliberate inaccuracies in there to keep people on their toes?

[KJDS]

This is a useful primer to RF power transistors.

http://www.nxp.com/documents/handbook/RF_Fundamentals.pdf

Any deliberate inaccuracies in there to keep people on their toes?

You think I've read it? I only had a quick glance through and it's got some pictures of the interdigitated gate and emitter that is used to provide a lot of parallel transistors on a die. It also gives some clues about how the internal matching is done so that the designer isn't trying to match to 2nH in series with 0.3ohms but instead get the luxury of matching to 1nH in series with 1 ohm.

[lewis]

You think I've read it?

I thought you wrote it!

[Raff]

Thanks for the link to the RF power transistor design stuff, I'm off to read it now. 

[Jibby]

In RF operation low gain ,low current are owing to many factors. Improper antenna load, mis- tuned tank circuit or mismatched  input to the final transistor.

[Raff]

Just a small update.

I have managed to remove 2 MRF646 transistors from old radio PAs I had lying around, made for a good practice run for removing the ones in the GE as the metal clad mica caps are in the way on the GE.

It seems the owner overdrove the poor little thing as well as winding up the power control on the board.

Its an FM only thing so is running class C, its off a repeater and expects around 125 mW on its input, I heard the owner put 5 Watts into it! 

I wont say what band or frequency it was being used on, only to say it was subject to some kind of "power war".

Thanks again for the input guys.

Raff. VK4KQ

[daedalux]

Rf power transistors damaged due to bad load impedance (too high or too low) is a very common issue. Good devices, like most hf transceivers have a system called ALC (automatic limitation control) connected to a bridge to measure SWR (static wavelenght relationship) at the output that negatively feedbacks the output when is mismatched to prevent overload or too high output voltage.

Anything that has not this system, or has a very conservative power use on the transistors, o an easily blown fuse or is very prone to breaking the output transistors, which is mostly seen in a too high Base voltage in BJT at work or blown shorted gate in mosfet.
The last transistor/couple in the chain is much more prone to get damaged than all the other stages.

I must say that fake transistors in rf power transistors is a big problem that anybody using them should investigate.

Also power transistors LIE a lot when checked with multimeter or any low power tester and tested cold. They look in better shape that they are.