TESTING RF TRANSISTORS

[unit_399]

I recently purchased a box of used Motorola RF Power transistors from an electronics surplus dealer in Bogota. He was closing shop, and I bought a lot of stuff for next to nothing.
The rf power transistors are MRF454, 455, and 455A. I checked them all (42 total) with a simpson 260 meter, and found a few with B-E and C-E shorts and/or leakage.
My question is ... Is there another quick and dirty test I can do to determine if they're good or not before I install them in some old amps I have??
Any suggestions will be appreciated. Thanx and 73s.

- 399

 

[unit_399]

bump!!!

 

[sonoma]

I have never had luck checking trans with a ohm meter myself. I use a regular trans tester. I have a sencore super cricket and a little one that I bought off ebay for 20.00 dollars and they seem to test good on those 2 testers. it will show pinouts and if they pass the test or not. maybe someone else will have a better answer for your test procedure.

 

[Crusher]

I bought a Sencore super cricket tf46 off of ebay awhile back. Since then I wonder how I got by without one. They are really nice and you can test for leakage and Beta testing. Which an ohmmeter or diode tester will not do.

 

[Captain Kilowatt]

My past experience testing RF transistors with a VOM has been pretty much as yours. Sometimes RF transistors will show a bit of B-E reverse leakage when the device is fine. Never could really figure that one out. Never will. It seemed as if the VHF RF devices were the worst for that.

 

[jazzsinger]

My past experience testing RF transistors with a VOM has been pretty much as yours. Sometimes RF transistors will show a bit of B-E reverse leakage when the device is fine. Never could really figure that one out. Never will. It seemed as if the VHF RF devices were the worst for that.

I've seen that too with vom meter, could it possibly be to do with internal emitter ballasting.

Some interesting info in this: http://www.nxp.com/documents/handbook/RF_Fundamentals.pdf

1998 Apr 09 15
Philips Semiconductors
RF transmitting transistor and
power amplifier fundamentals
RF power transistor
characteristics
2.1.1.1 DEFINITIONS

Section may be worth a read. The part under Vceo, there is always dc resistance between emitter and base caught my eye, but i might be misinterpreting it. I'm no expert on RF PA stages.

 

[unit_399]

I've seen that too with vom meter, could it possibly be to do with internal emitter ballasting. The part under Vceo, there is always dc resistance between emitter and base caught my eye, but i might be misinterpreting it. I'm no expert on RF PA stages.

If this resistance is the reason for my meter showing E/B and E/C leakage, then why do only 13 of the 42 transistors I tested show some leakage ?? Also, the amount of leakage varies from transistor to transistor.
I'm going to bite the bullet and order a Transistor/FET tester just to be on the safe side.

- 399

 

[Captain Kilowatt]

It's been a long while since I tested high power RF transistors but IIRC most show some sort of leakage. I fear I may have tossed many a good, VERY hard to obtain Marconi VHF RF output transistors for the very old Marconi DT-56 business band radios in the past.

 

[144inBama]

I'm going to bite the bullet and order a Transistor/FET tester just to be on the safe side.

- 399

The best investment and time saver I've bought in awhile is the DCA Pro (DCA75). I've found 2 bad transistors where the 10 ohm smokestack showed no sign of failure. It allowed me to do a more exact match on Hfe vs 2 "matched pair" sets as purchased, especially with the curve trace function. Granted you can't test them at over 12vdc to get super exact numbers but it's certainly close enough for a blow it up, fix it up self repair guy like myself. You can store (on the PC) data for each transistor, I use a fine tip sharpie to "code" each one so when I go to replace one or a set I can just go to my little chart, find the 2 letter/number combos and install. As mentioned the numbers aren't as accurate as $400 rig but from everything I've seen, the actual Hfe is usually higher than the reading on the DCA.

 

[nomadradio]

That's good to hear, that the DCA doesn't get spoofed by RF transistors.

Some of the thirty-buck (or so) chinesium testers we have tried will make a RF power transistor oscillate. When it does this, the tester displays genuinely hard-to-believe reports. Tries to tell me that a 2SC2879 transistor is a PNP device, not NPN the way it's built.

Haven't done any kind of coherent survey of which ones get fooled most easily.

YMMV.

73