Strange PL 400 outputs transistor failure

[premiumplus]

So I'm working on one of my Phase Linear 400 Series 1 amps, getting ready for installation of a dc protection relay, and did the current sharing test. All checked within 20%, but I decided to try to get it closer by swapping out transistors from another amp. I pull one and checking it with my dmm, I read a diode drop of 0.054 from base to emitter. Both ways. So I start pulling transistors and one after another they are reading base to emitter semi short circuits. Anywhere from 50 to 600 ohms. All in all, from both of the amps, I found 16 bad output transistors.
I couldn't believe that they functioned at all, and the funny thing is that at low volumes you couldn't hear a problem.
How in the world could an amp function with two thirds of it's output transistors compromised? The funny thing is that they all have normal forward diode drop readings, but there is the low resistance between the base and emitter of each one. I've never seen anything like it, and I'm just lucky that they failed the way they did, instead of the usual emitter to collector short.
I just ordered a bunch of MJ21196G from Mouser, so hopefully that'll square things away. I have good RCA 410's for drivers.

 

[Skratch]

Are they XPL 910 transistors?

 

[premiumplus]

Yes sir!

 

[Skratch]

They are most likely OK. Those are Darlington transistors which will show leakage but are still OK. IMHO they were the best transistor that the 400 came with. Be careful with the MJ21196's when using the original driver board as it may oscilliate.

 

[premiumplus]

There are eight others that are the same type, that have resistance in the meg ohms range between base/emitter and emitter/base. The sixteen that I am suspicious of all have dc resistance between base/emitter and emitter/base of between 50 and 600 ohms...why do you think that they're ok? If they are darlingtons, then shouldn't they all measure the same if they are good? A darlington pair typically has the collectors tied together, and the emitter of the top transistor goes to the base of the bottom one. Shouldn't there be a doubling of the diode drop between the base and emitter, and still have no conduction when reverse biased? Because what I'm seeing is as if there is a resistor connected internally between the base and the emitter. 50 ohms is really, really leaky, isn't it? I'm trying to wrap my brain around this one but I'm not succeeding too well!

 

[laatsch55]

Positive on base, hegative on emotter??

 

[premiumplus]

Both ways. The measurement is the same whether it's dc resistance or diode voltage drop. Using a Fluke model 87.
It electrically looks like a resistor between the emitter and the base, or the other way, base to emitter.

 

[laatsch55]

I can't remember at this moment what the deal withg Darlington's is, but don;'t throw them away.There is something weird about em and the way they test. When Joe get's back we'll have a professional explaination on em.

 

[laatsch55]

Or FaboNav----DENNIS!!!!!!!!!!!!!!!!!!!!!!

 

[premiumplus]

Cool. Thank you, sir! I'll hang on to them, and in the meantime, I've got new MJ21196G parts coming in the morning from Mouser.

 

[jbeckva]

Just pulled an XPL909 out of the bin and used the diode tester to check it. Looks normal to me?? Reads around .473 forward biased B to E, and B to C, nothing reverse biased and nothing either way C to E.

That's totally OUT of circuit, right?? And that's with the transistor cleaned up ... no elephant snot left??

 

[premiumplus]

Just pulled an XPL909 out of the bin and used the diode tester to check it. Looks normal to me?? Reads around .473 forward biased B to E, and B to C, nothing reverse biased and nothing either way C to E.

That's totally OUT of circuit, right?? And that's with the transistor cleaned up ... no elephant snot left??

Yeah, that's what half of mine check too. Around .460 to .500...totally out of circuit. The others measure as if they have a resistor tied between the emitter and the base.

 

[jbeckva]

Yeah, that's what half of mine check too. Around .460 to .500...totally out of circuit. The others measure as if they have a resistor tied between the emitter and the base.

Hmmm, yeah they just might! I am having a vague memory... I've seen somewhere that there are XPL's out there with an internal bias transistor in 'em... or maybe something else, but now that you mention it, seems very familiar.

Joe'll know...

 

[NavLinear]

Attached is a service bulletin addressing the Phase Linear output transistors and output driver transistors. Make sure you read the "Special Note" at the end of the file. The Phase Linear engineering department recommended that the only valid test for these devices was in circuit.

 

[premiumplus]

Well, that would answer the question I had regarding how the amp could possibly work with bad transistors, because it passed the current sharing tests. It still seems strange to me that there could be as low as 50 ohms resistance between the base and emitter of a good transistor!

 

[laatsch55]

Well Jeez, I could have done that. Was hoping for a treatise on the design, manufacturing and application of said devices along with all the inside tricks and hints....

 

[laatsch55]

OK, Stephens 400 has all XPL 910's. Let me pull a few and put them on the Sencore.

 

[premiumplus]

You rock, Lee! I knew I liked you!!!

 

[jbeckva]

Attached is a service bulletin addressing the Phase Linear output transistors and output driver transistors. Make sure you read the "Special Note" at the end of the file. The Phase Linear engineering department recommended that the only valid test for these devices was in circuit.

Ah, also identifies the 910's (missed that one) as darlingtons. Yeah... that would explain it. I'd be nervous with those still in use...

 

[laatsch55]

These I have measure .059 MV betwee B-E Base Positive Emitter --Neg. .473 MV B-C, nothing reversed on the B-C juntion. Neg on Base, Pos on emitter .059MV . On ohms scale 59 ohms B-C both ways, B-C junction 14.73 ohms, infinite reversed. On to the Sencore. This was with my Fluke 179.