Just completed an internal visual inspection of the failed MJ15001 using a 20X
stereo scope with alternate light sources. Unfortunately my "shade tree"
failure analysis/forensic lab doesn't include the capability to provide pics of
the 20X views.
My conclusions are based on what the 20X visual inspection revealed, and over
30 years of device/component failure anayses in a former life. In the interest
of brevity here on the list, I won't include details of what was observed, but
will offer detailed info via direct email (off-list) to anyone interested.
Suffice to say that I will not be replacing the failed MJ15001 with another
MJ15001. It is my opinion that the failure was due to a device manufacturing/QC
issue, and NOT an application-induced overstress of the part.
Since there was no evidence of electrical or thermal overstressing of the
transistor, I will replace the failed part with a JANTX 2N3055 which, although
not as robust (spec-wise), at least theoretically it will have been inspected
and screened to preclude the latent-failure conditions observed in the failed
MJ15001. Receipt of the JANTX parts is pending. Assuming they pass incoming
inspection and test, one will get installed in the Centurion, and after a
reasonable burn-in/shake-down period, I'll update with results.
73,
Allen--W7GIF
----- Original Message -----
From: A R<mailto:raf_3@msn.com>
To: Discussion of Ten-Tec Equipment<mailto:tentec@contesting.com>
Sent: Sunday, February 21, 2016 8:49 PM
Subject: Re: [TenTec] Centurion 422b troubleshooting help needed
Duane, Andy, and Carter:
Wow. Just confirmed that the TO-3 bias transistor (MJ15001) in my 422b is
shorted collector-to-emitter. Admittedly, 4 failures isn't a huge sample, but
it sure suggests that there's either a quality problem with the MJ15001's, or
the circuit (application) is overstressing their spec'd characteristics/limits.
Did you all re[place the faied transistors with MJ15001's, or some other
transistor? My 422 manual calls out a 2N3055, which is a "15A, 60V, 115W"
device, compared with the MJ15001's "20A, 140V, 250W" rating. Without knowing
the root cause of Q1 failures, I'm tempted to "socketize" Q1. At least
replacing Q1 again (if needed) wouldn't be quite the PITA (and board-stressing)
as it is with a soldered-in Q1. How much failure-free time on your amps since
replacement of Q1? Thanks for the feedback and good info!
73,
Allen--W7GIF
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