The 2N3055 was notorious for being available in cheep / poor quality units.
In addition to doping impurities as Alan pointed out, it could also be the
physical build inside of the casing.
In an earlier life, I was a Quality Control technician working on a NASA
project.
We used NASA-Spec components, which is much more stringent than MIL-Spec.
Each component had a serial number and was X-rayed.
Part of my job was reading the X-rays and determining if we would use the
individual device or not.
You won't believe the deviances from the norm that you find inside of the
casing.
A lot of the devices I "saw inside of" were pre-destined to early failure.
I threw out about 10% of the components I examined.
BTW, just buying NASA Spec, if you can find it, is no better than buying MIL
Spec.
The quality improvement came from the process of a human examining the X-Ray of
the individual silicon device and making the choice - go/no-go. The X-Ray was
included with each device.
"New" is no guarantee of quality.
Nor is buying from a good brand, for that matter.
The good brand just buys you a good warranty, not a guarantee, which means
replacement upon premature failure, not quality. THERE IS A DIFFERENCE!
73 - Rick, DJ0IP
(Nr. Frankfurt, Germany)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Gary J
FollettDukes HiFi
Sent: Monday, February 22, 2016 4:18 AM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Centurion 422b troubleshooting help needed
This transistor sounds like a likely candidate as the amp is acting like it is
keyed regardless of the state of the keying line.
Question is, why did it fail? A 2N3055 is good for 15 to 20amps so overcorrect
is not a factor. Is there adequate transient suppression around this transistor
to protect it from voltage transients?
Just curious…
Gary
W0DVV
> On Feb 21, 2016, at 8:03 PM, Carter <k8vt@ameritech.net> wrote:
>
> On 2/21/2016 7:42 PM, A R wrote:
>> My Centurion 422b amp is sick. With initial application of power, in
>> selected PTT and STANDBY modes (and un-keyed), the 3-500's are drawing 140
>> ma plate current (and barely perceptible grid current). Panel MODE led's
>> indicate proper mode when switched between STANDBY and OPERATE, and the
>> TRANSMIT led is unlit. When placed in OPERATE mode, and keyed (at the PTT
>> RCA jack), the TRANSMIT led is illuminated. Keying the PTT input when in
>> STANDBY mode does NOT illuminate the TRANSMIT led. So, the panel
>> MODE-indicating led's suggest that mode control functions are OK. The RF
>> input/output switching/routing is correct for all modes.
>>
>>
>> Have begun troubleshooting the QSK board, and Filament-AC board (with the
>> tube bias circuitry), and the HV filter board, but have found that the
>> componentry on those boards differs from that described in the 422
>> (no-suffix) manual. Although the manual available on the RKR website is
>> identified as a 422b manual, it appears to be for a 422 (no-suffix) amp. In
>> that manual, there are discrepancies between the schematic and board layout
>> and circuit trace views for the FILAMENT-AC, QSK, and HV FILTER boards. And,
>> the QSK board in this amp is a p/n 78247, not the 81542 called out in the
>> 422 manual.
>>
>>
>
> Sure sounds like the bias transistor on the Filament-AC board. Been there,
> done that.
>
> Let us know how you make out...
>
> 73,
> Carter K8VT
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