[TenTec] Centurion 422b troubleshooting help needed

[rick@dj0ip.de]

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 at 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 at 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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