[TenTec] Centurion 422b troubleshooting help needed

[Gary J FollettDukes HiFi]

Darned AUTOCORRECT!

“overcurrent”, NOT “overcorrect”!



> On Feb 21, 2016, at 9:17 PM, Gary J FollettDukes HiFi <dukeshifi at comcast.net> wrote:
> 
> 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 <mailto: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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