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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