[TenTec] Centurion 422b troubleshooting help needed

Mon Feb 22 02:46:57 EST 2016

I wonder what you mean by putting an RF choke on the output of an amp. 

There already is one, going directly from the center pin of the power output connector to ground. It is there to prevent presence of high voltage DC at the output connector should the plate blocking capacitor ever short. This is a safety device.

What type of RF choke do you use and specifically, where do you place it?

What is the “common mode current” pathway and how would this current affect lifetime of the keying transistor? Since the amplifier cabinet is a Faraday cage at HF frequencies, placement of external devices to suppress common mode currents would seemingly have no impact on devices contained within that Faraday cage.

I appreciate your sharing your experiences regarding the 2N3055 and your NASA experience. However, the Ten Tec amplifiers are a pretty small population. Statistically, even 10% fallout would be a pretty small number. There are likely several billion 2N3055 (and their cousins) transistors in service. This device has been in the marketplace successfully for over 40 years. They do fail from time to time but not in percentages even approaching 10%. Most cases of failure take place when they run hot for a long time (diffusion of materials within the transistor producing short circuit conditions) or when they are subjected to excess collector to emitter voltage (avalanche breakdown without current limiting).

Indeed, if anywhere near 10% of 2N3055 transistors failed in infancy, there would have been headline news about them, on a par with the electrolytic condenser plague of the late 1990’s.

In addition, the Ten Tec amp did not use the 2N3055, it used the MJ15001, which has a higher voltage rating (more than twice) and a higher current rating than the 2N3055, far more than one would expect to need for this menial application. 

In the absence of the issues I mentioned, a TIP 31C should suffice for this task at a lower cost.

From a personal scientific interest perspective, since I use high resolution 3D industrial Xray tomography from time to time for defect analysis, I wondered what you looked for in the X-rays of 2N3055 transistors to give you the go or no go acceptance criterion? What defect could you see via the 2 dimensional Xray methods available then that would predict device performance or lifetime?

Thank you for sharing,

Gary

W0DVN

> On Feb 22, 2016, at 1:04 AM, rick at dj0ip.de <Rick at DJ0IP.de> wrote:
> 
> The point of looking for the cause is well taken, but you guys may be thinking in the wrong direction.
> 
> There is a possibility that common mode current is the cause of the breakdown.
> To be on the safe side, I would place a good RF Choke directly on the output of the amp.  I do that all the time.  I also use one between the amp and the transceiver.
> 
> 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 5:25 AM
> To: Discussion of Ten-Tec Equipment
> Subject: Re: [TenTec] Centurion 422b troubleshooting help needed
> 
> I can only imagine that using legacy old inventory would be the best choice if indeed manufacturing issues are the root cause. These are readily available on the “world’s largest auction site”.
> 
> However, with a transistor with 20 amp capability is keying a line with a maximum of 1 amp of current, and there appear to be multiple examples of failures, I have to wonder where the problem lies.
> 
> One could speculate that using a transistor  in this capacity is a good idea at all. The reason is that, just as was the case with Cathode keying of old tube transmitters, the Cathode of a power tube in an amplifier can float to voltages that are pretty high, and this is what caused key clicks in those old Cathode keyed transmitters. If, by metaphor, the collector of the transistor in this keying scenario were to float up above the breakdown voltage of the transistor, then the transistor would act sort of like a Zener diode and pass current with power dissipation equaling the product of the Zener potential times the measured residual current flow, whatever that number might be. That transistor could run fairly warm…
> 
> It might be interesting to measure the voltage that appears at the collector during key up and determine if this is taking place. In addition, a current measurement of the current through the transistor (in a properly working amplifier) would be helpful.
> 
> I know a fix for the problem if it exists but won’t waste time with it until a need arises.
> 
> Gary
> 
> W0DVN
> 
> 
> 
>> On Feb 21, 2016, at 10:05 PM, A R <raf_3 at msn.com> wrote:
>> 
>> 
>> Gary,
>> 
>> No problem here. I have a built-in autocorrect "uncorrector". I read "overcurrent" despite the autocorrect's corruption.
>> 
>> I'm inclined to suspect the failures are due to a device mfg quality issue...like doping impurities, and probably lot-related. Wish I had the date codes from the failed devices (assuming that they were all MJ15001's). The one in my amp carries the "ON" logo, is from Mexico, and has a lot code of "BM0318". I believe that Harris also makes the MJ15001. Too bad Motorola is no longer mfg such devices here in the valley. Do we have any on-shore manufacturers of semi's anymore?
>> 
>> 73,
>> 
>> Allen--W7GIF
>> 
>> 
>> ----- Original Message ----- 
>> 
>> From: Gary J FollettDukes HiFi<mailto:dukeshifi at comcast.net> 
>> 
>> To: Discussion of Ten-Tec Equipment<mailto:tentec at contesting.com> 
>> 
>> Sent: Sunday, February 21, 2016 8:25 PM
>> 
>> Subject: Re: [TenTec] Centurion 422b troubleshooting help needed
>> 
>> 
>> 
>> Darned AUTOCORRECT!
>> 
>> “overcurrent”, NOT “overcorrect”!
>> 
>> 
>> 
>>> On Feb 21, 2016, at 9:17 PM, Gary J FollettDukes HiFi <dukeshifi at comcast.net<mailto: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> <mailto: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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