[TenTec] Centurion 422b troubleshooting help needed

Sun Feb 21 23:25:14 EST 2016

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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