[TenTec] Centurion 422b troubleshooting help needed

[A R]

Hank:

Thanks very much! You answered all my questions...even the one not asked, which was regarding the meter sensitivity. On vacation now, but when I return home next week I'll get after it.

Thanks again!

Allen--W7GIF


----- Original Message ----- 

From: HankP<mailto:pfizenmayer at q.com> 

To: tentec at contesting.com<mailto:tentec at contesting.com> 

Sent: Thursday, March 03, 2016 1:14 PM

Subject: Re: [TenTec] Centurion 422b troubleshooting help needed



I replied to this direct several times as he asked and have no idea if he received any info from me.

The most recent request was for calibration procedures for the meters . 

Here is the factory sheet sent to me in early 2005. 

>>To calibrate 422 Ig and Ip.

>>Feed 2.2 volts DC into cable plug #4 located on the HV Rectifier-filter board (81538) 

>>Unplug , feed 2.2vdec + to pin3 ,- to pins 1 and 4 of cable #4. Zero meters first. 

>>Set Ig to 240 ma.
>>Set Ip to 800 ma. 


About same time (April 2005)  they sent me an email .

>>We have made a modification to the bias circuit and the manual was never completely revised.

>>Fig 5-31 should read to plug 22 on QSK board. Diagram 5-1 is correct for plug 14.
>>Fig 5-7 Filaments get the ground through plug 3 on HV rectifier board. Plug 3 goes to 22 on QSK board. 

Now if the surge that blew the bias transistor was large enough - you MAY NOT  be able to reach calibration with the original cal trim pots and series resistors. 

I asked TT what the meters were supposed to be and was told they were 2 ma movements . I questioned that but was assured tey were 2 ma - so after a couple hours gnashing my teeth trying to get to the meter plugs , I  pulled off the front panel to be absolutely sure nothing else was hooked to the meters . When I did this I found the grid current (multimeter) was 1.10 ma full scale and was a 2468 ohm meter (2.715 volts across the meter  at 1.10 ma thru it) 

The plate current meter was 1.22 ma full scale  and 2511 ohms (3.064 volts across meter at 1.22 ma )Both meters were marked MODUTEC  9200551.

The meters are high resistance movements - they sent me new meters and the new meters measured 2.380 v for 1 ma FS(2380 ohms) and 2.383 v for 1 ma FS (2383 ohms)

Without going into too many gory details - I went thru this exercise twice - I had a 3-500 that would pop over from time to time just sitting there in standby .(First time 2 weeks after new- TT wanted it back - replaced the bias transistors and sent it back - will spare you the details of that )  

My conclusion was that during the current surge from a tube arc over or whatever - either the magnetic fields  in the box or current thru the meter movement was  demagnetizing or something along those lines to decrease sensitivity they CERTAINLY were not  2 ma movements. (TT kept saying they knew they had some problem with meters in shipping )To TT credit they sent me new meters twice for free) The second set of meters were 2.494V at 1 ma FS and 2.493 v at 1 ma FS (Dec 2005) 

So the last time - I decided to do some sort of protection to at least take care of over current thru the movements . This turns out to be very simple - just a 3.9 V zener across the movement limits the voltage to that and you get a bonus of some meter damping with the forward diode in the zener . You can just solder that across the movement  observing the polarity . (I cannot remember which was + side) Yes you could go lower than 3.9 but this protected it for me. 

The original sin of how and why the bias transistor(s) blow is another story. And  160 slump is yet another story. 

73 Hank K7HP 



To: tentec<mailto:TenTec at contesting.com<mailto:TenTec at contesting.com>> 

Subject: [TenTec] Centurion 422b troubleshooting help needed

   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.

Does anyone have a 422b manual, or know if one was ever published...or, even erratum for the 422 manual?

Has anyone experienced the Centurion failure mode as described, and found the cause/solution?

Any help is appreciated, and feel free to email me direct, off-list to avoid cluttering the list.

Thanks in advance, and 73...

Allen--W7GIF

 
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