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@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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