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There
is an apparent firmware design flaw in the outer oven
controller of the Z3801A GPS receiver. Power-on
cycling will not reset this error mode and no known receiver
command will correct it. However, a work-around first
reported by James G3RUH and Lyle KK7P has proven to be
effective in restoring those receivers exhibiting this
problem.
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Symptoms
of the problem
Sometimes
the Z3801A receiver will power-up in an error condition:
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Will not allow the outer oven to turn on and warm up.
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Will incorrectly always report that the oven is warming
up.
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The GPS Lock LED will not illuminate even though satellites
are correctly tracked.
The SatStat
receiver status display when this problem is present:
-------------------------------
Receiver Status -------------------------------
SYNCHRONIZATION ...........................................
[ Outputs Invalid ]
SmartClock Mode ___________________________ Reference Outputs
_______________
Locked to TFOM 9 FFOM 3
Recovery 1PPS TI --
Holdover HOLD THR 1.000 us
>> Power-up: OCXO warm-up
Holdover Uncertainty ____________ Predict --
ACQUISITION ............................................
[ GPS 1PPS CLK Valid ]
Satellite Status __________________________ Time ____________________________
Tracking: 5 Not Tracking: 3 GPS 22:22:57 [?] 10 Dec 2003
PRN El Az SS PRN El Az 1PPS CLK Synchronized to GPS Time
5 60 58 154 6 10 158 ANT DLY 0 ns
14 47 318 135 * 9 23 65 Position ________________________
18 32 189 140 25 12 270 MODE Hold
23 52 264 101
30 72 179 161 LAT N 36:07:29.825
LON E 139:44:35.388
ELEV MASK 10 deg *attempting to track HGT +27.15 m (MSL)
HEALTH MONITOR .........................................................
[ OK ]
Self Test: OK Int Pwr: OK Oven Pwr: OK OCXO: OK EFC: OK
GPS Rcv: OK
----------------------------------------------------------------------
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Sending
some receiver commands resulted in these messages:
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Sent:
:STAT:OPER:POW:COND? bit pattern:
001
Sent: :STAT:OPER:POW:EVEN? bit pattern: 000
Sent: :STAT:OPER:POW:ENAB? bit pattern: 111
Sent: :STAT:OPER:POW:PTR? bit pattern: 111
Sent: :STAT:OPER:POW:NTR? bit pattern: 000
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Also,
measuring test point TP104 showed a value of about 5 volts DC.
This is incorrect because when the oven is warmed up this value
should be about 14 volts DC.
Analysis
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The
controller thinks the oven is in a state of warming up,
but it is really not. The Z3801A depends upon a stored
value that somehow gets changed by mistake. Apparently
the oven control logic sometimes gets out of sync and
is confused if the oven is really allowed to be turned
on or not.
A ZERO logic level (near zero volts) on pin 8 of P2 will
unbalance the thermistor bridge of the oven controller
and will turn the oven heater voltage off. Forcing the
outer oven on by temporarily removing this incorrect control
signal to the outer oven circuitry will allow the oven
to warm up.
After oven warm-up, the Z3801A control circuitry on the
main board will then correctly assert the logic level
to the oven controller (greater than 2.4 volts DC) and
will permit the receiver to report the correct OCXO status.
The GPS Lock LED will then also illuminate properly.
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The work-around
Caution:
remove power from the Z3801A before working on the electronics.
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Temporarily
remove the wire on pin number 8 on P2. This is located
on the top mounted power supply and oven controller
board. This wire is typically red to purple in color,
and is located between the blue and the green wires
on the same P2 connector.
-
Connect
the power supply side on J2 pin 8 to a temporary +5V
signal using a suitable resistor value of 1K to 10K.
This now removes the bridge unbalance condition and
starts the outer oven warming up.
-
With the red wire removed, wait at least 30 minutes
or until the signal supplied from the main controller
board to P2 pin 8 asserts itself (goes >2.4 volts).
Your outer oven will be warm by now, and the test
point voltage at TP104 should be closer to 14 volts.
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Remove
the temporary pull-up resistor and reconnect the red
wire back to the original position at pin number 8
on P2.
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The Z3801A
receiver SatStat display after implementing this work-around:
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-------------------------------
Receiver Status -------------------------------
SYNCHRONIZATION .............................................
[ Outputs Valid ]
SmartClock Mode ___________________________ Reference
Outputs _______________
>> Locked to GPS TFOM 3 FFOM 0
Recovery 1PPS TI -55.8 ns relative to GPS
Holdover HOLD THR 1.000 us
Power-up Holdover Uncertainty
____________
Predict 60.5 us/initial 24 hrs
ACQUISITION ............................................
[ GPS 1PPS CLK Valid ]
Satellite Status __________________________ Time ____________________________
Tracking: 6 Not Tracking: 1 GPS 22:38:28 18 Dec 2003
PRN El Az SS PRN El Az 1PPS CLK Synchronized to GPS Time
5 40 47 44 18 11 182 ANT DLY 0 ns
6 28 145 101 Position ________________________
14 68 319 89 MODE Hold
23 41 234 112
25 26 287 100 LAT N 36:07:29.825
30 80 56 188 LON E 139:44:35.388
ELEV MASK 10 deg HGT +27.15 m (MSL)
HEALTH MONITOR .........................................................
[ OK ]
Self Test: OK Int Pwr: OK Oven Pwr: OK OCXO: OK EFC: OK
GPS Rcv: OK
Receiver
command results
----------------------------------------------
Sent: :STAT:OPER:POW:COND? bit pattern:
111
Sent: :STAT:OPER:POW:EVEN? bit pattern: 000
Sent: :STAT:OPER:POW:ENAB? bit pattern: 111
Sent: :STAT:OPER:POW:PTR? bit pattern: 111
Sent: :STAT:OPER:POW:NTR? bit pattern: 000
----------------------------------------------
TP104---+16.2V
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