732B/734A
Instruction Manual
5-16
+11
.
5V DC Regulator 5-22.
The input and output of the +11.5V dc regulator circuit can be easily checked at TP507
and TP508. The input should be nominally 14.87V dc. The normal range is 14.52 to
15.20V dc. If there is no voltage at the 10V output binding posts, check F501 first. If
okay, check the regulator circuit around Q504. The reference voltage used for this circuit
comes from U505, pin 6 and is +10.00V dc. Check U506 pin 3 to see that the feedback
divider is at the proper ratio, which is V3/V
out
= 0.870.
-6V DC Supply 5-23.
The -6V dc supply consists almost entirely of U511. Check the -6V output at U511 pin 5
or at J504 pin 6 to verify the output (voltage at J504 pin 6 will be approximately 0.4V
less magnitude due to the drop in R566). If it is bad, first disconnect J504 and recheck
the output. If it is now good, this confirms that something downstream from the cable is
at fault. If still bad, check to see that V
cc
and Common are correct on U511. If they are
correct, replace U511. Also check C524, C525 and C527 to confirm that they are not
damaged.
IN CAL LED Logic 5-24.
The IN CAL LED logic is controlled as follows: IN CAL LED(*) = TempHi(*) OR
TempLo(*) OR BattLo(*), where (*) = asserted low.
When the oven temperature is at nominal, TEMP-SENSE is approximately +8.14V dc. If
the oven temperature rises above 55°C, this signal voltage increases to above +9.02V
and a comparator in U507 changes state. Similarly, if the oven temperature falls to below
35°C, the TEMP-SENSE voltage decreases to below +6.98V dc and a comparator in
U507 changes state. If the VBATT line (battery voltage) falls to below +11.16V dc, a
comparator in U507 changes state.
Any of these actions (temperature too high or low or operating voltage too low) cause
the IN CAL LED to go out. You can check each trip condition individually by using a
variable source on the TEMP-SENSE line as follows:
1. Check to see that +10V-REF is very close to 10V. This comes from U505 and
should be quite stable.
2. Check VBATT to see that it is greater than +12V dc.
3. Disconnect J504 and connect a variable dc source to either J504 pin 5 or to TP513.
Vary the source and look at the wired-OR connection at TP512. Make sure you are
checking only one logic variable at a time. If there is a problem with any of these
comparators, replace U507.
The IN CAL LED is latched through the NAND gates in U509. If there is a problem
with this, check SW501 or U509. The external IN CAL indicator, available at J502 on
the rear panel, can be checked by varying the IN-CAL-LED(*) signal line and observing
that Q506 changes state, that current through the optoisolator changes state, and that the
output transistor in the optoisolator changes state. Be sure to use a 10 kΩ pullup resistor
between U513 pin 7 (or J502 pin 7) and U513 pin 8 (or J502 pin 3).
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