HP Car 3490A manual Ratio.Tboubleshooting, AmplifierZerc0ffset, 75SCHEMATIC.NOTES

SectionVII

7 . 67 RATIO.TBOUBLESHOOTING .

l-68.If ratio measurementsare not correct and the trouble cannot be correctedby the Adjustmentproce_ dure, or if the adjustmentscannot be made correctlv. the following troubleshootinginformation should be used. Check for obvious sourcesof trouble, such as burned or loose componentsor loosewires.Make sure the printed circuit board, Al3, is seatedfirmly in its connector, and that the cable, W8, and the three operationalamplifiersare seatedfirmly in their sockets.

7-69. ExternalReferenceAmplifier Checks

7-70. The Ratio Troubleshooting Tree, Figure 7_16, checks the External ReferenceAmplifier circuits. How_ ever, correct interpretation of the displaysymptomsis neededto determineif the trouble is in the ämphneror logic circuits. For example,if the displayreadsapproxi_ mately 75% of normal (l0V Input/tOV Exi. nef.

=07.5000), the trouble is probably in the amplifier circuits. If the displayreadsOverloadfor both positive and negativeinputs (at INPUT terminals),the trouble is probably in the amplifier circuits, but if it reads Overloadfor only oneinput polarity, the troubleis more likely in the logic circuits. If the displayed polarity is incorrect and the display reads Overload, check the polarity logic circuits.

7-71.AmplifierZerc0ffset.

7-12. lf the ReferenceAmplifier zero adiustmentsin steps a through h of the Ratio Adjustment procedure

cannot be made correctly,the trouble may be leakagein either AI3CR2 or 3. This condition can be most easily checked by unsoldering one lead of each diode and trying the zero procedureagain.If the amplifierscan 6e zeroed with the diodes disconnected,one or both of them is defective.

7-73. ReferencePolarityLogicChecks

7-74. ln the 3490A measurementsystem,if the Input voltage is positive,a negativereferenceis requiredior run-down,and viceversa. In ratio measurements,the applied External Referencevoltageis invertedand these two voltagesaresubstitutedfor the positiveand negative

intemal referencevoltagesThe.polarity

Reference Logic on the Ratio Assembly,A13, makesit possibleto use either polarity Externai Referenceinp,lt ,itt .itt u polarity Input voltageby selectingthe öorrectreference polarity for rundown. If a failure in the Reference Polarity Logic causesthe referencevoltaqe used for run{own to be the same polarity as the Input, the display will indicate Overloäd. If the 3490A operares correctly in dc voltage measurementswith inputs of both polarities. but reads Overload with one polarity input in ratio measurements,check the Reference Polaritl'Logic asfollows:

a - Set R{TIO switchto EXT REF l0Vandapply l0 \'to EXT REF terminals.

Model34904

b.Set FUNCTION to DC and apply full-rangeinput on either the 1 V or l0 V range.Selectthe combination of INPUT and EXT REF polarity that causesthe display to readOverload.

c.Set SAMPLE RATE to HOLD. Check logic levels within ReferencePolarity Logic circuits asindicated on the schematic diagram, Figure j - 22 . Losic HIGH= + 2 . 4V to + 5 V . LOW= 0 to + 0 . 6V .

7.75SCHEMATIC.NOTES.

7-76. tll.e following notes apply in general to all schematicdiagrams:

a.Partial reference designationsare shown. prefix

with asembly or subassemblydesigration(s)or both for completedesignation.

b.Component values are shown as follows unless otherwisenoted:

Capacitancein microfarads

Resistancein ohms

c.* Averagevalue shown. Optimum value selected at factory.

d.=I Denotesearthground.

e.I Denoteschassisor frame ground.

f.f Denotesfloatablecircuit ground.

.9. qP Denotesprintedcircuit assemblyground. - V

Denotesassemblyoutline.

Denotessubassemblyout- line.

Denotesmain sigral path.

k .Denotesfeedbackpath.

l.l------lDenotesfront panelmarkings.

Denotesscrewdriveradjust.

n.Wl Denoteswire color. Color code is the same

as the resistor color code. First number identifies the base color; secondidentifies the wider strip; and third number identifies the narrowerstrip (e.g. 924 = white, red, yellow).

" 3 Denotesschematicnumberon which con-

Y nection is maije.

p. A Referto manualbackdating,SectionVIII.

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