HP Car 3490A manual 21A. -to-C0NVERSt0ND CtBCUtTCHECKS, 3I .INTEGRATOR Troubleshooting

SectionVII

7-25.ReplacementofOCR30l.

7-26.If it is necessaryto replacethe referencetransis_

Model3490A

Table7-3. AC ReedRelayChecks.

tor-diodepackageAleCR3Ol, it is alsonecessaryto replaceresistorsA1R306,309,310,313,314and:t5, which are matchedto eCR3Ol.Matchedresistorsare suppliedwith thereplacementreference,

eCR30l.

7-21A. -to-C0NVERSt0ND CtBCUtTCHECKS.

7-28.Ttre Analogto-Dgital (A_to_D)Conversioncircuits consist of the Integrator, the x20 Amplifier, and the Zero Detect circuits. Some problern, in th"r, circuits Tay be detected by applying a full_rangeinput voltage. For example,the Integrator output *"öfo.. at AlTpl may be checkedwith a _ lOV input on the l0Vdc range. The center portion of the waveform (10 ms between_run-upand run_down) should be flat. if *ri, portion decays,this may_indicateleakagein the Integra_ tor capacitor,C2O3.If the waveform continuesto rise, the input switches e20,1 and e205 may not be

RangesSymptoms

Affected

100v lOverload- 1 V input on 100 V range 1000v resultsin 100 V display

I , I V I

I DisOlay is one-half the input value 1 0 v I

I

IV Low input impedance,approx. 2O kS-t

1 0 v I

100v Overload- I V input on 100 V range ro00v resultsin 5O V disptay .

1 V Displayis l /10 the input value too v

1 0 v Overload r000v

PossibleCauses

K2 Shorted

K2 open

K3 shorted

K3 open

K4 shorred

K4 open

completely cut off during this period.

7-29...Sometypesof problems,suchäs amplifier offset or failure, may be f9un9 UV setting the Sampte Rate control to HOLD and making dc voliage rn.urur"rn.nt, within the circuits. For inslance, boih inputs to an operational amplifier should be equal and near zero. To check the balance of a dual FST, such u, qZOO fo. example, first remove the microciriuit ampiifrer (U201 in this.case)from its socket.ConnectUotf,'pfifgatesto ground, and measurethe sourcevoltages,which should !" :gyd. Thesevoltagesmay be *"urirrä at pins 2 and 3 of the open U20l socket.

7-30. Mth .the instrument sampling and with a full_ rangeinput, the x20 Amplifier ouipuishould not exceed .2 to .4 V becauseof the diodesi"t*..n tfr" amplifier input and-output.Also with a full_rangei"p"t, tfräZt,r" detect reference voltage at pin 3 of*U203 should be approximately 1 50 mV during run_up una ,u"_Oo*n.," No^nJinearity of negative readiigs rnuy U, "uured by a defectiveamptifier,ü201.

7. 3I .INTEGRATOR TROUBLESHOOTING .

7-32. the integrator may cause noise on full scale readingsin the dc mode. If this occurs,check AlR207,

Q206, Q207, tJ2Ot and u202.If		theie i, noirc at low
input ievels (ie., 100mV input on l0 V dc range),check
AIC2O7. AlQ209 will be very noisy if pinch_äffvoltage
is not correct. To check for ä noisy eäOg, increusettre
-	supply and observenoise. The increase
!7y			in the
+ 17 V	supply increasesthe bias on aZOS,li-uoriuge,the noise
9Jt:qq.i" at a higher+ 17y		,upprv	e209
snoud	be

replaced. Readjust

the + 17 V supply per

Paragraph5-54.

7 . 33AC. CONVEBTERTROUBLESHOOTING .

NOTE

The indication on all rangeswith the input shorted will not be zero but should be'less than 50 counts.

7-34. Operation of the AC Converter circuits may be *::k.9,using the Troubleshooting Tree, Figure 7_6. Table 7-3 lists a number of symptöms associatedwith reed relay failures.

7-35.Cleanlinessof certain componentsand areasof the AC Converter Assembly are critical to the performance of the AC Voltmeter circuits. The assemblyshould be handled only by the board extractors, anä should be cleanedthoroughly following any repair.

7-36.Certaincomponentson the AC ConverterAssem_ bly must be spacedthe correctdistancefrom the printed circuit board for proper operation of the circuit. Table

74lists these componentsand the proper spacingthat must be observedduring repair.

. Table74. ComponentSpacing.

Designator	Spacing_fro m corn ponäit-c"nt",
A6-	ro l,rtnted Circuit Board
	Minimum	Maximum

c l	0 . 1in	O . 2in
c2
	O . 1in	u - 2 tn
c3
	0 . 1in	O.2in
c5
	0 . 1in	O.2in
R 1 A
	0.4 in	O.5in
R 1 B	0 . 1in	O . 2in
R44	0.4 in	0.5 in
R4B	0 . 1in	O . 2in
R4C	0 . 1in	O.2in

7 - 37OHMS. CONVERTEBTROUBLESHOOTING .

7-38. The Ohms ConverterTroubleshootingTree, Fig_ ure 7-7, may be used to locate the trouble if tn. Onr C-onverterdoesnotoperatecorrectly. Make surethe DC Voltmeter operation is correct before beginnrng the OhmsConverterchecks_

7-39. The cleanlinessof some areas of the Ohms C_onverterAssembly is important to proper operation. Handle the printed circuit board only'byihe extracton, and cleanthe repairedareathoroudily.

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