HP Car 3490A manual 13DC.ANAL0cCtRCUtTS, AZAsemblyExchange, DCAmplifierChecks, Wcc

SectionVII

b.If the problem is not with an option, determineif the faulty supply is in current limit (seeParagraph7-12). If so, determinewhere the short is by useof the power supplyjumpers and troubleshootthat section.

c.If the faulty supply is not in cunent limit, determineif the faulty supplyhasthe correctraw supply voltage (seeTable 7-l). ß the raw supply doesnot meet specification,troubleshootthis section.

d.If the faulty supply is not in current limit and the raw supply is within specification, troubleshoot the faulty supply.

7-11. lf the + 17V supply is out of specificationand cannot be adjusted per Paragraph5-54, troubleshoot accordingto Paragraph7-10, steps a, b, c and d. It is essentialthat the + 17 V supply be within specification since all other suppliesare referencedto this supply and will be affected.If oscillationproblemsareencountered on the +l1Y supply, changeAlClOg to 1000pF, 400 pF or remove from circuit. If there are ripple problems on the + 17 Y supply or any other supply, check to assurethe line voltage selection switchesare set to correspond with available line voltage. If ripple still exists, replace the filter capacitor of faulty supply. Ripple problemson the - 17 V supply can be causedby

ripple or low voltage of the - 30 V supply Qowervoltage than the -24Yrawvoltagesupply).

1-12. Nl power supply regulatorsarecurrentlimited so that if an excessiveload is applied,the regulatorvoltage output goes to neat zero.If this is suspectedto be the problem, the supply in current limiting can be deter- mined by measuringfor the current limit voltageacross the resistor which are both specified in Table 7-1. Provisionhasbeenmadefor isolatingcertainareasof tfue Main Circuit Assembly by removing designatedjump6r wires in order to locate the areawhereexcessiveloading occurs.Table 7-2 lists the jumper wire designationsand the circuits supplied through each. Thesejumpers are located on the Main Circuit Assembly, Al and the vertical board, AlAl. All of the Logic circuits are suppliedthrough only one jumper wire for eachvoltage. Consequently,jumper wires are providedin the ground circuits to isolated portions of the logic circuits.These ground jumpers are shown on the schematicdiagrams, Figure7-23 andT-24.

7 . 13DC.ANAL0cCtRCUtTS .

7-14.The DC Analog TroubleshootingTree,Figure7-5, covers the DC Amplifier, Integrator and Zeto Detect circuits, I l0 V References,the DC SwitchingLogic and kvel Translatorsand the PowerSupplies.

7-15.AZAsemblyExchange.

7-16.Cleanlinessof the A2 High ImpedanceAssembly and the proper positioning of components on the assemblyarehighly important to the performanceof the instrument. Also, A2U2 is very difficult to replace. Consequently,a rebuilt High ImpedanceAssembly,-hp- Part No. 03490-69502, has been made available to

			Model3490A
TableT-2. PowerSupplyJumperWires.
Voltag€	Wire	Location	Circuits Supplied
+ 1 7 V	WH	A 1	Logic
+ 1 7 V	w2	A 1 A 1	t 10 V Reference
+ 1 7 V	W6	A 1 A 1	Integrator,Zero Detect
- 1 7 V	wl	A 1	logic
- 1 7V	WEE	A I	DC Switching LevelTranslators
- 1 7V	W3	A 1 A 1	tl0 V Reference
- 1 7 V	ua	A l A T	Integrator,Zero Detect
+ 3 0 V	wz	A I	Integrator,Zero Detect
+ 3 0 V	WBB	A 1	DC Amplifier
+ 3 0 V	W4	A 1 A 1	t 10 V Reference
- 3 0 v	WAA	A 1	Integrator,Zero Detect
- 3 0 v	wcc	A 1	DC Amplifier
- 3 0 v	W F F	A 1	DC Switching LevelTranslators.
			A1CR403
+ 5	WJ	A 1	Logic, Display,Switches
+ 5	W1	A , ' 4 1	Al U4Ol , O\rerloadProtection,
+ 5	W7 A 1 A 1		DC Switching Logic
			Integrator,Zero Detect
- 5 V	WG	A 1	Logic
- 5 V	W5	AlA . I	Integrator,Zero Detect.

facilitate repair of your 3490A when the trouble is on the A2 Assembly.Contact your nearest-hp- Salesand ServiceOffice for details.

7-17.DCAmplifierChecks.

7-18. To check the DC Amplifier zero,setthe Function switch to TEST and select Range5. This grounds the amplifier input and sets the amplifier gain to 100. The front panel displayshould be 000.000 1 15 counts. To check for leakagecurrent in the input circuits,compare the numericalreadingin Test 5 to the readingin Test 6 with the input terminalsshorted.lf the readingin lest 6 is sömewhat higher (igroring the decimalpoint), check for leakagecurrent. Rememberthat no solderingshould be doneon the A2 Assembly(seeParagraph7-14).

7-19. If the DC Amplifier cannot be adjustedto zero in Paragaph 5-60 step b, it is possiblethat there may be leakage to Guard from some point in the circuits. Disconnectthe Guardshortingstrapfrom the input Low terminal.If the offset is removedor canthen be adjusted to zero, leakage to Guard is probable. Some of the possiblecausesare: l) Breakdownof the insulation for power transistorQl or CRl. 2) Leakageto Guard in the power transformer. Disconnectthe small orange wire from the transformer cable to the solder lug on the Guard shieldto check this. 3) If the instrumenthasone of Options 020,021,022,030 or 040, there may be leakageto Guard on the isolationassembly.4) A wire clipping or other metal chip may be wedgedbetweenthe Guardshieldand the Main Circuit AssemblyAl.

7-20. Normal operatingvoltageswrthin the DC Ampli- fier are shown on the schematicdiagram,Figure 7-30. To check the operation of various stages of the amplifier, hrst set the SampleRate control to HOLD

and disconnectthe Bootstrap Amplif-ier(white jumper wire, labeled "Bootstrap" on the Al Component Loca-