Greg,? ? ? ? ? ?
This meter was designed before electronic calibration where gain and offset constant are stored in memory to correct for circuit imperfections.? ?The first DMM Hp produced with electronic calibration was the Hp3468A for which I was the principle analog designer.? In this meter, not having electronic cal, the current source will need to have the converter currents very close to cardinal values hence the need for adjustment pots.? ? While designing DMMs at Hp, I never?gave this instrument serious consideration so never studied this particular design but upon looking over the schematic you posted, this converter is unusual for it SINKS rather than sources current to develop a voltage drop across the unknown which is measured with the voltmeter section of the meter.??
Before you go further, make sure the voltage across the range resistors is not shifting.? I am assuming the range resistors (R52 - R56) have not changed in value which you might want to check by pulling the left lead of R52 up (lead connected to?gate of Q15A)?and put the meter in the 10 Meg range when checking the resistors.? You commented that the range resistors? "all measure proper values; you can force the switches to "all open"? which essentially takes them all out of circuit and makes measurements possible without removing them"? so you may not need to lift the lead on R52.? These should be precision matched resistors - possibly the old custom made wire wound? Red Devils they used back then.? That is just a guess for I have never seen the inside of one of these meters.? Actually, if you can indeed force the range switches to an "all open" condition,??you might want to lift the gate lead of Q15A instead of? R52? for verifying the range resistor values and put a jumper there for other trouble shooting.
First connect a very high impedance voltmeter between the gate of Q15A and the output of U3 and step through the ranges.? This voltage across the range resistors needs to be solid, not changing, at close to 1.000 volt except on the 10 Meg range.? That voltage drops to 0.100 V on the 10 Meg range.? The importance that this voltage not change as you step thru the lower ranges cannot be overemphasized.? If this voltage is solid and the range resistors have not shifted in value, then the reference currents are correct and the problem has to be an unacceptable leakage path between this part of the circuit and the meter front panel input terminals.? A leakage path anywhere from the gate of Q15A to the front panel terminals will affect the lower valued sink currents used on the higher ranges more than it will the higher sink currents used on the lower ranges.? Such leakage paths will cause the errors to increase as you step up in range as the sink current pulled from the unknown resistor under test becomes less by a factor of 10 each time you change to a higher range.? Leakage currents are more likely to generate lower ohms readings than higher readings since the reference resistors are at a potential below ground reference and such currents will flow into the converter decreasing the reference current available to the unknown resistor being measured.
A critical component that could cause a significant leakage path is the AC feedback capacitor C1 that is used to insure proper operation of the differential FET pair Q15.??This capacitor MUST have a quality dielectric.? Any leakage due to this capacitor's equivalent shunt resistance will significantly compromise the operation of the reference currents.? ?Most of the protection circuit components (Q11- Q14, CR12 - CR14, R39 - R43) cannot create a leakage path unless there is a surface impedance problem with the PCB at the component lead thru-holes.? You might also check the PCB to make sure it is clean and clean with alcohol around the ohm's converter and DC input and wash with de-ionized water if possible if you suspect a surface impedance problem.? FR-4 seldom has leakage problems in its bulk.? ?Low level leakage paths will not cause a significant problem on this meter's accuracy since it is basically a 4-1/2 digit meter on all ranges except the 10 Meg range where it degrades to a 3-1/2 digit meter with a reference current of 100 nA.? This means that leakages less than 100 pA will not affect the readings.
The critical components in the protection circuit for leakage are CR13 and Q11.? Gate junction leakage on the Q11 JFET might be a problem if it sustained some damage due to transient ESD.? If you temporarily lift the gate of Q11 and the cathode of CR13, all possible leakage paths thru the protection circuit will be eliminated other than surface PCB shunt paths. The only possible leakage current path then would be thru the reverse biased gate of Q15A as gate current and the meter should work correctly if? Q15A is OK.? If the meter does not work correctly on all ranges when you lift those two leads, then you have a PCB problem or there might also be a leakage path in the voltmeter circuit since that circuit is connected across the unknown resistance under test to measure the voltage across it and any leakage there will also degraded the current sink reference values.
The differential pair should maintain a solid stable voltage at the gate of Q15A approximately?equal to Zener voltage of CR16 at pin 3 of U3 (- 6.2v) and not change as you step through ALL the ranges? EXCEPT the 10 Meg.? ?Connect a high impedance voltmeter between pin 3 of U3 and the gate of Q15A.? On the lower ranges, these two voltages should be within 0.05v of each other and jump to (+ 0.9v) on the 10 Meg range indicating the differential pair is balanced and operating properly.? The FET pair needs to be well matched since there is no DC feedback to insure balance of the differential circuit --? the differential pair is operating open loop at DC.? Any abnormal gate current from the Q15A JFET will also degrade the reference currents of the converter just as any other leakage current will.? The gate current should be on the order of pA.? Ideally one would like there to be no gate current at all.
[[ The voltage shown on the schematic above? CR16? should be? - 6.2? rather than? - 6.3? since? CR16 is a 6.2 volt Zener ]]
Properly operating and adjusted, the output voltage of U3 (pin 6) should be fixed at - 7.2v? on the ranges INCLUDING the 10 Meg range.? When the meter range is changed from 1 Meg to 10 Meg, the voltage at the gate of Q15A should drop from approx.? - 6.2v? ?to? - 7.1v.? That is to say, the voltage at the gate of Q15A needs to be? 0.1v higher than at the output of U3 when the instrument is properly adjusted and calibrated on the 10 Meg range.??
The feedback network of R58, R59, R60, R61 as shown in the schematic?fixes the U3 amplifier gain on ALL the ranges so its output voltage remains at? (-7.2v)? and also allows for fine adjustment of the gate voltage on Q15A? to set it to ( -7.1v)? on the 10 MEG range.? There should be no interaction when adjusting the R59 and R64 pots during calibration for the adjustments are indeed independent.? Any wiper arm setting on R59? for the 10 MEG range calibration does not change the parallel resistance of R59 and R60 in this part of the feedback network when a lower range is selected (1 Meg and lower)?since the pot wiper arm is switched out of the circuit on these lower ranges.??
NOTE:? There seems to be potential problem with the circuit involving U3.??The values as given in the schematic for the gain setting resistors in lower leg of the feedback voltage divider?around U3 are problematic if the nominal Zener?voltage of CR16 is lower than 6.373 volts when? R69? is 3480 ohms as indicated.? If the? CR16? Zener voltage is at the nominal value of - 6.2v? and? R69 = 3480 ohms, the output of U3 will be at -7.173 volts rather than -7.20v? when the? R64? pot is adjusted to its zero limit to provide maximum gain on U3.? The ohms converter will generate a smaller reference current than required for that value of R69.???If? R69 is reduced in value to 2200 ohms, then the adjustment range of the 1K pot,? R64,? will accommodate a? nominal 6.2v? Zener voltage for CR16.? To be within the adjustment range of the 1K pot, R64,? the Zener needs to have a 2% tolerance or better.
IMPORTANT:? ?Measure the Zener voltage of CR16 and if it is lower than 6.372 volts,? R69? will need to be reduced from 3480 ohms for the calibration adjustment range to work.
? ? ? ? ? ? ? ? ? ? ? ? ? ?The optimum value for? ?R69? =? 2200 ohms? ? ?----? see below
However, if instead of the converter having? R69 = 2200 ohms together with a 1K pot, there is just a 10 K pot, then this problem does not arise for the adjustment range would accommodate a range of Zener voltages between 5.6v and 7.9v.? For a single pot, a better choice would be a 5K value which would adjust in Zener voltages between 5.6v and 6.7v resulting in a smoother adjustment for the correct pot setting.? The Zener tolerance could also be relaxed to a 10% type with a 5K pot.
CONCLUSION:
This discussion should allow you to better understand the ohm's converter circuit and troubleshoot any problems you are having.? ?First check your reference currents for proper values by measuring the voltage across the range resistors to make sure it is stable at 1.000 volt on the lower ranges and 0.100 volt on the 10 MEG range.? If this voltage is stable and the range resistors are the correct value, then you certainly have a current leakage issue.? ?Check capacitor C1 first for leakage and the input gate current on Q15A.? Next check the protection circuit for leakage specifically around Q11 and CR13.? Then check the voltmeter input circuit for leakage.? That is about all there is to this.? The circuit is simple and very straight forward.? ?Wish you well.
George Hnatiuk? ? ? ? ? ? ? ? ? ? ? ?Feb. 13, 2019? 2:57am
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? ? ? DESIGN /? SELECTION? ?of? ?ADJUSTMENT? ?CIRCUIT? ?RESISTOR? ?VALUES? for? FEEDBACK? NETWORK? around? U3
? ? ? ? ? ? ? ? ? ? ? ? ?R58? =? 350? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? R60? =? 110? ? ? ? ? ? ? ? ? R63? =? 24.3 K? ? ??
? ? ? ? ? ? ? ? ? ? ? ? ?R59? =? 10 K pot? ? ? ? ? ? ? ? ? ? ? ? R61? = 3.9 K? ? ? ? ? ? ? ? ?R64? ?=? 1 K pot
??
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?R59 // R60? =? 109 ohms
? ? ? ? ? ?R69? to be determined? ( schematic gives 3480 ohms -- value too high )
In order to adjust the reference voltage across the range resistors to be? 1.000 volt? on the lower ranges so that the reference current will be a cardinal value of milliAmps ( 10mA, 1mA, 0.1mA? etc.), the relationship between the Zener voltage, Vz, the?feedback resistance, Rf, and the combined resistance of the lower leg of the feedback network, R1,? is? ? ?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?[[? ?Vz *? Rf? =? R1? ? ]]? ? ? ? ? ? since? ?1v? ?=? Vz * ( 1? +? Rf / R1 )? -? Vz
? ? Rf? =? R58? +? R59 // R60? +? R61? =? ?4359 ohms? ? ? ? --0--? ? ? ? ? ?R1? = ( R69? +? R64 ) +? R63? =? ( R69? +? R64)? +? 24.3K?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ??
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?R1? ? ? ?=? ? ?Vz *? Rf? ??
? ? ? ? ? ? ? ? ? ? ? ? ? ? ( R69? +? R64)? +? 24300?? ?=? ? ?Vz * (4359)
? ? ? ? ? ? ? ? ? ? ? ? ? ? ?( R69? +? R64 )? ?=? ? Vz * 4359? -? 24300
? ? ? ?so if? R69? =? 2200 ohm,? ? then? ? ? ?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ?( 2200? +? R64 )? ?=? ? Vz * 4359? -? 24300? ? ? ? or? ? ? solving for Vz:
? ? ? ? ? ? ? ? ? ? ? ? ? ?Vz? ?=? ?[( 26500? +? R64? ) / 4359]?
? ? ? ? ? ?R64? is a 1K pot:? ? ?0? <? ?R64? <? 1000? ? ? ? ? ? ? ? ?add? ?26500? and divide by? 4359? throughout the inequality:
? ? ? ? ? ? ? ? ?26500 / 4359? <? ?[( 26500? +? ?R64 ) / 4359]? ?<? ?(1000? +? 26500) / 4359??
? ? ? ? ? ? ? ? ? ? ? ? ? 6.08? ? ? ?<? ? ?[ Vz ]? ? <? ? ? 6.31? ? ?
?For a 2% Zener with nominal Zener voltage of? 6.2v,? ?Vz? =? 6.2v? +/-? 0.12v? rewritten as? 6.08? <? Vz? <? 6.32? ?which matches the designed adjustment range computed above of? ? 6.08? ?<? ? [ Vz ]? ?<? ?6.31? ? ?
? ? ? ? ? ? ? ? ? If? R64? is a 1 K ohm adjustment pot, then the proper choice for? ? R69? ?is? ?2200 ohms??
