While I have some hard data, I have realized a significant handicap.
Both of my 3478As are 2911 series Serial numbers, very late production. The only service manuals I can find apparently don’t cover these late models. For example the manual I have contains schematics for circuit board part number 03478-66501 Revision B and C circuit boards. My units have board 03478-66511 Revision A. My boards do not have JM403, so the signature analyzer I have coming will be useless for examining the A/D controller. Hard to tell what other changes were made to the later board, but not having a suitable schematic is troublesome.
For what it’s worth, here is what I have so far. Ohms troubleshooting instructions:
7-C-8. Inaccurate Readings on All or Some Ranges
7-C-9. This failure is normally caused when the ohms current changes value due to a load change. Do the following procedure.
a. Set the 3478A to the 2-Wire Ohms Function and the 3K ohm Range.
b. Connect a 3K ohm Resistor to the 3478A's INPUT Terminals.
c. Using a high impedance Digital Voltmeter (like the 3456A), measure the voltage across R205. Since the ohms current on the 3K ohm Range is 1mA, the voltage should be approximately .47V dc.
d. If the voltage is radically wrong, the ohms current is incorrect. Use the overload troubleshooting procedure (in paragraph 7-C-6) to determine the faulty circuitry.
I have several microvolts only across R205. So I go to the overload section:
7-C·6. Overload Readings on All Ranges
7-C-7. An Overload is normally caused by a high ohms current (or an open between the INPUT Terminals and the Input Circuitry). Make sure the DC Volts Function is operating correctly, before troubleshooting for an ohms failure. Do the following procedure.
a. Set the 3478A to the 2-Wire Ohms Function and the 3K ohm Range.
b. Connect a 3K ohm Resistor to the 3478A's INPUT Terminals.
c. Using a high impedance Digital Voltmeter (like the 3456A), measure the voltage between pin 17 and pin 19 of Ul02. Connect the low input of the voltmeter to pin 17 and the high input to pin 19.
d. If the reading on the test voltmeter is + 4V dc and the 3478A displays an overload, the Range Resistors in Ul02 may be too low. Replace Ul02 .
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e. If the reading is other than + 4V dc, the Ohms Current Source is defective. Go to paragraph 7-C-20 for troubleshooting.
I have 4.1Vdc but there is no OVLD display. Frustrated, I decided to check current directly. I am getting the appropriate current on each range, within the range of accuracy and noise floor at milliamp to microamp values afforded by my K2015 with 24 inch leads. I checked current with my K2015 in series with the 3478 inputs to the 3K resistor, as well as voltage drop across a .1 ohm, 1% resistor and the numbers are correct.
Lastly, the state of COTO relays K101, 102, 103 and 104 do not change on any ohms ranges. K101 and 103 are closed and K102 and 104 are open. The only thing that changes between ohms ranges is the voltage to the contacts.
So, unless someone knows if a service manual or schematics exist for later units with the 03478-66511 circuit board please share. My service manual is number 03478-90008. I have found one 03478-99008 but it has the same publication date so I doubt it is any different. I am assuming others I have seen available, 03478-90004, 90005 and 90006 are even earlier. If someone knows differently, please advise.
Thanks all for your feedback. Maybe some of this data will be useful to others with 3478As as well.
Jeff