22-08-2018, 10:14 PM
Hi Antony,
At least you were getting something on AC, up to a point.
Have you got the service manual showing the calibration procedure? There is actually no 'master calibrate' control for the AC ranges on the EA113 and no way to adjust FSD if it's reading incorrectly. The fact that all the AC voltage ranges were initially reading 10-11% high suggests that a resistor in the AC potential divider chain had changed its value, or else the gain of the AC amplifier is excessive. The presence of noise on the 0.01V range suggests that there might be a problem on the AC amplifier board. I'd suspect faulty decoupling (C22 or C24) or a problem with C8, C9 or C10, or even VT12 itself, but to be honest there are a host of potential components that could give rise to this fault. Check the obvious things first though, like are the switch contacts around SA4 all nice and clean?
Now that a 'new' fault has occurred, though, which is apparently placing an excessive load on your calibrator's output, you need to look for components within the input attenuator which are in circuit on all AC ranges.
As I read it, switch SA2 places three different values of series resistance into circuit, and switch SA4 varies the gain of the AC amplifier in four steps, together giving a possible twelve voltage ranges, of which eleven are actually used.
What I would do first is measure the DC input resistance of the instrument when switched to AC. You should see something close to the AC ohms-per-volt rating times the FSD of the voltage range in use. If it is much lower, you need to check for a leakage path to earth. On the highest seven ranges, switch SA2 earths the lower end of R65 and/or R66, so those resistors, together with R64 and all the capacitors associated with that network, should be the first ones to check.
At least you were getting something on AC, up to a point.
Have you got the service manual showing the calibration procedure? There is actually no 'master calibrate' control for the AC ranges on the EA113 and no way to adjust FSD if it's reading incorrectly. The fact that all the AC voltage ranges were initially reading 10-11% high suggests that a resistor in the AC potential divider chain had changed its value, or else the gain of the AC amplifier is excessive. The presence of noise on the 0.01V range suggests that there might be a problem on the AC amplifier board. I'd suspect faulty decoupling (C22 or C24) or a problem with C8, C9 or C10, or even VT12 itself, but to be honest there are a host of potential components that could give rise to this fault. Check the obvious things first though, like are the switch contacts around SA4 all nice and clean?
Now that a 'new' fault has occurred, though, which is apparently placing an excessive load on your calibrator's output, you need to look for components within the input attenuator which are in circuit on all AC ranges.
As I read it, switch SA2 places three different values of series resistance into circuit, and switch SA4 varies the gain of the AC amplifier in four steps, together giving a possible twelve voltage ranges, of which eleven are actually used.
What I would do first is measure the DC input resistance of the instrument when switched to AC. You should see something close to the AC ohms-per-volt rating times the FSD of the voltage range in use. If it is much lower, you need to check for a leakage path to earth. On the highest seven ranges, switch SA2 earths the lower end of R65 and/or R66, so those resistors, together with R64 and all the capacitors associated with that network, should be the first ones to check.
Phil (BVWS & IET member)
