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Old 1st May 2017, 2:11 pm   #1
Phil G4SPZ
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Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
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Default Avo EA113 electronic multimeter (1971)

This instrument was kindly donated by an old friend who is moving house and having a clear-out. It worked on some of the AC ranges, but the DC and ohms ranges wouldn't zero. The donor described having tried to replace a failed 68.38 Megohm multiplier resistor R81/R48 (with a 1 Meg, all he had...) so the back had to come off. Fortunately there was no battery corrosion and everything looked in pretty good condition, apart from some dubious soldering and evidence of incorrect reassembly.

I spent some time researching this meter as I'd not worked on one before. There is some excellent info available on the Forum and elsewhere, including a full service manual and some images of the insides which enabled me to reassemble mine (correctly, this time) as the positions of spacers etc is crucial. Some differences exist between the images I found and my meter, which has a single black component for R81/R48 whereas other examples have two separate blue resistors of 34.19 Megohms each.

I cleaned up the dodgy soldering and replaced the incorrect resistor with a 68 Meg thick film component that I just happened to have in stock. This checked out fine on my battery Megger at 500 volts, but I'm not sure of its actual voltage rating, so a permanent replacement with a higher voltage-rated resistor will have to follow in due course. After cleaning and adjusting the DC set-zero pot RV4, I was able to obtain stable zero settings on both left-hand and centre-zero positions. Most of the ranges now appeared to work, with the exception of the x100 ohms range.

All the resistors in the ohms circuit measured correct, but a dry joint was discovered at one end of R59 (10k 1%) and this was stopping the input voltage from the potential divider network reaching the DC amplifier. Re-flowing the joint cured the fault. After carefully zeroing the movement and the DC amplifier, the meter now appears very accurate on all ranges.

The sensitivity is 1 Megohm per volt on all ranges up to 300 and 1,000 volts, with a maximum of 100 Megohms input resistance on the upper three ranges. The centre-zero facility is also available on all DC ranges, except 300 and 1,000 volts. The reason is that the total series resistance of the multiplier chain (100 Megohms) is left in circuit on the 1,000, 300 and 100 volt ranges, the upper two ranges being obtained by reducing the gain of the DC chopper amplifier in two fixed steps. The centre-zero function is achieved by applying an adjustable DC bias to the input of the DC amplifier, which is sufficient to drive the movement to half FSD only when the DC amplifier is at full gain.

I'm obviously very pleased to have this meter and the repair has been an interesting exercise.

Phil
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Last edited by Phil G4SPZ; 1st May 2017 at 2:17 pm. Reason: Omissions
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