Avo EA113

[Sinewave]

Hello all.

I've managed to get hold of one at long last, however it doesn't appear to work.

When I switch on, the needle wants to hit far left, I can't zero at all or get any readings.

I have downloaded the service manual for it and am giving that a good read.

However, are there any folks here who's delved inside one of these?

Regards.

[Phil G4SPZ]

Hi Antony.

I think you're just going to have to bite the bullet, take the back off and take a few voltage readings around the DC amplifier. There is plenty of information to assist you in recent threads elsewhere. It will be worth it; when working properly, these meters are superb.

Good luck,

Phil

[Sinewave]

Hello Phil,

Yes I'll need to open the unit up and take a good look and make some tests inside.

I note some information in a thread of yours somewhere suggests checking the DC amplifier and to check are that both positive and negative supply rails are present and equal.

I look forward to getting this working as they do appear to be quite rare indeed. I've rarely seen them available and on the odd time I have, they have been quite expensive or appeared to be visually quite dishevelled.

[Sinewave]

Right, so some tests tonight tell me that....

This meter does likely have what appears to be the common DC amplifier fault.

I have it right now connected up to my calibrator and it is reading in AC.

Results as follows:

• 0.01V range appears too noisy to test.
  
• 0.03V reads 15% high FSD
  
• .01V range reads 11% high FSD
  
• 0.3V range reads 11% high FSD
  
• 1V range reads 10.6% high FSD
  
• 3V range reads 10.6% high FSD
  
• 10V range reads 10.7% high FSD
  
• 30V range reads 10.5% high FSD
  
• 100V range reads 10.3% high FSD
  

Then on the 300V range things took a turn and the calibrator didn't like it and shut down.

Quick check with a fluke shows the calibrator is fine to test the 300V and 1000V ranges, but something has occured inside the EA113 during the tests as now it appears to be shutting the calibrator down on any AC voltage range.

I think the EA113 could take a sinister turn if I tried it on the variac, as the calibrator appears to be protecting the EA113 from some kind of impending doom.

So, there's another little thing for me to work out.

[Sinewave]

Some AC current tests show some functionality on current AC.

Some of the readings are a bit silly, but here goes.

• 0.01mA range needed 407uA to read FSD
  
• 0.1mA range needed 489uA to read FSD
  
• 1mA range reading 40% low
  
• 10mA range reading 6% low
  
• 100mA range reading 2% low
  
• 1A range reading 1.6% low
  

3A range I'll need to test by plugging in another module (another night).

[Phil G4SPZ]

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.