Avo EA113 electronic multimeter

[Chris55000]

Hi!

I've just managed to get one to refurbish at a price I was willing to pay (two figures!!!) - in fact I'm not going to attend the Spalding DARS Rally as an EA113 was one item I was hoping to find!!!

Comes with original olive-green Instruction Manual which will be scanned and posted here for others to use!

Any tips to watch for?

Also, can anyone tell me how many different Circuit Diagrams are in exsistence and how to differentiate them please?

Additionally, can anyone PM me the "Fet Tester 467" Drawing Reference A6170-018 please to test the FETs as recommended please?

Chris Williams

New fully annotated circuit-diagram of Amplifier PCB to follow at a later date!

[Radio Wrangler]

To watch out for....

The battery life is remarkable (see, I just remarked on it) Keep an eye on them. Their electricity will last longer than their sealing given todays batteries.

Superb little meters

David

[Chris55000]

Hi!

Reasonable copy of S.M!

https://www.dropbox.com/s/8sd57pj6tu...D6578.pdf?dl=0

Chris Williams

[Phil G4SPZ]

Chris, it would be helpful if you would post a picture of the meter you have bought.

In my humble opinion, these meters are not candidates for 'restoration' unless you mean in the purely cosmetic sense. More sensibly, you would (a) see if it works, then attend to and rectify any faults; then (b) test it against some reasonably accurate standards. If the meter is in calibration, there is no need to test the existing FETs, because as I understand it the FET tester circuit is provided solely to enable the repairer to select, on test, replacement devices having suitable characteristics to replace either of the two FETs that are found to be faulty. You will only need to do this if step (a) above confirms that an FET is indeed faulty.

If the meter doesn't work, or works on certain modes/ranges, or is out of calibration, then you'll need to use normal fault-finding and rectification techniques.

These meters don't lend themselves to easy fault-finding, due to the difficulty in accessing the components. When you remove the rear case you will see what I mean. It will immediately become obvious that you won't want to start undertaking any unnecessary work, as the complexity of the dismantling and reassembly process - together with any interim testing - is daunting to say the least. Just find and repair the fault(s) then check the calibration.

[Sinewave]

Hopefully you've got yourself a nice one.

Recently there were some very rough looking ones on that famous auction website at eye watering prices for the condition.

I'm still on the look out for one of these but the ones I saw recently I couldn't justify the asking prices.

[Radio Wrangler]

I really don't understand what's inflating the prices. They've been silly for a few years?

Could it be something like the Avo VCMs which are well known to improve the sound of any valve tested on them?

I bought mine a few decades ago at M&B's. 20 quid seemed a lot back then, and I'd never seen one before but I read the specs and liked what I saw.

David

[Sinewave]

Originally Posted by Radio Wrangler

I really don't understand what's inflating the prices. They've been silly for a few years?

If something was really nice and rare, complete, with case and everything, one might pay a bit more.

But recently, the two or three EA113's I saw on eBay looked as though they were dragged from the bottom of a skip. One went for around £70 and it looked incredibly rough and no information on whether it was working or not.

[mole42uk]

Originally Posted by Chris55000

New fully annotated circuit-diagram of Amplifier PCB to follow at a later date!

I have just drawn it in kicad.

[mole42uk]

Here is the DC amplifier:

[Phil G4SPZ]

Hi Richard,

Thanks for the redrawn diagram. It is so much more helpful being able to see the component values on the circuit!

Regarding the voltages, I certainly agree with those you measured around VT2 and VT3, but are you quite sure about the 2.9 volts on the base of VT6? With no signal input, the voltage at both sides of R11 should be virtually zero w.r.t. the 0 volt rail, due to the clamping action of VT4. Did you by any chance measure the 2.9 volts with respect to the -ve supply rail rather than 0 volts?

It would also be interesting to know what type of voltmeter you used, and whether the EA113 was set to left-hand zero or centre zero mode.

Thanks again,

Phil

[Chris55000]

Hi!

I'll post pictures when it comes – the listing I bought it from didn't have very good ones! One of the three pictures (appeared!) to show the meter reading an AA cell, but how far it went it went up the scale I couldn't tell for certain!

(I remember an ex–colleague (passed on unfortunately!) used to say "you've been Photo– shopped!!!" when I received a piece of T & M distinctly tattier than the pictures!

Another case in point – I won a C3–U with (according to the pictures!) a semi–usable eye, but when it came it was far from gratifying – either the seller filched it before dispatch knowing what most EM34's are like and what they fetched, or used a doctored or stolen picture!

(I didn't know about S.N.A.D. at that time!)

Chris Williams

PS!

Richard's beaten me to the redrawing – I've only just started on a fresh drawing–sheet as I'd no A3 template in my circuit–drawing software!

[mole42uk]

You may be right Phil, I’ll check it.

The EA113 was set to 30V on left-zero. The meter I used to check voltages is a Keighley 175 DMM.

[mole42uk]

An update for you Phil, the voltage I'd erroneously marked as 2.9V was actiually 0.003V - an error I put down to not noticing that the Keithley 175 was set to 'autoranging'. Either that or I was careless.

All the measured voltages were taken with the -ve input of the Keithley connected to the negative end of C2 which is the 0V rail.

I've added a few more voltage measurements to the circuit and include it here:

[Phil G4SPZ]

Hi Richard,

Many thanks, those voltages make a bit more sense. However I may now have to re-think my ideas about the operation of the VT5-VT6 stage. VT6's base-emitter junction is reverse biased with the voltages shown, if the 0.5 volt reading at VT6's emitter is genuinely positive, so VT6 will be cut off, rather than conducting as I'd first assumed. If VT5's base voltage is -0.8V, VT5's emitter voltage should be around -1.4V, so how can +0.5V appear at the other end of RV4 if VT6 is cut off? I could perhaps understand if it were minus 0.5V...

The picture is complicated by the presence of the set zero pot RV4, and its setting will alter the voltages. Basically, I'm just too lazy to dismantle my meter again and check the voltages myself! It's all working perfectly at the moment...

The annotated circuit is really useful, and I have saved a copy for future reference but in the hope I'll not need it! One last point is that VT4 is drawn the opposite way up on your version compared with the original Avo circuit diagram, which showed the transistor connected with its collector (rather than its emitter) linked to the 0V rail. I did wonder initially whether this was a mistake on the original diagram, but as you have numbered the leads, I'm assuming that's how your example is wired. It certainly makes more sense than the form of connection shown on the Avo diagram, which caused me no end of confusion to start with.

Thanks for taking the trouble to draw it all out.

[mole42uk]

Hello Phil,

I suspect that the DC conditions are only partially useful in understanding the circuit operation when looking at a circuit with a chopper in the signal line. I think I might have another go through with the 'scope and get some signal images for each point which should be more useful. It might take a while though, all this investigation of my one working meter isn't allowing time to fix the poorly one!

Looking at my version 2 of the circuit I think that VT4 may be incorrectly drawn as it appears to have -1.8V on the base. I'm not sure that voltage is correct because I can't see a DC path from TAG20 (-batt).

I'm halfway through the AC amplifier circuit but I might have to investigate the DC one before I finish that!

[Phil G4SPZ]

Originally Posted by mole42uk

I suspect that the DC conditions are only partially useful in understanding the circuit operation when looking at a circuit with a chopper in the signal line... I think that VT4 may be incorrectly drawn as it appears to have -1.8V on the base. I'm not sure that voltage is correct because I can't see a DC path from TAG20 (-batt).

Hi Richard,

I agree that some DC voltage readings are probably misleading.

The signal coming from the multivibrator (VT10 collector) should be a square wave but not necessarily a 1:1 mark:space ratio nor symmetrical about zero, which is why you're seeing a net negative DC voltage on a DVM. Once it has passed through C32, which together with the forward-biased B-C junction of VT4 acts as a differentiator, the positive-going pulses will be loaded down so the DC voltage reading will appear more negative, as you've measured.

The actual signal on the base of VT4 will probably look like a series of spiky positive half-cycles and square-ish negative half-cycles. It will be really interesting if you can post some scope traces!