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Old 1st May 2017, 2:11 pm   #1
Phil G4SPZ
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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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Old 2nd May 2017, 5:35 pm   #2
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Default Re: Avo EA113 electronic multimeter (1971)

Never seen that model before. When something so useful can be repaired fairly easily, it's daft not to, isn't it? Great stuff.

Regards,
Paul
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Old 2nd May 2017, 8:01 pm   #3
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Default Re: Avo EA113 electronic multimeter (1971)

Excellent fix there Phil, that must have have been very satisfying to bring it back to full usefulness. I think that they're a really good case of something that combines being a nice piece of industrial design and managing to be genuinely useful as well,

Colin
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Old 2nd May 2017, 8:10 pm   #4
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Default Re: Avo EA113 electronic multimeter (1971)

Good repair Phil. I fancy one of these but they're always pricey when they come up for sale. I've never seen one in the flesh, are they around 1/2 the size of the AVO 7/8?

Thanks - Andrew
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Old 2nd May 2017, 8:46 pm   #5
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Default Re: Avo EA113 electronic multimeter (1971)

With a sensitivity of 1meg per volt that might even be suitable for the very basic testing of the voltage cycling of a petrol engine lambda/oxygen sensor! (if the needle can respond quickly enough..)
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Old 2nd May 2017, 10:51 pm   #6
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Default Re: Avo EA113 electronic multimeter (1971)

Phil, I have a few NOS 68Meg VR68 resistors here.

http://www.vishay.com/docs/28907/vr25vr37vr68.pdf

If you want one I can pop it in the post. FOC+ postage, just PM me your details

Al
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Old 2nd May 2017, 11:13 pm   #7
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Default Re: Avo EA113 electronic multimeter (1971)

Quote:
Originally Posted by Andrewausfa View Post
Good repair Phil. I fancy one of these but they're always pricey when they come up for sale. I've never seen one in the flesh, are they around 1/2 the size of the AVO 7/8?
I've had one since the 80's when it cost me about 25 at M&B's in Leeds.

I've seen the prices they've been fetching and gabberflasted is an understatement. People have gone silly.

1) It's a very nicely made instrument
2) It has much greater impedance than passive instruments
3) A set of 4 AA batteries last for many years, so don't use Duracell
4) It goes down to 10mV FSD DC and 10uA thanks to its chopper amplifier.
5) I put mine on a Datron standards lab calibrator and its accuracy was fine.
6) AC measurements see flat to around 200kHz so it's an audio millivoltmeter
7) It isn't good at low Ohms
8) You seem to get less battery current if you not only turn it off on the main switch, but also select battery test on the left switch.
9) Just AA cells, no funny 15v batteries.
10) I wouldn't risk mains into it set to current or Ohms, I don't think it's as bomb proof as a odern Fluke DMM.

The black cased ones seem rarer than the blue cased one.


So, it's a very very good analogue meter.

Put one on your 'don't miss the chance if you see one' list but don't sell your first-born

David
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Old 3rd May 2017, 12:03 am   #8
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Default Re: Avo EA113 electronic multimeter (1971)

Thanks for all the interest, and the kind comments! I was going to paste a picture showing the relative sizes, but David has beaten me to it, and provided lots of other relevant information too. However it's worthwhile to look at the EA113 against the Avo 8 Mk V with its later style of movement. They can't be the same, as the movement in the EA113 is 3.5mA FSD rather than the Avo 8's 37.5uA, but they do look physically similar and I suppose it would have made sense from a manufacturing point of view.

Alistair, yes please, one of those 68 Meg resistors would be ideal, thank you very much for your generous offer. I've PM'ed you with my details.

Philpott, you asked about the speed of response of the pointer, and that's perhaps the major shortcoming of this instrument. My example does respond pretty slowly, about half as fast as an Avo 8 Mk V, so if you're trying to follow rapidly-changing quantities the EA113 would smooth things out rather too well. There's a 10uF tantalum capacitor across the input to the DC amplifier, and I guess this makes everything move rather slowly, including when setting-up the master zero and gain presets.

Having said all that, I agree with Colin that it's a stylish piece of design which is also ergonomically easy to use, probably even easier than the traditional Avometer with its twin range selectors. Mine has made itself at home on the test bench where it has proved useful a couple of times already.

Phil
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Old 3rd May 2017, 2:33 am   #9
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Default Re: Avo EA113 electronic multimeter (1971)

That's a nice meter, and a good repair. I was given a very badly abused one of these a long while back, the covers for the terminals are missing, the 2 smaller knobs are broken, and the front case is broken in half with a bit missing. It looked most un-promising, and I was going to offer it here for free to whoever wants it, or worse still, bin it! In the end I decided to try it out, and surprisingly it works quite well, despite it's condition. I've since glued the front back together, and I'm contemplating giving it a coat of paint, if I can find something that won't chip off easily. I decided to keep it after that, and maybe loose a broken Avo 8 and a GEC selectest. I'll have to look for that manual you mention, as the spacers in mine are just where they looked like they fitted!

Regards,
Lloyd.
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Old 3rd May 2017, 6:04 am   #10
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Default Re: Avo EA113 electronic multimeter (1971)

One thing I forgot to say is that they are very susceptible to RF. If you use one to perform any adjustments on a transmitter, readings can go all over the shop. Having it measuring something else when it's your over on HF can also be entertaining.

David
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Old 3rd May 2017, 7:56 am   #11
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Default Re: Avo EA113 electronic multimeter (1971)

Hi Lloyd,

The service manual can be downloaded free from the excellent Elektrotanya website here: https://elektrotanya.com/avo_ea113_e.../download.html

It contains exploded diagrams and guidance on the positions of shims and spacers. The design requires the case to be clamped by the four screws without putting undue stress on the seal area between the case and the front panel. If incorrectly fitted, the corners of the case can be cracked, as my example shows

Good point David about RF immunity. I once wasted some time trying to trace the cause of drifting zero in another electronic voltmeter, only to discover that it was picking up the transmissions from the mobile phone in my pocket!
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Old 3rd May 2017, 11:22 am   #12
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Default Re: Avo EA113 electronic multimeter (1971)

I have three of these. One is permanently on my bench, one is "First reserve" the other is in the infirmary.

I have half a re-drawn circuit diagram on my drawing board at the moment since the only ones I found are hopelessly low resolution.
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Old 3rd May 2017, 11:41 am   #13
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Default Re: Avo EA113 electronic multimeter (1971)

I have an original user handbook, from which I scanned the circuits at 300dpi. Printed out at A4 size, they are clear enough to read.

PM me if anyone would like a copy of the scans.
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Old 3rd May 2017, 11:57 am   #14
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Default Re: Avo EA113 electronic multimeter (1971)

I was able to scan an original (did you lend it to me, Phil, I forget?) but it's the "Fig.4: Instrument Circuit Diagram" that is difficult - even in the original manual, using an illuminated magnifier, some if the print is very hard to read accurately.
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Old 3rd May 2017, 1:16 pm   #15
Phil G4SPZ
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Default Re: Avo EA113 electronic multimeter (1971)

No Richard, it wasn't me. I have taken a photo of the main circuit diagram which is perfectly legible, but the thumbnail loses the resolution. Let me have your e-mail address by PM and I'll send it as a full-sized attachment.

Phil
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Old 3rd May 2017, 1:19 pm   #16
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Default Re: Avo EA113 electronic multimeter (1971)

Hi Phil,
Thanks for pointing me to the manual, hopefully I can get this bashed up thing working to spec! I was surprised to see that the bit of vero-board on the back of the meter movement is actually an original part, I thought someone had been bodging mine when I looked in it. I have noticed the movement is out of balance, but it doesn't stick, so the pivots and jewels are good. Certainly more promising than my poor old Avo 8, which had a sticking problem, and while trying to sort it the coil went OC

Regards,
Lloyd.
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Old 3rd May 2017, 4:43 pm   #17
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Default Re: Avo EA113 electronic multimeter (1971)

Aargh! O/C moving coils tend to be terminal... the movement in my EA113 is out of balance too.

Richard, I've e-mailed you the diagram. Fingers crossed!
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Old 3rd May 2017, 11:14 pm   #18
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Default Re: Avo EA113 electronic multimeter (1971)

Quote:
Originally Posted by Phil G4SPZ View Post
...it's worthwhile to look at the EA113 against the Avo 8 Mk V with its later style of movement. They can't be the same, as the movement in the EA113 is 3.5mA FSD rather than the Avo 8's 37.5uA...
Update - I just spotted my own deliberate mistake According to the components list, the EA113's movement is indeed much the same as the one in the later Model 8s, with a 37uA FSD.
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Old 19th May 2017, 11:18 pm   #19
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Default Re: Avo EA113 electronic multimeter (1971)

Alistair, the 68 Meg resistor was waiting for me when we arrived home this afternoon. Many thanks; see post here!

https://www.vintage-radio.net/forum/...=60832&page=14

I'll fit it shortly, making it up to the actual specified value of 68.38 Megs +/- 0.5% by adding a selected 390k resistor in series. The 100, 300 and 1,000 volt ranges do indeed read fractionally high with just 68 Megs in place.
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Old 30th Apr 2018, 11:20 pm   #20
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Default Re: Avo EA113 electronic multimeter (1971)

Thank you for re-opening this old thread, Bill.

There’s been a more recent repair episode involving this particular instrument, and the fault turned out to be due to a single component, so I thought I’d add it to my previous report.

Over time, I noticed that on DC voltage ranges the meter started drifting from zero, and this was specifically temperature-related. On cold days, the pointer would drift down below zero, but on warm days it would drift up well above zero. On DC, there is no external ‘set zero’ control, and so I was forced to use the meter in ‘centre zero’ mode. The meter remained accurate, but this problem bugged me.

It took me the best part of a year to get around to fixing it, much of that time spent studying the circuit, reading up on chopper-stabilized DC amplifiers, and trying to fathom out just how the DC amplifier worked. I appealed for advice on the Forum, and fortunately both Peter Munro and Ron Bryan respectively came up with some missing pages from the manual and some suggestions.

I initially started out looking for heat-sensitive components, but quickly found that even the radiant heat from my fingers in the vicinity of VT5 or VT6 was enough to upset the zeroing. Placing a finger on either transistor sent the pointer racing off above or below the scale end stops, so it seemed that the chopper stabilizer wasn’t working. I calculated from the circuit values that the astable multivibrator that drives the chopper circuit should be running at around 600Hz, and with the aid of a simple signal tracer I found that it was indeed running as it should.

DC voltage checks showed that there was a negative potential of a few millivolts at the emitter of the chopper demodulator VT4, whereas with no input this should have been zero. Ron had pointed out that bipolar transistors used in this type of circuit are often connected in inverted mode (i.e. emitter acting as collector and vice-versa) as this achieves a very low Vce(sat) figure in the microvolt range when the demodulator transistor is turned hard on by the square-wave drive from the multivibrator. I also reasoned that, with zero input, there should be no AC at all coming out of the second voltage amplifier VT3, but sure enough, using the signal tracer there was a pronounced 600Hz buzz at its collector. Not only that, but the same buzz could be heard across the 64uF electrolytic emitter bypass capacitor C2...

Bingo! An emitter bypass capacitor should behave as a virtual short-circuit at the lowest frequency of operation. I temporarily lashed-in a replacement capacitor across C2. The buzz immediately vanished, and the pointer returned to zero. I snipped out C2 and replaced it with one of the same value, 47uF (although the Avo components list states 64uF) and the drift vanished. When I measured the old capacitor, it read just 270pF! After careful adjustment of RV4 and RV1 the meter was perfectly stable, and well within the +/-1% accuracy quoted in the manual. The other big improvement is in the pointer’s speed of operation; no longer is it sluggish, and it now behaves like any other electronic multimeter.

I took the opportunity to replace R81 and R48 with the new 68 Megohm resistor kindly sent to me by Alistair D exactly a year ago, in series with a 390k resistor selected from my own stock to bring the combined value as close as possible to the required total of 68.38 Megohms. This restored the 1,000 volt range to full accuracy.

Needless to say, I am absolutely delighted to have been able to restore this desirable meter to full working order by the identification and replacement of a single faulty component.

Having spent so long studying the operation of the DC amplifier in order to understand its method of operation, I can’t resist sharing it with you for the benefit of anyone else who has the misfortune to have to repair one of these meters. Feel free to skip this bit if you’re losing the will to live, but prior to this little project I knew nothing about chopper-stabilised DC amplifiers!

The DC amplifier circuit occupies part of the ‘amplifier board’ which sits behind the meter movement and in front of the range board, making access very difficult. The DC amplifier itself is a chopper-stabilised analogue operational current amplifier with a basic gain of 37, i.e. 1uA input gives FSD on the 37uA movement. The gain is set by the feedback resistor R10 in conjunction with the series multiplier resistors on the range board, resistors R8 and R9 being switched in to reduce the amplifier gain on the 300V and 1,000V ranges.

VT1 is a shunt chopper modulator which is driven by a square wave from the astable multivibrator VT9 and VT10, running at around 600Hz. On the positive-going multivibrator pulses arriving at VT1’s gate via C32 and D3, VT1 turns hard on and shunts the input voltage to zero. On negative-going multivibrator pulses, the input is coupled via C2 to the base of VT2. VT2 therefore sees a succession of positive pulses proportional to the magnitude of the input voltage. VT2 and VT3 form a conventional AC amplifier, with base bias and DC negative feedback via R4. C2 bypasses VT3’s emitter resistor R7, which is returned to the -2.8 volt rail.

An amplified version of the input pulse train appears at VT3’s emitter, and is AC-coupled by C3 to the chopper demodulator transistor VT4. VT4, connected in ‘inverted mode’, shorts the AC signal leaving C3 to ground in synchronism with VT1, resulting in an amplified positive-going train of pulses at its emitter, which is filtered and smoothed (integrated) by R11 and C5. The resulting DC potential across C5 is applied to the base of VT6 which, in conjunction with VT5, form a long-tailed pair or differential amplifier. VT5’s base bias is fixed, and the current through tail resistance R14 is substantially constant. RV4 provides the means of setting the zero (with no input) by adjusting the ratio of current flowing through VT5 and VT6.

With no DC input, the voltage at the junction of C3/VT4 emitter is zero (or just a few microvolts above zero, due to the clamping action of VT4) and hence the voltage at VT6's base is also zero with respect to the 0 volt rail. However, VT6 is conducting, due to its emitter being returned to the -2.8 volt rail via part of RV4 and R14. VT5 is also conducting, to a degree determined by the position of the 'set zero' pot RV4. VT7 and VT8 also conduct, to the extent that the net potential difference across the meter, i.e. between the junction of D4 cathode/R17/R16 and RV1/R20, is zero. Note that the circuit diagram contains an error here; “SB4” should read “SB6”, and “SB3” should read “SB5”. These refer to switch contacts shown on the instrument main wiring diagram, and this error caused me a good deal of head-scratching at first! Essentially, SB6 goes to the movement’s negative terminal, and SB5 goes to its positive terminal. Resistors R17, RV1 and R20 form an adjustable shunt across the meter movement, and hence RV1 is the main ‘set calibration’ control for the whole instrument. Once set, with 30mV applied on the 30mV range as described in the manual, the calibration should be correct across all ranges.

With a positive-going DC input present, VT6 conducts harder and the current through VT5 falls, causing VT7 to turn on harder. The current flowing through emitter-follower VT8 falls and the potential at its emitter and at the cathode of D4 moves more negative, causing current to flow via R20 through the meter and moving the pointer clockwise. The same mechanism applies with a negative-going DC input. The useful centre-zero function is provided by R1 and R2; a fixed bias is applied via the centre-zero switch which effectively makes the movement sit at mid-scale. The instrument then responds to input voltages of either polarity.

There is a feedback mechanism whereby any drift in the output under no input conditions sends an opposing potential back to the input, via the gain-setting resistors R8, R9 and R10, which act to correct the drift. The output of the DC amplifier is a current drive of up to 37.5uA to the meter movement. If drift occurs anywhere in the DC-coupled stages VT5 to VT8, a proportion of this drift is applied via R10/R9/R8 back in anti-phase to the input. As the entire amplifier is inverting, this negative feedback acts to stabilize the DC operating conditions of the amplifier and render an external ‘set zero’ control unnecessary.

Capacitor C2 had gone open-circuit, allowing AC feedback, causing the DC conditions to float about and permitting the multivibrator pulses to appear as an output at VT3’s collector. This manifested itself as the original symptom of temperature related drift. The thumbnails show the circuit diagram and the offending faulty capacitor.

Phil
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