Avo H.I.108

[derek.ireland]

Hello.

Can anybody help with some info on this meter? It reads OK on DC, but reads low on AC both voltage and current. I try not to touch pots if I don't know their job, but my fingers are getting itchy.

Thanks for any help, Derek.

[pmmunro]

Derek,

Welcome to the forum. We always like to help if we can but I think we're going to need more information.

From the type number and the mention of pots, I assume that this is a fairly modern meter, but it's not one I recognise from the type number or find in the catalogues to hand, although it does feel slightly familiar.

Could you give some more information please - a photograph of the front and back would be useful and anything else you know about it.

If it is one of the later meters badged "AVO", information may be difficult to find as these were imported and circuit details are not generally available.

PMM

[Dekatron]

Derek, I own one with similar problems but I never got around to repair it fully. What I found though is that the switches were heavily oxidized and that some of the trimmer capacitors had come loose - there are some on the back of the front panel inside (or near) the screening box.

My H.I 108 only worked on the battery test setting before I fixed the loose trimmer capacitors, then the DC ranges started to work but the AC ranges were still not working properly - there was no signal passed from the inputs to the PCB with the transistors and opamps on so the problem was definately somwehere in the input stage with the switches.

I think that you need to check if you have any broken connections inside the screened box and also check what signal you get when it enters the PCB, apply a signal at the input and then check it with an oscilloscope to see how it looks when it enters the PCB, that might help you to see if the problem lies in the PCB or in the screened box.

Don't touch any of the trimmer capacitors or potentiometers until you are absolutely sure that the meter is out of calibration. As always you should check the capacitors, especially the electrolytics to see if they are working properly - you could temporarily replace the ones you think are broken or poor to just check if that is the problem and put them back if nothing changes.

Do you have the manual and circuit diagram?

[derek.ireland]

A few pictures of the AVO. I've done some checks, the pointer seems out by 6mm approx on all AC ranges, DC spot on (err- ish )
she runs on 3 x 9volt batteries - 27 volts. Clean inside, switches clean and OK. I can find nothing on the internet, perhaps it is a model AVO want to forget Thanks for any help you can offer

[pmmunro]

Derek,

Now it makes sense but unfortunately I can't help directly with a manual.

Telford Electronics is listing an operator's manual which may include a circuit diagram. http://www.telford-electronics.co.uk/manual.php?alpha=a

I don't have access to my catalogues at present but I think this unit is from the 1960s. Judging by the type of instrument it is and the quality of construction, it would not have been cheap and so will not have sold in large numbers.

Have you been in touch with Megger in Dover? I don't think it would be unfair to say that corporately the company does not have too much interest in AVO products, especially this old. They have no commercial value. However there are still porobably some of the staff who care a bit more about such things.

PMM

[Dekatron]

Here is the manual including calibration instructions.

The AC-amplifier is almost identical to the design that AVO use in their VCM163 valve tester, it is simple but works very well. It uses a few electrolytic capacitors that go dry over the years and then work better as resistors than capacitors. The H.I. 108 also uses resistors manufactured by Velwyn which sometimes haven't fared well over the years, neither the black model nor the silvery model. But before you replace any resistors you must measure their value as some are adjusted at manufacturing time and they might not have the value stated in the component list - some are even mentioned in the calibration instructions as possible to adjust.

The circuit isn't that complicated but calibrating it for frequency response is not a trivial task as it includes applying calibrated voltages of varying frequencies and adjustment of trimmer capacitors to get the correct frequency response.

In my instrument the heavy trimmer capacitors had come loose, possibly from a drop on the floor as the handle was also loose on one side and bent. The meter had survived and all is well on the DC and Ohms ranges now but there is still something not right with the AC ranges as no signal is passed from the input stage to the AC-amplifier, possibly some broken switch or wire that I didn't have the time to check, I was satisfied when the DC range worked as that was what I was interrested in right then.

If you read the manual you also see that you might have to adjust the zero setting of the needle with power on, this since there is a small drift in the amplifier which might pick up hum and amplify it so the needle moves.

What happens if you zero the needle with power on and then measure an AC voltage or current? Do you get correct readings then?

[derek.ireland]

Thanks for the manual Martin it will be invaluable. Should be OK to sort it out now. Will keep me busy now winter nights are here.

Once again thanks for everyone's help, Derek.

[Dekatron]

Derek, it would be very kind of you if you could update the thread when you have your H.I. 108 in working order so me and other people can learn from your repairs.

/Martin