My newly acquired Avometer 8 Mk.6, dead on AC

[WilsonLaidlaw]

My newly acquired Avometer 8 Mk.6, after cleaning battery compartment and terminals with a fibreglass contact brush plus Servisol and cleaning the 1 amp fuse and contacts, is now working properly on the DC ranges, both volts, amps are working well. I bought a new BLR154 battery and put in a 1.5V D cell and the ohms are also working on all three ranges and seem accurate. However, AC ranges are not working at all. My suspicion would fall on the OA95 diodes. My question to the experts is: how commonly do the diodes on an Avometer fail? I don't feel it is the switch as it feels very crisp and positive. I will open it up anyway and clean the contacts with Servisol. I can buy Mullard OA95 diodes for around £2.50 each, so no great hardship if that is all it is.

Wilson

[paulsherwin]

The first thing to do is obviously to test the existing diodes. They don't fail very often, but you've no way of knowing what the meter's been used for during its life.

[WilsonLaidlaw]

Thanks Paul. I have cleaned all the internal switches again with Servisol with no change. I presume these are the diodes on the AC board to the left of the movement. The lower one might be slightly discoloured at the right hand side. Given the cost of replacements, I might as well go ahead. Those are the only components other than the switch, I can think of which would disable all AC voltage measurements.

Wilson

[ajgriff]

Why not test the diodes before ordering new ones?

Alan

[deswradio]

Interestingly, I had the same problem with my Avo Multiminor quite some years ago - both diodes had failed (and I don't recall doing anything terrible that could have caused the diodes' demise...). Replacement with a couple of junk box Ge diodes did the trick and accuracy was preserved. But it really is worth checking (with a second multimeter!) before replacement.

Best wishes
Des

[WilsonLaidlaw]

Originally Posted by ajgriff

Why not test the diodes before ordering new ones?

Alan

Alan,

You can't check them without unsoldering them from the AC board (I have tried with my Metex digital multimeter) and given that new ones are just £2-00 each, I might as well replace anyway. Unsoldering diodes often seems to damage the heat sensitive germanium, as it always seems to take more and longer heat to unsolder than to solder. I only have a standard soldering iron in the UK, my nice thermostatic adjustable heat soldering station with desoldering wicks, is down at my French house.

Wilson

[ajgriff]

Originally Posted by deswradio

But it really is worth checking (with a second multimeter!) before replacement.

Best wishes
Des

As the AVO's resistance ranges are working a second meter shouldn't be needed unless I'm missing something. Wilson shouldn't even have to reach for the soldering iron as the diodes can be tested in situ.

Alan

[ajgriff]

Originally Posted by WilsonLaidlaw

Alan,

You can't check them without unsoldering them from the AC board (I have tried with my Metex digital multimeter) and given that new ones are just £2-00 each, I might as well replace anyway. Unsoldering diodes often seems to damage the heat sensitive germanium, as it always seems to take more and longer heat to unsolder than to solder. I only have a standard soldering iron in the UK, my nice thermostatic adjustable heat soldering station with desoldering wicks, is down at my French house.

Wilson

Ok. Perhaps I'm mis-reading the circuit diagram. As you say new diodes are not expensive. Hope the replacements will do the trick.

Alan

[WilsonLaidlaw]

Update folks: One of the OA95 diodes was open circuit but I also found a broken wire from the AC board to the transformer, which you have to remove along with the AC board to get to the soldering side. There were also a couple of suspicious looking soldered joints on the AC circuit board, where the solder only went round a fraction of the component wire. All redone with decent tin/silver/gold Mundorf solder.

I am delighted to say that the meter is now 100% functional on all AC, DC and resistance ranges. If this Model 8 Mk.6 lasts as long as my Model 7 did (50 years in my hands), it will see me out. The only fettling required to make it prefect, is to sort out a slightly sticky cut out switch, that needs a few prods to get the needle back to zero, if it has been depressed in the case.

Wilson

PS. I treated myself to another soldering station which was on special offer on Amazon for Prime day at just £26. Seems to work perfectly and the tip temperature checks out with my IR thermometer within a couple of degrees. I think soldering temperature sensitive components like diodes using the high melting point Mundorf Sn/Ag/Au solder, you really need a temperature controlled iron.

[deswradio]

Excellent stuff - congratulations! I've heard of quite a few of those old Ge diodes failing.

Enjoy the Avo!

Best wishes

Des

[ajgriff]

Good news and well done!

Alan

[WilsonLaidlaw]

The cut out switch on my Avo 8 Mk.6 is sticking a bit. It often needs quite a few jabs before the needle zeros on the meter if the cut out has been pushed in. I gave it a squirt with Servisol switch cleaner hoping to wash out any old gunk but no improvement. I have to make sure to put my small zip up polybag with all the probe accessories, down the top front of the meter when putting it into the leather case (very smelly) so that the cut out button cannot contact the case. If I do this every time, the meter stays zeroed perfectly. Anyone else had this problem and have you found a cure?

Last week I got a mains shock off the leads and on inspection, I found a longitudinal crack on the red lead plus there was a previous repair of soldering iron damage. They have therefore gone in the bin. I have replaced them with a Hirschmann (German made quality!) PMS 4 lead set for low voltage use. This comes with three sets of probes, push button micro hook, heavy duty crocodile clips and a pair of regular probes, although the micro hook probes do protrude a little bit out of their insulating sleeves, so can be used as regular probes as well. Very well made leads, where unlike some leads from the far east, I feel the insulation is trustworthy. Bought from RS components. I would still like to get a set of new high voltage leads as well - any recommendations?

Wilson