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Old 11th May 2019, 11:21 pm   #1
Phil G4SPZ
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Default Avo EA113 faulty resistance range fixed - again

Almost exactly two years ago, I posted this Success Story and since then the instrument has given excellent service as my main workshop meter.

This week, a fault appeared, namely the loss of the 'ohms x 100' range. I studied the circuit diagram and homed in on R59 (10k) which on inspection had a very obvious dry joint at one end, shown by the white arrow in the attached picture. I re-soldered the joint and all was well again, but a little bell rang somewhere in the deep recesses of my memory that I had done this before. Sure enough, I had re-made this same joint exactly two years ago, and I mentioned it in the original posting.

I can't explain why this joint has failed again. I'm not known for making dry joints. Perhaps there is some residual contamination present which led to the joint failing? I will put a note in my diary to open the meter up in May 2021 and check it again!

Phil
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Old 12th May 2019, 5:45 am   #2
mole42uk
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Default Re: Avo EA113 faulty resistance range fixed - again

Very odd. I must open mine and see if there’s any evidence of a similar problem.
The only thing I can surmise is that it may be a heat related problem, if the resistor warms, expands and then contracts it may disturb the solder.
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Old 12th May 2019, 7:09 am   #3
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Default Re: Avo EA113 faulty resistance range fixed - again

Morning Richard. Yes, that’s a valid suggestion under normal circumstances, and the solder does indeed appear to have flowed away from the component leg back towards the pool, but in this location the resistor is dissipating virtually no power. It’s very odd, but it’s easy enough to keep an eye on. Fortunately this resistor is easily accessible, unlike so many other components...

Did you manage to make any progress repairing your faulty EA113s? ISTR you have one working example and two faulty ones.
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Old 12th May 2019, 7:20 am   #4
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Default Re: Avo EA113 faulty resistance range fixed - again

Hello Phil,
Unfortunately my stash of EA113s is still the same - one working and in fairly constant use, one with a faulty AC or DC, I don't remember which, and one that is quite dead.
I have been diverted by a number of other 'projects' - the Gould DSO took a lot of time, and now I have a Radford LDO3 on the bench with an intermittent fault.
I will have a look at my EA113s and see if the same resistor exhibits any solder issues.
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Old 12th May 2019, 7:24 am   #5
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Red face Re: Avo EA113 faulty resistance range fixed - again

Quote:
Originally Posted by mole42uk View Post
I have been diverted by a number of other 'projects’...
Happens to me, too, all the time! Good luck,
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Old 12th May 2019, 7:35 am   #6
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Default Re: Avo EA113 faulty resistance range fixed - again

Any other joints looking iffy?

It must be quite difficult to get any heat into a 10k resistor with only 4 AA cells to go at. You don't have a goblin infestation problem you haven't noticed? Tiny hands, tiny soldering irons and an unfriendly sense of humour...

Good mystery, Phil. Our equivalent to a locked-room Agatha Christie.

David
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Old 12th May 2019, 10:11 am   #7
The Philpott
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Default Re: Avo EA113 faulty resistance range fixed - again

Out of all the yards of solder we use, i suppose there will be the occasional inch that is short on rosin.. Or a duff reel that has less Pb in it than the label declares..Mixing that with older solder could move the %age of Pb into the 'bad' zone?

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Old 13th May 2019, 9:38 pm   #8
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Default Re: Avo EA113 faulty resistance range fixed - again

Here's photos of the same resistor on my three EA113s, none seem to have a problem with the solder (in order, working, faulty, dead):
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Old 14th May 2019, 10:59 am   #9
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Default Re: Avo EA113 faulty resistance range fixed - again

Certainly no visible problem with the soldering there. Mine had a visibly obvious dodgy joint. As David says, one of life’s mysteries, of which when dealing with Avos there are many.
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Old 14th May 2019, 2:52 pm   #10
mole42uk
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Default Re: Avo EA113 faulty resistance range fixed - again

I wonder if the rivet is giving a problem? Mine seem to have through-hole rivets a bit like some Philips machines, in the Philips CD players (104?) there's a well-documented issue where the rivet develops a dry joint to the print....
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Old 14th May 2019, 4:37 pm   #11
Phil G4SPZ
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Default Re: Avo EA113 faulty resistance range fixed - again

Hard to see a rivet on my example, but the range board is quite thick and double-sided so it probably does have rivets.

I know some problems did arise many years on with the early plated-through holes on some PCBs in 1970s gear too.
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Old 15th May 2019, 7:42 pm   #12
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Default Re: Avo EA113 faulty resistance range fixed - again

Phil's original picture shows a fibreglass board while Richard's pictures show two SRBP and one fibreglass board of a different type to Phil's. It seems probable that these different materials may be used with different connection techniques which might be significant.

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Old 15th May 2019, 10:16 pm   #13
Phil G4SPZ
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Default Re: Avo EA113 faulty resistance range fixed - again

Peter, you’re absolutely right, and your keen eye has spotted the deliberate mistake... er, well, my mistake anyway. I uploaded the wrong picture in post #1. That was one I found whilst researching a couple of years ago. The actual picture of my meter is attached, together with a close-up of the repaired dry joint at the upper end of the 10k resistor.

My PCB is a conventional SRPB type and closer study shows that it definitely incorporates rivets for through-board connections. The instrument’s serial number is 1377.

The glass fibre board must have appeared in later production.

Phil
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Last edited by Phil G4SPZ; 15th May 2019 at 10:19 pm. Reason: Clarification
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