Avometer 8 Mk5 shunt resistor replacement.

[pietrafesa]

Dear friends, good afternoon.
This is Roberto, IZ7VHF, from southern Italy.
I bought an Avometer 8 Mk 5, and I found DC current measurements wrong.
I opened the meter and found cracks on PCB tracks, shunt resistors R9, R10 and R11.

Have you an idea how may I replace these?
May I cut-out tracks and put new resistors, same values?

I will appreciate your help.
Thank you very much indeed.

Roberto.

[threeseven]

Hi Roberto, welcome to the forum,
Those cracks are actually trimming cuts to calibrate the shunts for the appropriate ranges. This is the technique used by the factory. It's common to other instuments even now but it's done by laser. Is the inaccuracy to the same degree on all DC current ranges?

[pietrafesa]

Dear sir, first and foremost thank you for welcome me here.

Yesterday late evening I measured resistance inside Avometer 8 Mk5.
I used my calibrated Fluke 87V to make this measurements.

I putted the fluke terminals (probes) inside the Avometer plug holes, to measure resistance. I have attached a printscreen of an excel file I made to write all measurements.

As you can see by picture, the problem is on 3 PCR (Printed Circuit Resistance), exactly R9, R10 and R11.
Ok for the laser-techinc used by manufacturer in order to calibrate Avometer, but why I have so different R9 & R10 resistance values

About current readings when I use Avometer, these are good within a full-scale of 10A, 1A, 0.1A, 10mA, error about 1%.
On a full-scales of 1mA I have an error about 5%.
On a full-scales of 0.3mA I have an error about 15%.
On a full-scales of 0.05mA I have an error about 25%.

Thank you all for all effort you can made to help me to comprehend this phenomenon
and my congrats for this well-made forum!
Sincerely,
Roberto.

[WME_bill]

Avo8 Universal shunt resistance values. I wonder if your trouble is switch contact resistance, as the resistance errors seem largest on the high current = low resistance ranges are most affected. For the 50ua range, perhaps you should confirm that the calibration of that range is correct. It is probably irrelevant to worry about resistance values on that range. If it is wrong, then all the other current ranges will be out.
Your comment about the low current ranges being greatly in error seems strangely at variance from your table of Resistance errors. Or maybe I am reading it all wrong. wme_bill

[pmmunro]

Roberto,

I think you need a slightly different approach to your problem. The fact that the error is greatest on the 50 microampere range suggests that the problem starts here and it will always affect all the other ranges.

WME Bill's suggestion will point you in the right direction. The first thing you need to check is that the basic movement in responding correctly. It should display full scale deflection when a current of 37.5 microamperes flows - the chain of resistors which make up the current shunt (the chain is known as a "universal shunt") is connected in parallel with the movement on all current ranges and the chain draws another 12.5 microamperes to make up the total of 50 microamps. You need to be aware that the movement and the universal shunt are in connected in parallel and this will, of course affect the resistance readings. The voltage across the movement for fsd is 100 mV.

The moving coil has a thick film resistor - it looks like a miniature tile - in series to give a total resistance for the moving coil and the thick film resistor of 2667 ohms. The thick film resistor is used to minimise the effect of temperature on the overall resistance of the movement. It is known as a "swamp resistor". It is not easy to measure the resistance of the movement with an auto-ranging digital multimeter as the DMM will pulse the resistor under test to help it find the correct range to use. These pulses will cause the moving coil to rotate causing a back electromotive force to be generated due the the electrodynamic characteristics of the coil and magnet.

The resistance ranges of your Fluke 87 are not really suitable for measuring the low value sections of the universal shunt as the test lead and contact resistances of the probes will be similar to the value of resistance you want to measure. For these low resistance you really need what is known as a "4-wire" technique. Essentially what this means is passing low current of known, constant value through the resistor and measuring the voltage across it, which will be a value of millivolts.

Once you have the correct reading on the 50 microampere range, you will be able to check the other ranges.

It might help if you had the service manual as well as the user manual. Send me a private message if you need the service manual.

PMM

[pietrafesa]

In the meanwhile I'm waiting for repeat measurements this evening, may you please let me know how may I clean contacts? I have not a specific cleaner in my house, neither I can buy in a local shop, so if you can suggest me a "simple-product" to clean these contacts (inside & outside) , I thank you very much.
Roberto.

[pietrafesa]

Dear PMM, thank you very much indeed for exhaustive answer.
I will investigate more deeply on my Avometer, and I'll notice you the results.
I will write you private to ask you user manual. (I just have service manual).

Thank you a lot.
Roberto.

[WME_bill]

Avo 8 cleaning switches.
s you ask what to use, I will suggest my favourite. It is not marketed by anyone, so not advertised, and even people on this forum spurn it.
But it has a good pedigree. I found it many years ago in a Marconi Service manual (or it might have been a Tektronix one)as a switch and contact cleaner.
Use white spirit (refined kerosene for paint thinning) with a scap of vaseline (petroleum jelly) dissolved in it. The mix does not want to be too greasy, just enough to coat the contacts after cleaning. Apply with a small brush, and scrub it gently over the contact surfaces.
I store it in a small wide necked jar. Throw the mixture away when it becomes dirty.
I have also used Isopropyl Alcohol (Propanol) instead of the white spirit. Avoid other chlorinated hydrocarbons, such as Tri-chlorethylene (often as Drycleaning fluid) or Xylene or Acetone as they can damage plastic parts. And never WD40 (silicone lubricant), as it is an insulant, and leaves a gummy residue which is the very opposite of what you want.
The Electrolube EML or Servisol proprietory cleaners are often easier to apply as they are pressurised and can be squirted into awkward corners, but otherwise very similar. wme_bill

[pmmunro]

I have always used isopropyl (propan-2-ol) alcohol (http://en.wikipedia.org/wiki/Isopropyl_alcohol#Uses) alone for cleaning Avometer contacts, without any lubricant. I usually use a cotton bud (US cotton tip) to apply the cleaner.

I very much agree with WME Bill that WD40 should never, ever be used. I would also strongly adise against any spray cleaner as the mist can settle on the turns of the spiral hairsprings in the movement and cause them to stick together. If you have a suitable switch cleaner in spray form, spray some into a small container and use it as a liquid cleaner.

If isopropyl alcohol is not available, ethyl alcohol , sold in the UK stained with methyl blue, and named "methylated spirit", (alcool a bruler (F), brennspiritus (DE), without dye) is a possible substitute.

The switch arms in the Mark V, 6 & 7 Avometers are a glass reinforced plastic around which any agressive cleaner is to be avoided.

PMM

[pietrafesa]

Dear friend, thank you very much for suggestion.
Next week I will buy an isopropyl alcool, paying attention it doesn't contain any kind of lubricant, and carefully try to clean contacts.
After I will notice you the results of new measurement.

Thank you all for your effort to help me.
I appreciate.
Sincerely,
Roberto.

If anyone of you may send me a Avo8 Mk5 user manual, I thank him very much.

[Paul Adams]

Originally Posted by pietrafesa

If anyone of you may send me a Avo8 Mk5 user manual, I thank him very much.

You will find the service manual Here


Paul.

[pietrafesa]

Dear friends, happy Sunday all.
I bought Isopropyl Alcohol (Propanol) and I have cleaned all contacts.
Now my Avomenter 8 Mk5 is working within tolerances, no more than 2% errors, depending by full scales. I think this is solution.With this post, I would kindly thank you all very much indeed for your suggestions.
I wish you all the best.
Sincerely,
Roberto.