avometer model8 mk1 reading high - Page 1
Forum section: electronics / testgear

author: m3vuv
author_rank: Super Contributor
timestamp: November 05, 2020, 11:39:17 am
content: hi all,ive just got one ov the above,on the dc volts ranges it reads high,ie ,an 8 volt input reads about 12v on the meter,ive not checked the other ranges yet,i will post in the repair section too,has anyone any idears where to look?,ive read it could be the universal shunt,any idears where its located?,looks like the meter swamp is u/s as well,ive the service manual but cant find anything related,any help welcome to get this dinosaur usable is most welcome,cheers Paul m3vuv.

author: tggzzz
author_rank: Super Contributor
timestamp: November 05, 2020, 11:48:57 am
content: The answer is in your other thread

author: Specmaster
author_rank: Super Contributor
timestamp: November 05, 2020, 12:01:55 pm
content: The manual provides clues of what to check for readings that are too high or to low but with the 8v showing more like 12v, then I would guess that you or someone else has made the mistake of spraying the bottom set of switch contacts with switch cleaner. By bottom set, I mean the set that on the bottom when the meter is laying on its face with the back cover removed. The "ring doughnuts" that form the fixed contacts will absorb the cleaner fluid and provide tracking between other contacts which will screw up the calibration so check there for leakages.

author: m3vuv
author_rank: Super Contributor
timestamp: November 05, 2020, 02:05:46 pm
content: i dont think thats the cause,its cleen as a whisle and dry inside,the copper spiral coil was loose and unsoldered from its wires,due to an overload i imagine,i need too find the universal shunt as its called tho,i suspect shunt windings s/c.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 06, 2020, 09:47:40 am
content: HiThe first thing to do is to put the AVO on the 50 micro Amp range with another AVO or similar in series and then apply a 50 micro Amps current to check that they both read 50 micro Amps. This leaves just the basic movement and the swap resistor in use.George G6HIG

author: m3vuv
author_rank: Super Contributor
timestamp: November 07, 2020, 06:14:02 am
content: Hi,just to say george,i dont have a 50 microamp supply but i have the 5 transistor esr meter from here i built,that has a 59ua movement,would it be viable to set the esr meter for full scale,then connect my avo in place of the esr meter movement with the avo set to 50ua,is my logig right?,cheers Paul m3vuv.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 07, 2020, 09:11:07 am
content: HiThe easiest way to get a 50uA supply is with a 1.5VDC battery with something like 30K Ohms, made up of a fixed resistor and a pot, in series. use your ESR meter to adjust it to give 50uA and them substitute the AVO.The idea is to use just the basic movement and the swamp resistor in the AVO. This is all low impedance circuitry so I doubt if contamination is the cause of your problems, but it may be.George G6HIG in the current home of Megger, nee AVO.

author: tggzzz
author_rank: Super Contributor
timestamp: November 07, 2020, 10:23:36 am
content: Quote from: George Edmonds on November 07, 2020, 09:11:07 amDon't substitute; the two meters may have slightly different characteristics that change the current flowing.Do connect the two meters in series and that will automatically cause the same current to flow through both.Twiddle the external pot until the golden meter indicatesaround50µA, then twiddle the UUT until it reads thesame.That procedure works at all currents and with radically different meters, e.g. one digital and one analogue.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: November 07, 2020, 01:55:32 pm
content: HiActually with a 50uA current source resistance of some 30K Ohms it does not matter if the two meters are in series or not.It is always better if they are, but this may not always be practical.George