Ocuda CT-100, Analog multimeter

[karesz*]

I agry absolutly with your opinion Harry.
We have presented yet ca 3 different circuits in similar equipments(I didnt compare in details the element values), but they are 2KOhm/V---50KOhm/V solutions, also in differing systems of R-values! I think, as you wrote, a back drwoing -minimum comparing and "errating" the proportions of resistor networks- is absolutly needed...
Karl

[Herald1360]

Originally Posted by Herald1360

Be wary about any resistor values from "Vetter" TMK meters. Everything will revealed proportionate to the movement sensitivity.

I seem to have been visited by the predictive text fairy!

Hopefully it still made some sort of sense. "Vetter" was just a typo for "better".

[omarbeyaty]

thanks karsez and Chris, and yes karesz I will be wary, as Chris said i will draw the circuit diag. of this meter with the known res. hope we find a solution. I have changed different set of resistors and the reading deflected way far, i measured the 1.5 Vdc battery the deflection shows 4.6 V.

[omarbeyaty]

HI GUYS

I finished drawing the okuda ct-100 multimeter diagram take a look on it please with your opinions

[Herald1360]

R6, R7 and R8 look like the components of a universal shunt, though only one of its current ranges (250mA) is actually used. It's not clear from the circuit, but I think there must be a high current low resistance shunt between the 20A and COM sockets. If this has burned out or got a bad connection that's probably what fried the shunt resistors. Or perhaps trying to measure a high current without moving the meter lead to the coreect 20A socket!

The circuit itself is a bit of a nightmare to follow- I suspect a lot of effort went into achieving minimum component count which can lead to some strange configurations where different ranges share parts with each other.

I think to get anywhere with the current shunt values, we need to know what the raw sensitivity and coil resistance of the actual meter are. Measuring these needs to be done with care to avoid allowing more than about 500uA ever to pass through the bare meter. It will be possible with judicious use of a 1.5V cell, a 3.3K resistor and a reasonable DMM. Set the pot RV1 shunting the meter to maximum resistance or better, just disconnect the meter from circuit to remove any shunting effects.

This could turn into a challenge of the "just because it's there" sort- the meter itself isn't intrinsically worth the effort! Just the sort of exercise we love

[karesz*]

Omar,
because your meter is a 2K/V type; R12 must call 50K, than R6 and movements resistance in sum has to be 500 Ohm. I think R6 can be at 200 Ohm_movement inself maybe 300-330 Ohm?
How looks R8, it must be a peace of thick, 5-7cm long (Manganin) "Wire", maybe some winding in longcoil form or a cooper strip on PCB?, but as value ~ some ten milliOhms, can we have a photo of them please...
What about AC 10V please, "some detail" is missng from that point_I see, that is to connect to 10V DC .
Karl

[omarbeyaty]

YOU ARE RIGHT, it is 50k and i fixed the drawing as attached. R8 is as you described , but shorter, its pic. here. 10DC is a dead shorted to 10AC,as I see, the combination of the resistor will act as divider to reduce the voltage to what the pointer coil need. thanx for your help Karesz

[omarbeyaty]

Originally Posted by Herald1360

R6, R7 and R8 look like the components of a universal shunt, though only one of its current ranges (250mA) is actually used. It's not clear from the circuit, but I think there must be a high current low resistance shunt between the 20A and COM sockets. If this has burned out or got a bad connection that's probably what fried the shunt resistors. Or perhaps trying to measure a high current without moving the meter lead to the coreect 20A socket!

The circuit itself is a bit of a nightmare to follow- I suspect a lot of effort went into achieving minimum component count which can lead to some strange configurations where different ranges share parts with each other.

I was wondering like you about existence of small value resistor between the common and +20A dc, but there is no such thing, only a 1.5 mm thickness bus soldered to both of them

[TonyDuell]

I think you'll find that 1.5mm thick wire _is_ a very low value resistor which forms the shunt on the 20A range. I would think that there are wires connected to each end of it that go to the rest of the meter circuitry.

[omarbeyaty]

exactly, that is obvious from the diagram, and it plays a role in the 250 mA scale while the thick wire allow the huge current to pass through as its resistance very low

[Herald1360]

Yes that 1.5mm wire is probably not copper, but a resistance alloy. It won't need much resistance to drop a few hundred mV at 20A and its presence in circuit on other ranges will have negligible effect on accuracy.

[omarbeyaty]

correct, it is kind of alloys not copper.

http://www.mediafire.com/view/1ideya...204_193257.jpg

[karesz*]

Hi Omar,
Prima, that is the thick-some cm long "wire" what i wrote (I marked it falshly as R8) too, please drow it in- correct your schematic with it.
Btw: I would dewind R8 and measure the resistance(s-if must be from all peaces) and substitute it with 2-3 as composit...
Otherweise can you borrow please an better Ohm-meter_ or VOM?
If yes, check the coil resistance from Okuda`s movement please, or measure at fullscale (battery and some potentiometer in serie and adjust to FSD) the voltage on the movements coil_ than we can calculate the R1-R7... Than can you measure an value of "Zero Adj" potmeter too.
Than please put the symbol of your asked gray_ unknowed, CEC marked, component into the circuit too.
Tnx - greetings, Karl

[omarbeyaty]

I Appreciate your efforts, i reflected your thoughts on the drawing . R8 is just a melting wounded resistor i tried to decompose it but the resistance wire split to parts and couldn't do the resistance measurement. after the adding the updates on the drawing what is your thoughts about the probable values...thank you

http://www.mediafire.com/view/4898uh...205_211943.jpg

[omarbeyaty]

update

I just found the operation manual of Okuda CT-100. take a look on it and will find resistors values, is it the actual values? thnx
http://www.mediafire.com/view/g6am8ne1g882d6y/4.jpg

[Herald1360]

I can't see any useful information in that user manual other than the 2K/V sensitivity figure that we already know. There's no circuit information, at least in the picture.

[omarbeyaty]

under the red line might be a useful infos as they mention the resistors values

[The Philpott]

The numbers given are those half-way up the scale, not the values of the actual resistors/s inside the meter.

I have had this situation once or twice where i have used the 'centre scale' numbers to guess at the actual resistors to use, and experimented to find the correct ohms value. This only works if you have another good meter to work with, preferably a digital one. I hope you manage to get the numbers you need as there can't be many people with the patience to mend this meter. Good luck!

[omarbeyaty]

THANKS philpott...ill use variable resistor (per ohm scale in the device ) and try to adjust it to read a unkown resistor . when the needle shows the exact value of this known resistor ill read what the value of the variable resistor and replace it with fixed one and so on for the other scales.