Maxwell multimeter

[mikew8760]

I have an old Maxwell meter, model MX 25-304. The switch contacts seemed to be a bit dicky, so I thought a little cleaning would help.  The switch is a circular plastic disc with a number of contacts on each side that run over tracks on a top and bottom pcb. Unfortunately the little leaf spring contacts are not fixed to the plastic disc and most fell off! The position of the contacts is crucial to the operation, and there must be thousands of incorrect positions.

I am wondering if anyone has such a meter and can show the correct positions for the 'bottom'.   I have attached a photo of the top-side switch, which is correct. I need the pattern for the bottom.

Keeping my fingers crossed.....

[theHWcave]

Can you not figure out the contact slot position from either the traces (marks) left on the bottom PCB or maybe slots in the knob itself have marks (under a magnifier) indicating if a spring was inserted?

[mikew8760]

Hi,
Thanks for taking the time for replying.  But I've studied the problem in the hope I may find a solution, but I'm no Sherlock.  The main pcb seems to be the one that switches the values from the inputs, i.e. volts, amps etc. The top set of switches seem only to produce the logo for the unit on the LCD.  There are 12 circular tracks and 6 leaf contacts. So it may have been all 6 are on one 'side' of the plastic disc, but that doesn't work. The tracks are arranged into 'sectors', with seemingly random positioning of the gaps - which dictates how inputs are selected to the circuitry.
So all the tracks show witness wear, and the fine shim contacts leave no mark on the disc; so I'm stuffed.
I am left with the conclusion that only the correct positioning will work.
I know it's a long shot to find this combination, but otherwise the meter is junk!

[wraper]

If it's the same on opposite disc then you only have 5 positions on each side of the disc and same position on opposite side should remain not occupied except one. I suggest initially put all of the spring contacts to one side, do not put 6th contact first. Then change position of each spring contact to the opposite side by one at a time and see how it affects functionality. If something starts working properly or just shows some signs of being better, leave it there. Make only safe measurements, like resistance, capacitance or low voltage through some current limiting resistor. Once all of them are placed for best operation, find a place for the last contact.

[shakalnokturn]

Upload some photos of at least the two sets of circular tracks, preferably all four sides of PCB and the front of the multimeter with function positions.

There are some "Sherlocks" here, I'm sure it's easier than you think.

[theHWcave]

electrotanya has the schematic for this meter. With that it might be possible to establish which traces of the PCB should be connected at which switch position. Its a cool meter, does inductance as well as capacitance which is unusual. 

[wraper]

If another disc is the same as on the photo and uses 6 spring contacts, total possible variants should be 37 if I calculated correctly, not thousands.

[wasedadoc]

The schematic indicates there should be 18 "circles", not 12 and you should have 9 contact springs to insert. 8 of them are wide enough to bridge 2 circles and the other one is wide enough to bridge 3 circles.  Two of the circles have no gaps. The wider contact bridges the full circle and the neighbour circle that has a break between every position of the rotary knob.  The middle of that contact sits on the full circle.

[mikew8760]

I recently stumbled across Elektrotanya a while ago, and I've seen the schematic. I have the impression there are a number of MX-25 304 meters, with this same model number, but with some variant addition. I am unable to reconcile this schematic with my model. I think the  "tracks" shown is misleading as there are only 12 on this pcb. Also, there are 12 springs, 5 for top side and 7 for bottom. (There are 10 tracks on top in smaller diameter circles.)

[mikew8760]

Thanks for your input, but I'm intrigued by your figure of 37 combinations.  The switch has space for 12 positions for the spring leaves and 7 springs.  By using nCr formula I get 792 permutations. Admittedly not thousands but too many for me to try them all!

Regards,
Mike.

[mikew8760]

Hi,
I've added the two pcb's showing the tracks. and the cover.

Perhaps some super-sleuth might have an idea; I'm at an impasse......

[wasedadoc]

More questions:

1.  Clearly the photo of the bottom board rings and the front cover are in alignment.  Meaning that clockwise on the photos is also physically clockwise for both.  Is the same true for the photo of the top board or do we need to mirror it left to right?

2.  What is the alignment between the pointer on the selector knob and the springs?  When the knob is pointing "north" are the contacts on a north-south line?

If the answer to that last question is yes, I'd be reasonably confident that the 5 small arcs on the bottom board outer ring from 6 o'clock towards 8 o'clock are the 5 capacitance ranges.  So one of the springs should be in the appropriate place to bridge those to a neighbouring ring when the knob points to those positions.  There is no ring further out so it must be the next one inwards.

1 down, 5  to go.    They go one or the other side of the centre.  32 possibilities.

[wasedadoc]

Continuing from my last post, looking at the next 2 rings inwards there is a gap for the two positions before (clockwise) 12 o'clock.  It does not seem probable to have that gap in the middle of the resistance ranges.  But 180 degrees opposite are the temperature and 20 Amp positions.  Those two ranges are different from each other and their neighbours so I think the contacts should be on the other side of the centre from those for the outer pair.

[wasedadoc]

Moving in to the next pair, that is rings 5 and 6 counting from the outer one.   I see gaps which would line up with the positions for hFE, temperature and continuity. I would place the contacts to be in the half that makes it be in those 3 gaps when the selector knob is pointing to those 3 functions.

[wasedadoc]

Moving in again to rings 7 and 8 counting from the outer one, I see a gap that would correspond either to the 20 Henry or the 10 MHz position.  As there are more than one inductance range but only one frequency measuring range, I would assume that the contacts should be over that gap when the knob pointer is at 10 MHz.

[mikew8760]

Many thanks for you taking time to help solve the riddle!  I've spent some time looking at it without coming to any conclusions.
I have the position of the leaf spring contacts for the smaller pcb, it's exactly the same as the one shown in the first post above. You are correct that this pcb would be turned left/right.

The plastic wafer that holds the contacts has a square hole for the knob spigot. This means that there are 4 ways the wafer may be fitted. I reckon this might be a simple fix by finding the position where the values around the cover match the LCD display. Although this might not be a total solution if the small pcb is also involved in switching inputs.

I shall now study your deliberations very closely and see if I can make some progress. Once again, I take my hat off to you for your perseverence.

Best regards,
Mike

[wasedadoc]

I am 99% convinced that the selector knob and the contacts holder disc should be aligned such that when the pointer is at 2M the contacts are on the line between 12 o'clock and 6 o'clock on the bottom PCB.  If you look at the arcs of its 2nd ring (counting from the outside) they are a very good match for the class of measurement. Volts, current, capacitance, inductance, diode/continuity, resistance,

As the holder disc is symmetrical there are two possible orientations (180 degrees apart).  But the contacts for the top board are not symmetrically placed so you need to determine using it which of the two possibilities is correct.  It looks to me that for the top board switching, the two inner contacts should be on the two inner arcs when the knob is pointing to the resistance ranges. Once confirmed I suggest marking the disc near the knob pointer so you do not lose track as you proceed.

Then you can place the outer 4 contact pieces for the bottom on the sides as I surmised earlier.  That leaves the inner two.  4 possibilities to test by trial and error.

[mikew8760]

I have been looking at the bits following your descriptions.  However, something I had not noticed before; I have 12 leaf contacts, 5 should populate the top side of the wafer - which is correct. The remaining 7 should fill the other side, but there is only 6 spaces, without any position "duplicating" an already covered two tracks. If that is actually correct then I'm stuck for any reason how it would work and how?

Any ideas for further down the rabbit-hole?

Mike.

[shakalnokturn]

The schematic could match more than it seems. (Without going s far as saying it matches completely.)
One could easily fit 12 contacts on that schematic on the 2nd and 8th columns there seem to be 3 runs to link. That could either be done with wider contacts or by having a 1/2 position overlap on the opposite side of the wheel, which is physically possible here.

What the schematic does not account for is routing so switch runs that look continuous on schematic may have been segmented for layout considerations although electrically identical to schematic.

What is unclear to me in 2nd colmun of contacts on schematic is the lower portion that's not quite aligned, did they not have room to draw that separately but it should be considered separate from the "3 runs" portion above?
If so, it fits 12 contacts.
From left to right...
1st column: 1
2nd: 3 (upper portion 2 opposite with 1/2 overlap? 1 for lower portion considered as separate runs)
3rd: 1
4th: 1
5th: 1
6th: 1
7th: 1
8th: 2 (opposite with 1/2 overlap?)
9th: 1

[wasedadoc]

1. The schematic appears to be for a completely different multimeter.  Even though the same model number the range switch layout is completely different.  See th4e drawing on page 5 of https://www.maxwell-digital.com/products/25304/downloads/25304_user%20manual_en_hu_ro_sk.pdf.  I think Maxwell do not make these meters.  They are just OEMed form other manufacturers and the old one that the OP has and the modern one are different designs from different manufacturers.

2. I have come to the conclusion that there are two contact bridges on the two inner rings.  As this a manually ranging meter, not autoranging, I have been wondering where that switching is.  I think the single lands of half the inner ring do that.  But that would only work for half of the positions.  That's why I think there is another contact bridge 180 degrees opposite it.

3.  That leaves just the last bridge to position.  I think it goes where I've shown in the attached photo.  The yellow patches are where I think all 7 contact bridges should be when the selector knob is pointing to 2M.

[mikew8760]

I am, again, amazed by your brilliance!  I have put your layout into effect and had some good reults; close, but no cigar, yet! 
At the moment, after getting the spring contacts on the top side in the correct order - you'll appreciate the top and bottom contacts must be 'in phase'. So now the LCD shows correct unit logos, but some with 'anomolies', like a minus sign, battery sign and Hz and H together. Also, the display fails to show any correct input values. For example, resistance ranges show 0.0 or OL, voltages variable, small values - 0.01 or 0.33  etc etc.

I am beginning to think there must be some fault(s) in the circuitry, although it was fine before I decided to clean the contacts.

And now, to add insult to injury, my Riden RD6006 has gone on the blink; display shows output voltage as zero and no amount of twiddling the knob has any effect!  There's quite a lot on this unit in the forums, but can't find a schematic.

Regards,
M.

[wasedadoc]

OK, let's back up a bit.  I was quite confident with my first deductions to place the 4 contacts for rings 1 to 8.  Obviously there must be a contact on ring 12, the one nearest the centre.  And that can only be a bridge between 11 and 12.  Only two positions for that bridge and to me the the top one looks more likely.  So remove the two with red blobs and leave the yellow ones in the attached.  Test that.  Of course some things will not work but it will be encouraging if there are no "wrongs".  Wrongs such as two or more incompatible icons enabled.  Do the decimal points occur in the expected positions on at least some selector positions?  If not or missing altogether on some selector positions please give full details.

[mikew8760]

I'm not sure your present conclusion is incorrect. All along  I have made an assumption that the two sets of tracks on the two separate pcbs work in isolation - that is provided the arrangement of spring contacts are correct all is well? But I have a suspicion that they have some interaction and thus the "incorrect" arrangement will not work properly. I shall try changing the orientation of the bottom contacts 'as is' and see how that affects things and if not correct try your latest figure.

Watch this space!

Best regards,
M.

[wasedadoc]

Yes the upper and lower contact assemblies work in unison.  It is ESSENTIAL that the two holder discs are in the one and only one correct angular orientation with respect to each other. The square shaft would mean there are only four possibilities.

[wasedadoc]

I have worked out the orientation of the top contacts.  See the attached where your photo has been mirrored (left-right) and rotated 90 degrees anticlockwise and then some switch positions marked.  You can see that the arcs in circle 5 (counting from the outside) fit perfectly with the ranges, For example all the k resistance ranges make the same connection.  All the M resistance ranges make the same connection.  All the V ranges make the same connection, but the mV range has its own one.  All the mA current ranges make the same connection but the A (20A) has its own.  Similarly for the nF and uF ranges of capacitance.  And for the mH and H ranges of inductance.

Looking at your photo in the very first post, when the knob pointer is in the 2M position the contacts for the top board should be in a horizontal (or very near) row with the 2 contact bridges on the left and the 3 on the right.

(When the pointer is at 2M the contacts for the bottom board should be in the vertical column as worked out earlier.)