Maxwell multimeter

[wasedadoc]

I think the bottom contacts should be inserted like this.  '1' means contact in slot.  '0' means empty slot.  Top to bottom when pointer at 2M

1
1
0
0
1
1
0
1
1
0
1
1
centre
0
1
1
0
1
1
0
0
1
1
0
0

[mikew8760]

Many thanks, again, for taking the time and effort on my problem.  I was very hopeful that your ideas would be the solution, but this still eludes me. I've tried the two last ideas, but no breakthrough.  I cannot expect anyone to spend valuable time any further, but I shall continue occasionally giving it a poke.

For what it's worth I present a few observations that  might give a clue to the underlying problem, these for the suggested arrangements of contacts:-

1. None of the functions indicated actually work!  e.g. all the voltage ranges give OL rather than 0.0.
2. The buzzer sounds at the buzz/diode position, but also at 20k, 200mH, 20mH, 2mH and 200uF.
3. The kHz position also gives 20H on the display.

I don't think there is a fault with the electronics of the meter, it was working fine apart from intermittent switch contacts, but it is a possibility.  A shame the controller chip is a "black blob".

Any more ideas  welcome......

[wasedadoc]

You will have to wait until I recover from this head cold that is preventing my brain from operating at full potential.

[mikew8760]

Sorry to hear about your affliction! Hope you get well soon.   
Thanks for all your help, but at the moment I have had a bit of a disaster!!
One of the spring contacts has pinged off into the ninth dimension, despite being very careful in handling them.
I see that many meters use very similar switches and contacts, but so far I've failed to find any source of such items.
 
So no rush now......

M.

[wasedadoc]

Turns out I have Covid19 and it isn't throwing in the towel yet. Post again if you do find the original contact. If you do not find it the meter is a goner.

[mikew8760]

Oh, that's a real bummer!!   Here's hoping you make a speedy recovery without any lingering symptoms.

I've made a little contact from a thin Be/Cu blade from a relay. Not ideal, but will siffice until I get a proper one. (I think there are some cheap Chinese meters that use the same ones - but take forever to deliver.)

I've had a look at some images on Elektrotanya which looked suspiciously the correct layout, but did not work. Then closer look showed the pcb's given were different from mine! The top set were correct, but I have already seen that elsewhere.

So at the moment I'm fresh out of ideas.....

Regards, and thanks for your help.

[wasedadoc]

Please confirm that my interpretation of your photo of the top PCB is correct.  Attached is a repeat of an image I posted previously.  I have written "Display" and "Sockets" at two of the edges.  Do these indeed correspond to the sides of the board where the display and input sockets are?  (The image is deliberately flipped about a horizontal line through the middle.  That does not affect my question.)

[mikew8760]

Thanks again for your continued doggedness! Trust you are now back to 100%?

Yes, the position of these contacts is exactly correct.  If you look back it's the same as the one in the photo I first posted and which gives the selected units on the LCD.  I think there is also some input to these from the bottom side of the wafer switch - which, of course, is the answer to the problem. Perhaps a pointless observation, but unless the arrangement of these contacts is exactly correct the result is gobbledegook. I had expected/hoped that a near correct positioning would give a partially correct output, but it seems not.

Regards,
M

[wasedadoc]

A small change from my previous suggestion for the bottom contacts.  Top ones as before.

If this does not work you'll need to do a bit of leg work. Short the common and Volts sockets. Make a list of the 30 positions.  For every position, what is written on the dial, what numbers including decimal point if any, and all other symbols (eg A, V, k, M etc) on the display. If there is a beeper does it beep on, and only on, the Continuity position?

If you are using a DIY contact to replace the one you lost, put it in the position for the outer rings.  The angular spacing at the inner rings makes the contact position and size more critical.

[wasedadoc]

Another update which with yesterday's should make a dramatic improvement.  I reviewed my work on the top contacts and concluded that I got the orientation wrong.  The disc should have the 3 contacts on the left side when the knob is pointing to 2M.

The attached shows your original photo of the contacts side of the top PCB with the ranges going round in the opposite direction from the case markings.  From that it is clear that when the board is flipped over (left to right) with its rings downwards, the rotating contacts will need to be 3 on left of centre and 2 on right.

[mikew8760]

Hi there,you have been busy!  I've only just looked at your update, so the following may well be redundant? 

Attached is a pdf with the meter readings as you suggested. The numerical values displayed are misleading, as the number seems to be random - possibly just an input to a floating gate.  However, the Buzz/Diode works, sort of. Short on the meter inputs gives the buzzer, but no diode voltage. Also, I tried a 1kHz signal on the 10MHz position and was surprised to see 0.909 with the decimal point slowly moving from right to left position.

I shall now study your latest post and report back.

Many thanks for all the effort you have taken.

Regards,
M

[mikew8760]

I've checked out the arrangement of the top side contacts and it is the same as I have been using. I can see there must be a correct orientation of the top contacts, the bottom contacts and the switch.   I have assumed that if the top contacts are correct then the LCD symbols will appear properly and with the switch correct will correspond to those on the case. But even if the bottom contacts are correct but 180 degrees out will not produce the correct values on the LCD if the top and bottom contact positions need to be in 'sync'?

I have also assumed that if the bottom contacts are in correct positions, but not in sync with the case markings, then meaningful values would be shown on the LCD. The present arrangement does not do this, BUT it suggests that some contacts are correct since the buzzer operates as it should when inputs shorted, but there is no diode voltage shown although there is 2.5V on the terminals. (If the short is removed then the BUZZ symbol disappers.) Also, the 10MHz position appears to show a correct value when 1kHz applied; and it shows the decimal point moving from right to left to give the corect value. I shall try a range of frequencies to confirm this. Perhaps I'm clutching at straws but it suggests that some bottom contacts are correct - but which ones?

Best regards,
M

[wasedadoc]

Regarding the bottom contacts, see the 5 with white dots on the red contacts.  The one for the two inner rings I have high confidence in.  The other four I have very high confidence in.

Obviously ring 10 (counting from outside) must connect with either ring 9 or ring 11.  I've yet to find what I consider compelling evidence for one or the other though I do strongly favour the contact being below centre.  That leaves the 7th contact which is still even more of a mystery.

For the top set, if you truly have the contacts in the correct slots of the disc and the LCD symbols match perfectly with the case markings, then you can be confident the rotational orientation is correct.  There is no symmetry which would give 100% match in any of the other three possible rotations.

[wasedadoc]

If my reasoning is correct this bottom layout should, at least, get the decimal points correct for all the ranges.

[mikew8760]

Unfortunately I suffered a mild seizure a couple of weeks ago, so things have taken a back seat, at least for the time being.
Thanks to everybody who has spent time and effort in helping; I still hope to find a solution to the problem in dues course.

Bye for now.......

[Hydrawerk]

I have never heard of Maxwell DMMs. https://www.maxwell-digital.com/index.php?page=products&cat=multimeters
The MX 25-304 looks like it was inspired by Unitrend UT70A. Ranges are similar although it is redesigned a lot.

[wasedadoc]

Mike, best wishes for a speedy recovery.

[wasedadoc]

I have never heard of Maxwell DMMs. https://www.maxwell-digital.com/index.php?page=products&cat=multimeters
The MX 25-304 looks like it was inspired by Unitrend UT70A. Ranges are similar although it is redesigned a lot.

The OEM is Victor.  See the 9808 at the top of the PCB in the photo in the first post.  Victor have been through several iterations of meter with 9808 in the model number.  Maxwell have badged at least two different meters with the same MX25-304 number.

[mikew876o]

Hi, I'm back!
After wasting the whole summer, doing more 'outside' things I now wonder if you are willing to take up the cudgel?

I've been trying out the last pattern of contacts, but still not correct.
But thanks again for all the trouble you have taken so far.

BTW  I've been unable to logon, perhaps I have been deleted, so re-registered under a similar pseudonym.

[wasedadoc]

Welcome back.

I had high hopes that the last set of contacts would be very close if not perfectly correct.  What ranges are working 100% and what are the faults on the ones that are not?

[mikew876o]

I am very grateful for you considering to continue helping.  I've attached a .pdf summarising the responses for each switch position for the last setup.  The displays shown are for the meter sockets shorted and left open circuit. I've tried applying voltages and resistors, but results are no different. This has made me wonder if there is a serious fault somewhere on the main pcb, but without some form of schematic it would take more tracing than I fancy at the moment.

There are a few curiosities:  in many positions the display shows a minus sign. The 10kHz position gives zero, but in a previous setup worked OK. A thermocouple gives correct temperatures and the hFE socket gives true values.

Finally, one point has puzzled me; there are 7 contacts and "only" 6 pairs of tracks. The latest arrangement has one leaf 'straddling' an adjacent track. I've always assumed the contacts cover one of the six pairs, even if two cover the same tracks but in different places.

Perhaps perusing the list of values may spark an idea!

[wasedadoc]

I tried searching for 9808 which is the model number of the OEM (Victor) and found a discussion on a Russian site with photos including the one I attach here.  This looks identical to your PCB and the selector dial markings correspond too.  Using Google's translation I read that some of the posts were about the contacts falling out and asking for info how they should be placed.

Although the photo shows both discs with the contacts in a North-South line, I think we have established that they are 90 degrees apart.

Give it a try.  Bottom disc with contact for two outside rings should be at the top when switch in the 2MOhm position.

https://forum.cxem.net/index.php?/topic/195985-%D0%BC%D1%83%D0%BB%D1%8C%D1%82%D0%B8%D0%BC%D0%B5%D1%82%D1%80-victor-vc9808/#comments

Post on first page by boris_ka

[wasedadoc]

Another post on a Russian site with photos. Post by 4 words.

https://www.nn.ru/community/techno/tech/?do=read&thread=2747380&topic_id=64375777

Both sets of photos do say that the two lines of contacts should be North-South.  Was my earlier conclusion of 90 degrees rotation incorrect?

[mikew876o]

Well, I thought I had scoured every corner of the internet without any real success.  I found a Hungarian site, Elektrotanya?, which you may know, but not the Russkis! I can only admire your persistence.

I shall apply the patterns forthwith and hope very much they are the sought  after Pot of Gold. 

Watch this space......

Best regards,
M.

[mikew876o]

Hooray!!!!  Mr Wasedadoc  please award yourself a Gold Star.  In fact, award 5 Gold Stars.   

The arrangement of contacts has raised the meter from the grave.  I am extremely grateful for your tenacity; I thought I would never live to see a successful outcome.  I've checked out all settings, apart from capacitance and induction, which I'll do this evening.

Again, many thanks for your help - I'm sure I would never have been able to unravel the conundrum.

Best regards,
M.

P.S.    Большое спасибо России!