Author Topic: UNI-T (UNI-Trend) UT90C review.....I killed it!  (Read 31933 times)

MJLorton

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Re: UNI-T (UNI-Trend) UT90C review.....I killed it!
« Reply #30 on: April 20, 2012, 05:18:49 AM »
It shows 0.0000 on ohms.

I noticed the volts reading is also 0.000.  With your ohms reading 0.000, it almost looks like the input jacks, COM and voltage, are shorted together?

In any working multimeter, if you put the probes together you would get 0.000V and 0.000 ohms (or close to it).  When the probes are apart, you should get a little bit of ghost voltage for volts (minor fluctuation) range and of course 0L for ohm range.

So there might be a shorted component somewhere?  Anyway, if you have time, post the photos.  I will take a look to see if any component is the likely suspect.
Thanks very much for the offer...I'll try get some hi-def photos up over the weekend if possible.
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MJLorton

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Re: UNI-T (UNI-Trend) UT90C review.....I killed it!
« Reply #31 on: April 26, 2012, 06:57:19 AM »
It shows 0.0000 on ohms.

I noticed the volts reading is also 0.000.  With your ohms reading 0.000, it almost looks like the input jacks, COM and voltage, are shorted together?

In any working multimeter, if you put the probes together you would get 0.000V and 0.000 ohms (or close to it).  When the probes are apart, you should get a little bit of ghost voltage for volts (minor fluctuation) range and of course 0L for ohm range.

So there might be a shorted component somewhere?  Anyway, if you have time, post the photos.  I will take a look to see if any component is the likely suspect.
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MJLorton

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Re: UNI-T (UNI-Trend) UT90C review.....I killed it!
« Reply #32 on: April 26, 2012, 06:58:09 AM »
It shows 0.0000 on ohms.

I noticed the volts reading is also 0.000.  With your ohms reading 0.000, it almost looks like the input jacks, COM and voltage, are shorted together?

In any working multimeter, if you put the probes together you would get 0.000V and 0.000 ohms (or close to it).  When the probes are apart, you should get a little bit of ghost voltage for volts (minor fluctuation) range and of course 0L for ohm range.

So there might be a shorted component somewhere?  Anyway, if you have time, post the photos.  I will take a look to see if any component is the likely suspect.
Play, discover, learn and enjoy! (and don't be scared to make mistakes along the way!)

retiredcaps

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Re: UNI-T (UNI-Trend) UT90C review.....I killed it!
« Reply #33 on: April 26, 2012, 05:44:45 PM »
Okay, I have quick 5 minute test for you to do.

Circled in red is the Volts jack.  Circled in black is the COM jack.  Circled in blue are diodes (5 total).  Circled in yellow are transistors (3 total).

All tests can be done "in circuit". That is, there is no need to desolder unless the readiings indicate they may be shorted.

1) Test the diodes with the diode test function of a working multimeter.  As you know a good diode should read between 0.4 and 0.8V one way and "out of range" the other way.

A shorted diode will probably reading the same voltage both ways (something low like 0.2V both ways).

2) Test the transistors with the resistance function of a working multimeter.  Number the transistors pins 1, 2, and 3.  Measure the resistance between pins 1-2, 1-3, and 2-3.

An ohms reading of less than 30 ohms suggests a shorted transistor.
« Last Edit: April 26, 2012, 05:46:17 PM by retiredcaps »

MJLorton

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Re: UNI-T (UNI-Trend) UT90C review.....I killed it!
« Reply #34 on: April 27, 2012, 09:06:12 AM »
Thanks.....

I did the checks on the diodes....all ok.

Then I did the checks on the transistors....and got very excited....there was a short on 2-3 or 1-2 depending numbering sequence....on all of them...

I pulled out some new NPN transistors to do an "out of circuit" test to confirm I had it right....and yes...no shorts there.

I preceded to warm my soldering iron to remove and replace the 3 transistors....when I noted they are shorted on the underside of the board with solder...????

See attached picture.
« Last Edit: April 27, 2012, 09:10:59 AM by MJLorton »
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retiredcaps

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Re: UNI-T (UNI-Trend) UT90C review.....I killed it!
« Reply #35 on: April 27, 2012, 06:51:19 PM »
It is a little difficult to see due to the low resolution of the photos if they are soldered together, but I believe you.  What are the part numbers on the transistors?  I'll take a look at the datasheets to see why they may be soldered together?

By the way, here is another way to test those transistors using the diode test function (34 second video)

http://www.youtube.com/watch?v=8VK_4gZU__I

You may want to test those 3 transistors again using the above method and on a couple of new transistors.
« Last Edit: April 27, 2012, 07:09:20 PM by retiredcaps »

MJLorton

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Re: UNI-T (UNI-Trend) UT90C review.....I killed it!
« Reply #36 on: April 29, 2012, 05:45:54 AM »
It is a little difficult to see due to the low resolution of the photos if they are soldered together, but I believe you.  What are the part numbers on the transistors?  I'll take a look at the datasheets to see why they may be soldered together?

By the way, here is another way to test those transistors using the diode test function (34 second video)

http://www.youtube.com/watch?v=8VK_4gZU__I

You may want to test those 3 transistors again using the above method and on a couple of new transistors.
Thanks very much, that video put the transistor testing with the diode function into perspective staight away. The NPN's on the board have a different pin out to the ones I was using as a reference. I'm about to de-solder one of the transistors so I can test it out of circuit and will feedback.
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MJLorton

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Re: UNI-T (UNI-Trend) UT90C review.....I killed it!
« Reply #37 on: April 29, 2012, 06:20:01 AM »
I removed the transistors from the board and tested between base, emitter and collector and a 0.7 voltage drop showed correct operation. No shorts.

I show the pads they were removed from and you can see how the 2 pins of each transistor were shorted.

Pity!! I thought we might find the fault....but I learnt something through the process so thanks very much for the guidance. 
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retiredcaps

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Re: UNI-T (UNI-Trend) UT90C review.....I killed it!
« Reply #38 on: May 10, 2012, 12:25:19 AM »
I just watched your test a transistor and diode video.  Great job.

If you are willing to test a bit more, take a high resolution picture of the components underneath the lcd.  I can look for their datasheets and see if any of the ICs are shorted.  Sometimes VCC will be shorted to GND. 

I have more ideas, but obviously since I don't have the board in front of me, I can't just test things when I want to.

BTW, I ran into some Sperry brand multimeters.  Sperry is a known name in North America.  Their multimeter suspiciously look like Uni-Ts.  Even the manuals have the same 30V maximum "warning" when measuring frequency.

http://www.sperryinstruments.com/product/digital-multimeters

Hmm, does this screen look familar?

http://www.sperryinstruments.com/images/applications/dm6400_2_lg.jpg

MJLorton

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Re: UNI-T (UNI-Trend) UT90C review.....I killed it!
« Reply #39 on: May 10, 2012, 07:02:54 AM »
I just watched your test a transistor and diode video.  Great job.

If you are willing to test a bit more, take a high resolution picture of the components underneath the lcd.  I can look for their datasheets and see if any of the ICs are shorted.  Sometimes VCC will be shorted to GND. 

I have more ideas, but obviously since I don't have the board in front of me, I can't just test things when I want to.

BTW, I ran into some Sperry brand multimeters.  Sperry is a known name in North America.  Their multimeter suspiciously look like Uni-Ts.  Even the manuals have the same 30V maximum "warning" when measuring frequency.

http://www.sperryinstruments.com/product/digital-multimeters

Hmm, does this screen look familar?

http://www.sperryinstruments.com/images/applications/dm6400_2_lg.jpg

Thanks very much...but certainly inspired by your guidance and assistance.

What a laugh...Sperry...certainly a re-branded UNI-T....I'd love to make a study of how all the re-branding in different industries works....can be very confusing to Joe public and often a money spinner....

Ok...let me get a good picture taken of the bits under the LCD and I'll post it soon.

Cheers,
Martin.
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MJLorton

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Re: UNI-T (UNI-Trend) UT90C review.....I killed it!
« Reply #40 on: May 11, 2012, 08:30:47 AM »
Here we go.......but take a look at the caps along side the IC....I think they look a little suspect?? Only noticed them in the pictures when posting this...
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retiredcaps

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Re: UNI-T (UNI-Trend) UT90C review.....I killed it!
« Reply #41 on: May 11, 2012, 07:19:24 PM »
Here we go.......but take a look at the caps along side the IC....I think they look a little suspect?? Only noticed them in the pictures when posting this...

1) The cap sleeves look all melted somehow. 

2) I found the datasheet for FS9721_LP3.  I'm reading it now to try and understand how it works.

3) While I do that, I have 3 more things for you to measure.  All are resistance readings.

i) Marked in yellow are PTCs.  I believe they should all be around 1K ohm from reading the datasheet.  You may have to desolder these 3 to get a accurate reading?

ii) R26 (in blue) should be a 10M ohm resistor.  Measure this in circuit and verify that it 10M.

iii) R31 (in red) is a 10K SMD resistor.  Measure this in circuit and verify that is 10K.

4) BTW, rumor has it that the Fluke 15B and 17B use the same FS9721-LP3 processor.  Certainly the specs for the Fluke 15B and 17B match up with the FS9721-LP3 datasheet.

5) A really good SMD soldering video is here

http://www.youtube.com/watch?v=3NN7UGWYmBY

6) Of course Dave Jones also has a 3 part soldering video and demonstrates various techniques for SMD.

MJLorton

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Re: UNI-T (UNI-Trend) UT90C review.....I killed it!
« Reply #42 on: May 14, 2012, 04:00:52 AM »
Here we go.......but take a look at the caps along side the IC....I think they look a little suspect?? Only noticed them in the pictures when posting this...

1) The cap sleeves look all melted somehow. 

2) I found the datasheet for FS9721_LP3.  I'm reading it now to try and understand how it works.

3) While I do that, I have 3 more things for you to measure.  All are resistance readings.

i) Marked in yellow are PTCs.  I believe they should all be around 1K ohm from reading the datasheet.  You may have to desolder these 3 to get a accurate reading?

ii) R26 (in blue) should be a 10M ohm resistor.  Measure this in circuit and verify that it 10M.

iii) R31 (in red) is a 10K SMD resistor.  Measure this in circuit and verify that is 10K.

4) BTW, rumor has it that the Fluke 15B and 17B use the same FS9721-LP3 processor.  Certainly the specs for the Fluke 15B and 17B match up with the FS9721-LP3 datasheet.

5) A really good SMD soldering video is here

http://www.youtube.com/watch?v=3NN7UGWYmBY

6) Of course Dave Jones also has a 3 part soldering video and demonstrates various techniques for SMD.

Thanks very much for this, I will tackle the testing later in the week...a little busy at the moment.

I'll  take a look at the video and follow-up on Dave's videos...I thought he might have something on that topic.

Cheers,
Martin.
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Re: UNI-T (UNI-Trend) UT90C review.....I killed it!
« Reply #43 on: May 19, 2012, 08:48:05 AM »
Here we go.......but take a look at the caps along side the IC....I think they look a little suspect?? Only noticed them in the pictures when posting this...

1) The cap sleeves look all melted somehow. 

2) I found the datasheet for FS9721_LP3.  I'm reading it now to try and understand how it works.

3) While I do that, I have 3 more things for you to measure.  All are resistance readings.

i) Marked in yellow are PTCs.  I believe they should all be around 1K ohm from reading the datasheet.  You may have to desolder these 3 to get a accurate reading?

ii) R26 (in blue) should be a 10M ohm resistor.  Measure this in circuit and verify that it 10M.

iii) R31 (in red) is a 10K SMD resistor.  Measure this in circuit and verify that is 10K.

4) BTW, rumor has it that the Fluke 15B and 17B use the same FS9721-LP3 processor.  Certainly the specs for the Fluke 15B and 17B match up with the FS9721-LP3 datasheet.

5) A really good SMD soldering video is here

http://www.youtube.com/watch?v=3NN7UGWYmBY

6) Of course Dave Jones also has a 3 part soldering video and demonstrates various techniques for SMD.

Ok...here we go:

PTC1: 1K
PTC2: 567 ohm...out of circuit
PTC3: 589 ohm...out of circuit
R26: 100 ohm...out of circuit
R31 SMD: 10 M ohm...checked several times...

I watched Dave's soldering tutorials....very helpful...I have now acquired a solder station so I'm setting up for the SMD work soon.


Thanks again...

Cheers,
Martin.

retiredcaps

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Re: UNI-T (UNI-Trend) UT90C review.....I killed it!
« Reply #44 on: May 19, 2012, 11:31:57 PM »
PTC1: 1K
PTC2: 567 ohm...out of circuit
PTC3: 589 ohm...out of circuit
R26: 100 ohm...out of circuit
R31 SMD: 10 M ohm...checked several times...

Okay, I'm using this datasheet as a reference.

www.ic-fortune.com/upload/Download/FS9721_LP3-DS-20_EN.pdf

With the above, I was using the datasheet's pcb designators (page 19) as described by the various diagrams/figures, but it looks like the UT90C is using a variation.

R31 is good as your 10M ohm resistor.  As long as R26 matches the color code for a 100 ohm resistor, it is good.

PTC1 at 1K is good.  PTC2 and PTC3 are lower than what I expected (1K) unless they are in series, but I don't think the low values are the cause of the UT90C not working.  If one of the PTCs were completely open, then it might explain why the UT90C is not working.

Let me think about this for a few more days.  Unless I spot something obvious, I think the FS9721-LP3 might be bad.