UEI DM393 - repair advice all segments on all the time

[retiredcaps]

I have an UEI DM393 (the true RMS version of the DM391 that Dave reviewed in the $100 shootout).  I'm not the original owner so I don't know what went wrong with it or how it got into the following state.

Unfortunately, when I turn it on, all the segments are on full display and never disappear.  When turning the range switch, all the segments are on.  The backlight key works and the continuty buzzer works, but if I try to take a reading, obviously, I can't tell what is on the lcd.

Now this multimeter uses the FS9711_LP3, the successor predecessor I believe to the FS9721_LP3 rumored to be in the Fluke 17B.

Look at the datasheet for the FS9721_LP3 (I couldn't find one for the FS9711_LP3), I see that pins 48 to 65 control the lcd display and all the pins are designated as output (pages 6 and 7).

I have read through the datasheet a number of times over the last 2 weeks, but would like some clarification or confirmation on my understanding of the following:

Q1) From diagram 1 (block diagram), I'm assuming the microprocessor commands what segments are on and off based on the rotary switch and switch button.  If correct, would something shorted or malfunctioning with the rotary switch and push buttons cause all the segments to be displayed?

Q2) Is it possible that the microprocessor is shorted or malfunctioning and just not turning off the segments after a self test?

Q3) Is it possible that something from the external regulation network is shorted and sending the wrong voltages to the microprocessor which causes the all the segments to come on?

Q4) Is there an application note for this chip that is available so I can understand it more?

I have tested as many discrete components on the board as I can and so far I found nothing out of the ordinary.  I removed BD1 as it tested shorted in circuit only to find out it was good out of circuit.

Suggestions are welcome. 

[ElektroQuark]

Could the micro drive a transistor to light all the segments and it be stuck?

[ModemHead]

The backlight and buzzer functions are processor-controlled, so the fact that they work seems to indicate the clock is OK and the processor is running.  This also means it is probably beyond any self-test function in the firmware that might purposely turn on all segments.

In order for an LCD segment to come on, its segment input and corresponding backplane (common) input must be driven with a low-frequency (approx 50 to 100Hz) signal in opposite phase.  The signals will be in-phase for segments that are to be off.  Are the LCD segments all full-on with normal contrast?  If they're fuzzy or dim, it suggests that something has gone wrong with this scheme.

I think your Q3 is the most relevant, and hence I would try to check the the RLCD input to the chip. If I understand correctly, it is the drive voltage for the LCD.  The data sheet says it should be +3V with respect to the VSS pin.

Do you have a scope? It would be interesting to know if the LCD COM signals look like those in Diagram 5 of the data sheet.  That might provide a clue.

[retiredcaps]

EdoNork, Modemhead,

Thanks for the suggestions.  I have a few things to get done first in the next 24 hours before I can try the suggestions.  I don't have a scope so my findings will be limited.

[retiredcaps]

Here are my findings.

1) All the segments are full-on with normal contrast for all ranges.

2) The voltage to RLCD is 3.214V with respect to VSS.  A little higher than the specified 3V in the FS9711_LP3 datasheet.

3) BD1 is shorted from + to - pins (circled in red)

4) I believe 1D is a NPN transistor as per datasheet

http://www.datasheetarchive.com/smd%20TRANSISTOR%201D-datasheet.html

5) All 1D NPN transistors are shorted collector to base - i.e. 0.02 ohms (circled in yellow).

6) I just happened to have a Fluke 85 III open and noticed it had a bridge rectifier just like the UEI DM393 BD1. 
On the Fluke, the + and - are not shorted.

[ModemHead]

The bridge near the current shunt is just a safety component to limit the voltage drop across the shunt, regardless of polarity. For testing purposes, the meter should operate without it.  There was some mention of this application of diode bridges in another thread here recently, but I don't remember where.

I will guess that the SMD NPN transistors are being used as zener diodes, and you may find that the collector and base are actually tied together with copper.

If RLCD is 3.2V and the LCD segments look good, then we'll have to assume that the processor is driving them correctly, and because it thinks that it should be.  And if the processor is running at all, then we'll have to assume it is running correctly otherwise this repair exercise is moot (unless you're prepared to solder in a new one.)

Those assumptions lead back to your original Q1 and Q2 questions.  Something must be causing the proc to stop at the point where all segments are on.  Check for shorts and problems around the HOLD switch?  It invokes the LCD segment test.

Edit: oops, sorry, reading the wrong datasheet.  I don't know if this one has a segment test, check all the mode switch inputs for problems.  Basically everything in Diagram 20 of the datasheet.

[retiredcaps]

For testing purposes, the meter should operate without it.

Good because I had already removed it prior to knowing its function!

I will guess that the SMD NPN transistors are being used as zener diodes, and you may find that the collector and base are actually tied together with copper.

Yes, I thought that was wierd that they were all shorted out together.

If RLCD is 3.2V and the LCD segments look good, then we'll have to assume that the processor is driving them correctly, and because it thinks that it should be.  And if the processor is running at all, then we'll have to assume it is running correctly otherwise this repair exercise is moot (unless you're prepared to solder in a new one.)

I looked at the price of a new processor and the FS9711 isn't available on ebay.  The FS9271 is for $14.88 + $5 for s/h, but that is too rich for my blood.

Those assumptions lead back to your original Q1 and Q2 questions.  Something must be causing the proc to stop at the point where all segments are on.  Check for shorts and problems around the HOLD switch?  It invokes the LCD segment test.

Edit: oops, sorry, reading the wrong datasheet.  I don't know if this one has a segment test, check all the mode switch inputs for problems.  Basically everything in Diagram 20 of the datasheet.

I'm going to take the range switch out and have a look. 

This is a good exercise for me to learn more about troubleshooting so I don't mind investing the time to learn something new.  Then I'll start bidding on the Fluke "as is/parts" auctions!    Just kidding.

Thanks for your feedback and suggestions.  It is greatly appreciated.

I'll update this over the next few days.

[T4P]

If you can find a decent taobao agent in the states there are far cheaper FS9721s on taobao (4.8USD per piece)
But FS9711 seems to be non-existent

[njht4]

Hi I have the same problem with a 391 uei,all segments on,my query is, in the user manual it mentions fusiable resistors inside the meter.What I want to no is where are they.

[retiredcaps]

On my still unfixed DM393, there are no fusible resistor or spark gaps despite what the manual says

http://www.instrumart.com/assets/UEI_DM391_393_Manual.pdf

However, there are two PTCs.

[njht4]

Hi thanks for the reply. I will have to go on searching.I hope someone will find the answer to this problem.Thanks again