Multimeter DT9205A problem

[Andrew Borg]

Hi guys I have this Multimeter with auto switch off. The problem is that I cannot switch it on.
Once in a while it switch on but it had become rare now to switch on.
Any help please What's wrong with this multimeter?
Thanks

 

[(*steve*)]

You've probably already tried, but have you replaced the battery?

 

[OLIVE2222]

If the battery is OK you can have a look on the schematic power down circuitry:

http://www.dzkf.cn/upimg/allimg/081010/112594C0A-1.jpg

 

[Andrew Borg]

You've probably already tried, but have you replaced the battery?

Hi Steve and Olive2222
thanks for response. Yes I've replaced the battery and even took off the fuse to reboot as I read somewhere but still same problem.
I will have a look at the schematic and see
Thanks guys

 

[Andrew Borg]

I've looked at the schematic but don't know what's wrong. Tried to clean switch but I think there's something with the chip.

 

[KrisBlueNZ]

Thanks Olivier for finding the schematic!

Here's the power-down circuit.



It's controlled by the DPDT swtich at the left. When it's in the position shown, the OFF position, C19 is charged up to the battery voltage. When the switch is turned ON, C19 is connected to R53, and via D14, it is used to provide initial power to IC2A.

IC2A compares the voltage on R53 with a fixed threshold of about 18% of the power supply voltage (set by R52 and R51). When the multimeter is first switched ON, the R53 voltage should be a lot higher. IC2A will drive its output high, turning ON Q2, which turns ON Q3 and feeds the battery voltage to the V+ rail.

After a while, C19 discharges through R53 and its voltage drops below the 18% threshold. When this happens, IC2A's output goes low, Q2 and Q3 turn OFF, and the multimeter powers down.

There are lots of things that could go wrong with this circuit. A good place to start would be to measure (with a different multimeter!) the voltage across C19 when the multimeter is switched OFF (should be equal to the battery voltage), then when the multimeter is first switched ON. It will drop somewhat, but should remain much higher than the 1.6V threshold for several minutes at least.

If that's OK, measure the voltages on C18 (the V+ rail) and IC2 pin 1 (the comparator output) immediately after you have switched the multimeter ON. If they are both more than a few volts, measure the collector and base voltagres on Q2 and Q3.

Post the results here and we can work from there.

 

[(*steve*)]

For simplicity, follow Kris' instructions.

Spoiler

My first test would be to check the voltage across pins 4 and 8 of the HA17358 (as a side note, it looked like pin infinity at first!).

Does this voltage rise to 8V or so on power on, and stay there, or does it fall back to 0V immediately (perhaps you won't even see it going above 0V.

If you see voltage across here (say 7.5V or more), then the rest of the multimeter is receiving power and the fault is not in the auto power off circuit.

I would be hoping the fault IS in this circuit (and hence that you see the voltage across the op amp falling to zero quickly).

This circuit, if it is at fault, should be easy to repair as noted by Kris.

 

[Andrew Borg]

Thanks Olivier for finding the schematic!

Here's the power-down circuit.

View attachment 14479

It's controlled by the DPDT swtich at the left. When it's in the position shown, the OFF position, C19 is charged up to the battery voltage. When the switch is turned ON, C19 is connected to R53, and via D14, it is used to provide initial power to IC2A.

IC2A compares the voltage on R53 with a fixed threshold of about 18% of the power supply voltage (set by R52 and R51). When the multimeter is first switched ON, the R53 voltage should be a lot higher. IC2A will drive its output high, turning ON Q2, which turns ON Q3 and feeds the battery voltage to the V+ rail.

After a while, C19 discharges through R53 and its voltage drops below the 18% threshold. When this happens, IC2A's output goes low, Q2 and Q3 turn OFF, and the multimeter powers down.

There are lots of things that could go wrong with this circuit. A good place to start would be to measure (with a different multimeter!) the voltage across C19 when the multimeter is switched OFF (should be equal to the battery voltage), then when the multimeter is first switched ON. It will drop somewhat, but should remain much higher than the 1.6V threshold for several minutes at least.

If that's OK, measure the voltages on C18 (the V+ rail) and IC2 pin 1 (the comparator output) immediately after you have switched the multimeter ON. If they are both more than a few volts, measure the collector and base voltagres on Q2 and Q3.

Post the results here and we can work from there.

Hi Kris thanks for this info.
I checked the C19 it was without charge. I notice that when I shorted the bottom pins of the switch it switched on. When checking the cap after this it had charge in it as you've said.
So what is the fault about not charging the cap?
Thanks

 

[(*steve*)]

It sounds like the "upper" contacts (or at least one of them) isn't making contact.

 

[shrtrnd]

Yep. Clean the on/off switch conacts.

 

[Andrew Borg]

Hi Steve and Shrtnd thanks for response. To clean on/off switch with spray solvit would be ok?
Thanks guys

 

[davenn]

dunno what solvit is ?? google show a lot of different solvit's, none were cleaners

use a contact cleaner or something like WD40

Dave

 

[Andrew Borg]

Hi Dave I used WD40 but invain. The thing now is that I changed the switch; and while working I hit the board and not working now .
I noticed that C14 is missing. What part has to be there?
Thanks.

 

[(*steve*)]

C14 is listed as "223" which means 22 * 10³ pF which is 22000pF, or more commonly these days 22nF

 

[Andrew Borg]

Thanks Steve I'll will do one and see hoping that it will come alive again

 

[shrtrnd]

Sometimes manufacturers make 'all-purpose' boards, but don't install some components in instruments that don't have certain 'extra' options.
If a part is missing, it doesn't necessarily mean that one should be there for the model/revision/ or options in your instrument.
You may have knocked something loose on the board when you hit the board, but it's more likely you just didn't reseat everything correctly when
you reinstalled it. Displays are particularly touchy about improper re-seating. Try taking the board out again, and being particularly careful when reinstalling it
that all contacts are properly made.

 

[AdamF]

Yep. Clean the on/off switch conacts.

I just bought one of these and came with the exact same problem.

Just checked the DPDT and one of the switches wasn't working; reason why C19 did not charge when OFF.

Used a contact cleaner without even unscrewing the board, and now it's working like new... well, it is new.

Thanks a lot!!!

 

[GayanDSilva]

As per the cct diagram i serviceable my unserviceable DT9205A DMM in very short time.
In my case the problem was in the power switch (2P2D) and the capacitor (47uF/16V). I replaced both so found DMM serviceable. The logic is when power switch on charged cap (47uF/16V) has to discharged and give the voltage to non inverting pin of the op-amp and 10M resistor.
when soldering the switch be careful to replace the switch correctly considering the pin configurations. (N/O & N/C positions must correct)

 

[Andrew Borg]

As per the cct diagram i serviceable my unserviceable DT9205A DMM in very short time.
In my case the problem was in the power switch (2P2D) and the capacitor (47uF/16V). I replaced both so found DMM serviceable. The logic is when power switch on charged cap (47uF/16V) has to discharged and give the voltage to non inverting pin of the op-amp and 10M resistor.
when soldering the switch be careful to replace the switch correctly considering the pin configurations. (N/O & N/C positions must correct)

Thanks Gayan for your reply but it's a bit late cause I already scrapped it off cause could not find the fault.

 

[gribouilli]

hello, I have the same multimeter, I had the same problem, I started testing as notice in the post, and after a moment of lucidity, I did a test of my battery voltage, (9.37v ) then a test on the power cable of the battery, 0v.
I "opened" the case of the pin of the battery te hop cable poorly welded XD