Tektronix DMM4050/Fluke 8846A Guard Crossing Restarted Error +370

[Johnny10]

Tektronix DMM4050 Guard Crossing Restarted Error +370


Anyone familiar with this error?

I can't measure anything from front or rear posts.
Manual lists error with no further information.

[TiN]

I saw this one on my dead 8846A, when ethernet cable was plugged in (when meter worked a little).
But no idea what's the root cause.

[commongrounder]

I have this same meter in my shop.  I haven’t experienced this error, but it seems to imply a communication problem between the in-guard and out-guard sections of the main board.  Does the meter always hang at this point?  Might check and see if anything is blocking the optical coupling across the guard gap.
I do hope you can solve the problem.  It’s a great meter! 

[Johnny10]

OK So I am doing a repair on this DMM wondering if anyone has theirs open?

I have a blown diode? CR16 next to the LF353 opamp.

Checking power rails, seems something is dragging down my 15V and -15V rails. This could be part of problem.

Would like to know value.
Thanks

[Johnny10]

better pic

[picburner]

I read "A7", a BAV99?

[malaino]

Hi, I have the same problem. When I try to measure high values ​​of resistance I get the same error.

Have you replaced the A7 diode?
Does it work?

[Johnny10]

Changing the A7 diode made no difference to Guard Crossing error.

Are you saying you get "Guard CrossingRestarted" error only when you try and measure high resistance or whenever DMM is powered on?

[malaino]

Exactly, only the error appears when I try to measure high resistances.

[Kleinstein]

Getting the error only when measuring high resistance is odd, as there is not that much different in high resistance mode compared to normal readings. If at all the current consumption in the input referenced part would be higher in low resistance more, but not that much.
Normally the Ohms part should be very well separated from the data transmissions to keep the accuracy of the Ohms ranges. The only difference in the high ohms mode could be a relatively high voltage at some points. If there is a design fault or a defect there could be rare cases where this higher voltage could cause a latchup and this way higher than normal supply current (dragging down the supply), that could effect the rest of the operation. With a weak supply a latch-up may not blow the parts involved.

Is the error coming when using the high resistance range or is it only coming when the resistor also is high (or open circuit) ?

[malaino]

Yes it is, Only when I measure low or normal resistance values ​​is when I avoid the error.

[Urs42]

I got a Fluke 8845A with the same error. Im my case the error dosn't cause any troubles, the multimeter does work perfectly. I get this error sometimes  when i start the multimeter with the hardware power switch on the back. I can trigger the error by blocking the light from the IR LED DS2 in the second picture.

Will test some capacitors on the PCB.

[Johnny10]

I haven't gotten any further on my units since the holidays last year.

Once I found my units were not identifying Inguard software or hardware... I shelved them and started working on other projects.

[neosenshi]

I have a Fluke 8846A that would randomly error out.  I would get either 300, 301, or 370.

When I checked the output of the opamps in the Rx section of the optical circuit, the Inguard opamp's output was only 1.9V vs Outguard at 3.3V. Replacement of the IR LED (DS3) on the Outguard side brought the Inguard opamp's output back to 3.3v. I used an IR LED from an old camera illuminator. You do have to remove the small shield from over the Rx circuit to check at pin 1 of the opamp.

[Johnny10]

I will check my notes on this again. I did switch IR LED out, not sure anymore what the opamp output was.
Thanks for the info.

[Jonhys]

Hello .. Good evening,
I have DMM4050 with Q73 completely burned, would it be possible to get the reference of this component?
Thank you very much in advance