Fluke 8845A 6.5 digit Multimeter smoking transformer

[murta79]

Hello,
I got this Fluke 8845A that I am troubleshooting, from the scrap bin 
The symptoms thus far is as follow. When plugged in the first time the screen was showing "OVERLOAD" and the transformer got very hot, to the point that streams of smoke came out and I quickly shut off the power. My first thought was that the voltage selector was in the wrong position, but it is in 240V which is correct here in Sweden.

I have done some basic troubleshooting:
- I disconnected the power-plug that goes to the FPGA-side of the board and measured the voltages on the "communication/VFD-display-side", these voltages looks OK (...and also no smoke when I only have this side connected).
- I then tried and connect only 1 of the secondary voltages on the FPGA-side at a time. I found that if I leave +/-20 volt disconnected (violet cables) the instrument stops showing "OVERLOAD" (The transformer still gets hot but at least no smoke this time either).
- Voltages on the secondary side of the transformer:
Violet_1 = 16,4VAC
Red_1 = 13,9VAC
Violet/White = 0V
Violet_2 = 12,3VAC
Red_2 = 12,6VAC

Measuring resistance (on PCB) between +/-20V->0V gives Mohm readings and between +/-15V->0V gives 50-200Kohm readings (fluctuating).

Grey_1 = 3,7VAC
Grey/White = 0V
Grey_2 = 3,7VAC

This is as far as I have come. I was hoping someone here has some good ideas on how to proceed in my troubleshooting. Regarding the transformer, I´m pretty sure some of the windings on the +/-20 volt has shorted out, probably because of a faulty component on that rail, maybe also at the same time shorted windings on the +/-15V ?
The question is, is it necessary for me to buy a new transformer to continue the troubleshooting or can I use the one I got until I figure out what other components are faulty on the board and then buy a new transformer?

[murta79]

..Forgot to mention, I did look trough a bunch of other posts on this forum regarding this multimeter but I could not find any with similar problem symptoms as mine. If anyone knows of any posts that can help me (on this forum or other), please notify me. 

[andy3055]

Check the power supply caps and diodes first. Something is shorting.

[strawberry]

add some load (resistor ~1k/incandescent bulb ~50W) in series with multimeter cord to save transformer from destruction
observe voltage drop

[bdunham7]

First, the 'OVERLOAD' warning is very common when the unit is turned on with nothing connected.  It is just the result of noise and autoranging, a quirk to be sure but one that should be easily resolved by shorting the inputs or selecting DCI (current) immediately upon turning it on.  Not having power to the guarded power supply also might cause the 'OVERLOAD' to go away, but for now I would just not worry about it.

If the transformer is smoking, can you measure the amount of current the unit is drawing?  And if that is excessive, what happened to the fuse?  Does the unit appear to have been worked on or is there any physical damage? 

Your transformer has not likely been improved by smoking, but if the current draw is normal (low) when the secondaries are disconnected, then it may be OK enough for now.  Possibly the unit was not set for 240V, connected to 240 volts long enough to overheat or overvoltage and damage a diode, regulator or other component in the guarded power supply and then finally set to 240 volts, but too late and without replacing the fuse--which is 1/4A slo-blo for 120V but only 1/8A slo-blo for 240V.  Check which you have...if you see a 1/4A fuse in there, this is the likely scenario.

If it sort of works but draws too much current, you'll have to map out the guarded power supply because AFAIK there are no schematics available.  I've worked on these a bit, but don't remember the exact setup for the guarded PSU.  Perhaps post some good photos?

[Ugur]

Is it possible to verify that your regulated voltages are correct and stable? 5V,-5V,15V,3.3V,1.5V,55V,20V-20V,7Vref

If not, check the ohms between ground and regulated voltages.

[manicdoc]

add some load (resistor ~1k/incandescent bulb ~50W) in series with multimeter cord to save transformer from destruction
observe voltage drop

This is an absolute MUST. NEVER take unknown equipment and plug it unprotected into the mains, it's quite likely you will make matters worse. Always do a resistance measurement of the mains side before plugging in and ascertain you have the right resistance for expected power consumption - that will dismiss absolute shorts in the winding, plus check there is high resistance to earth.

Next, measure the resistance across the secondary side windings and check its proportional to expect voltage per winding if you can. If it is low, could be bad winding or bad diodes, cap, or regulator. if it is open, could be bad winding...

Also check the resistance across the rails as delivered from the power supply with it disconnected from what it's supplying..  if these are short, you need to get rid of the short before you proceed. Also, this is a good time to check the resistances of what it is supplying in isolation, as a short could be there as well.

Basically powering it up usually comes last before taking a whole load of measurements to reduce the likelihood of what you experienced. This also assists in your understanding of what the power supply is doing in a safer way. Plus you can use this as an opportunity to do ESR measurement of caps to see if any have gone bad. I usually spend a good 30 minutes checking power supplies in isolation before any power gets applied, it's time very well spent.

Now you might be lucky and have got away with this, but it certainly reduced the life of the transformer if that was smoking.

[bdunham7]

Now you might be lucky and have got away with this, but it certainly reduced the life of the transformer if that was smoking.

A replacement transformer will be needed IMO at some point because inguard isolation depends on its integrity and being smoked could affect that.  If it is still producing the correct voltages, I'd wait on replacing it until the other issues are dealt with.  Testing to make sure that all the inguard, outguard and primary windings are at least nominally isolated is probably enough to proceed for now.

[alm]

I would expect the primary-side fuse to be sized to protect the transformer. Was the fuse correct for the mains voltage?

[strawberry]

I would expect the primary-side fuse to be sized to protect the transformer. Was the fuse correct for the mains voltage?

nails are not marked with mains voltage 

[Kleinstein]

The main purpose of the primay fuse is to prevent a fire after the transformer is blown or when used with the wrong mains voltage setting.

The normal cheap fuses are not very accurate: they may blow just above the nomunal current, but could also last long at twice nominal. The tolerace is too large to really protect the transformer without-over sizing the transformer and use a relatively small fuse. It may help with the wrong mains setting, but subtile over load, like just 1 winding seeing a near short (e.g. Ta capacitor causing a short) can over load it. With frequent hot - cold cycled the transformer should also be the primary source of failure, like getting some windings shorted out.

[strawberry]

fast /slow /delayed
and cheap made out of ill-proper materials like aluminum