Fluke 1520 MegOhmMeter service manual ? No high voltage output.

[Falkra]

Hi,

I can't find the service manual or some schematics for the Fluke 1520 (quite obsolete now).
There is a Calibration manual in addition to the user manual but I don't see the Service Manual.

If anyone has a link or has it, please let me know.

Thanks in advance.

[retiredcaps]

You likely won't find any service manual or schematic for any newer Fluke products.  Maybe we can fix whatever is wrong with your 1520 without one?

Post a detailed description of what is wrong with your meter.

[Falkra]

Hi, thanks for answering. 

It is a discontinued product and Fluke says the newer ones are better and don't give any service manuals directly.

The problem is the high violtage part doesn't work : the insulation testing doesn't work (at 250V/500V/1kV) so it doesn't measure anything. Every other functions are working (measuring ohms, low ohms, voltage).
The display is fine, backlight is ok, it beeps, you can hear the relay click for low ohms and it beeps but doens't output high voltage (I measured 0.000V). The fuse is OK and the HV diodes look ok. I hope it is not the tiny transformer...

I found some pictures here, the pcb is a bit different (some components change in my version) :
http://schneiderelectronicsrepair.nl/?p=179

I have two 1520 meters with the same issue.

[Falkra]

The fuse, MOVs and the passives I checked looked ok.

I can hear a high pitch noise (low volume) when the test button is pressed for insulation testing.

Is there an easy way to check the tiny transformer ? The two wires beep in continuity test (orange and white here) :
http://schneiderelectronicsrepair.nl/wp-content/uploads/2015/06/1520_3.jpg

[Electro Detective]

It is a discontinued product and Fluke says the newer ones are better and don't give any service manuals directly.
I have two 1520 meters with the same issue.

The newer ones are not 'better',

smaller and lighter perhaps, big price tag, full of inferior spec multimeter features you may not need, but not 'better', that's just sales BS    

The 1520 is in a working class of its own 

I would check the selector dial area for dirt, dry tracks and bad contact if you haven't already. 
Check the input sockets are not dirty or turned black and sooty.
See if the glass fuse is ok, and for any small board cracks

You might get lucky otherwise it may be the HV capacitor and diode array thingies
 
It's unlikely BOTH would have a tiny transformer problem   

They are worth fixing but not at Fluke prices,
or sell them as 'parts or not working' on Ebay and take a look at the Fluke 1507 which you may like too.
Not as 'nice' a meter, no bargraph, no low ohms, but you get a 50v to 1000v range, and option for auto timed insulation tests

1507 is easy to use (no menu BS) and works great if you can't fix or get another 1520 

[Falkra]

I'll check that, thanks for your answer, there was some dust on the inferior part of the pcb, and I'll re-do the solder joints on the outputs, one "may" be suspicious.
The dial works fine but I'll give him some contact cleaner and see.

I have a 1503 for home experiments (checked a few things with it), very nice one that I got at low price (ebay luck).

[Electro Detective]

I edited my comment above for more suggestions

1503 is nice too, 1507's lil brother   

[Falkra]

The glass fuse is ok (looks ok, continuity test is fine).
Edit : In the calibration manual it passes the test ok (lo Ohms, short leads and press test, correct value).

I've redone some soldering joints that looked suspicious, nothing changed (apart from levelling up my soldering skills).
The two grey diodes near the transformers test fine OL / 0,5V in reverse.

The HV capacitor, is it the arc spark gap one near the output ? (1,5kV is written on it)
Edit : that one is not shorted, reads 60 pF (had to use my peak LCR meter, Peak ESR tester did'nt see it).

[Electro Detective]

Strange that both 1520s are wacked out

With a bit of luck if you find the problem with one meter, hopefully the other will be the same fix

You checked the HV switch and relay points etc ?

All the rubber buttons pop up ok and don't stick to the gold board contacts? That's all I can think of tbh, it's been a while since I serviced mine.

If you hit a brick wall, let it go for a day or two (remove batteries and leave inputs shorted in case the discharge function is confused)
and come back fresh for Round 2   

Good luck mate 

[Falkra]

The relay's coil needs 5V to switch, it works and you can hear it for lo-ohms mesurement. Between the coil 108 and it clicks when injecting 5V (I tested with 20 mA).
The buttons are fine, and the pads are ok, the carbon contacts appear to be ok.

HV switch ? The "test" button ? It is ok.
The mosfet also looks ok.

I must have forgotten something essential. But it's an interesting challenge (I'm a beginner) I'm learning a lot. 

[Falkra]

Hmm that relay may be suspicious.

There are 4 pins (not 5). 2 aligned with the coil which reacts when you inject 5V, I can hear it.
The other two pins don't make continuity, even when I inject 5V (!).
It is hard to measure : 2 hands and 4 pins... 2 pins for the DMM on continuity mode and the 2 other pins with the 5V.  But I'm quite sure and there were no beeps.

But the relay only clicks when using test button for Lo-Ohms, which works perfectly fine. If I remember well the relay doesn't click when using HV.

[Electro Detective]

I got mine out of the work truck before I head off, and did some quick numbers in case they are of any use 

yeah the relay works/clicks only for low ohms Test, which puts out about 240 ma at 5.3 volts DC  tested direct into a Fluke 87V,
but should be same results with many decent 10 megohms input multimeters

whereas HV 250/500/1000v is only about 2 ma current, which most insulation testers are rated into a short circuit

and about 2.4 ma for the Continuity Ohms or beeping/buzzer at 5.3 volts DC

HV must get triggered by something else along the test switch path/s

Does HV beep when you hold down Test, or Test and Lock, or Lock and Test etc  ?

Batteries, 6 volts getting to the board, battery test function ok?


Found these sites claiming they had a service manual  http://mauritron.com/manuals/html/fluke.html

www.emanualshop.com/Fluke/Fluke-1520-Service-Manual.html

Best to verify in case it's a scam or BS or the owners/CAL manuals etc

If it is legit, it's worth getting if you can get both meters going.



The mystery continues... 

[Falkra]

Hi, thank you for your answers, I am currently away and the meters are at home, so I will have toi wait to check more things.

HV beeps when I hold down Test, or Test and Lock, or Lock and Test, this is fine.
Batteries are news and the bettery test reports 100%, I'll check for the 6 volts getting to the board.

[shakalnokturn]

It is a discontinued product and Fluke says the newer ones are better and don't give any service manuals directly.
I have two 1520 meters with the same issue.

The newer ones are not 'better',

I doubt the new ones are better, and it's starting to look like the old 1520 was bad enough as it is.

I've looked into this myself by now:
Battery feeds a LTC1044 charge pump used as a positive voltage doubler, this in turn supplies an UCC3805 through a BJT transistor when "test" is pressed.

The UCC3805 datasheet from TI states that:
"(2) In normal operation, VCC is powered through a current-limiting resistor. Absolute maximum of 12 V applies when VCC is driven from a
low impedance source, such that ICC does not exceed 30 mA (which includes the gate drive current requirement). The resistor must be
sized so that the VCC voltage, under operating conditions, is below 12 V but above the turnoff threshold."

ICC measured in circuit is well below the 30mA limit (closer to 2mA), with nice new batteries the LTC1044 will deliver as much as 12V reaching the absolute maximum though. There is no current limiting other than the saturated BJT switching this power supply, source is low impedance having tantalum and ceramic decoupling caps. I'm not saying this is a problem as the 30mA limit is far but for a serious company (that will not deliver service information) like Fluke this certainly could look like a datasheet overlook...

PCB revision is: FLUKE-1520-3001 REV 006

Attached are current measurements (differential voltage) across 22.1? MOS gate resistor from UCC3805, nothing big just for reference.

[shakalnokturn]

Gate discharge...

[shakalnokturn]

UCC3805 has died again, once more output pin is internally shorted to ground, apparently this has nothing to do with Vcc being close to limit, as I had zenered it down to 10V.
Further following tracks around revealed that the HV is taken through the rotary switch on this meter, the tracks there and back seem to be on a buried layer and run close to the UCC3805, as failure always occurs on 1kV test voltage I'm starting to think that maybe the HV is getting through the PCB material to the top layer where the UCC3805 is and destroying it.
Not sure why it would kill the UCC3805 but not the IRF9410 though, I've tried adding a zener protection to UCC3805 pin 6.

[spinella]

Hi, I resume this post for a similar reason ... I've a 1520 rev 7 with a blown component .... it is the small 0805 discrete at the right side of the
mosfet, circled in red.
It is burned and became black, so I can't recognize what it was ... it seems a capacitor, like the brown parts in other parts of the
pcb, but the resistance now is near 0 ohm ... besides it is in series to the drain ... so it seems strange that it is a capacitor ...
If someone can help with just a picture or a measure  to understand if it was a R or C that would be wonderful ...
thanks

franco

[shakalnokturn]

My own photos taken when I repaired mine are not that good however it does look like a plain dark grey component without marking.

First guess would be a ferrite bead or given proximity to MOS a thermistor.

I still own a 1520 so I'll keep you updated with a decent photo or measurement sooner or later.

What you should be worrying about though is what went wrong, the burn mark is not on the component itself but rather between tracks or vias.
On my failed 1520's I was suspecting that the PCB was experiencing HV breakdown, yours seems to further confirm this possibility.

On mine for some reason the HV overshoots quite a lot before settling to the right voltage, this ought to be looked into to avoid the HV arcing through the epoxy.

It would be nice if owners of little-used 1520's could confirm theirs also overshoot.

Edit:
@ Franco: BTW I'll only get back to you on this if you post back here first, too many new members don't follow thier requests. There's no urge for me to open my 1520 right now, if there is no rush for you either, I'll leave it be.

[ddcc]

@spinella I repaired a 1520 with a similar problem, this is a design flaw where the HV is too close to the adjacent via and is arcing through. On subsequent revisions, they've changed the board layout to fix it.

As a workaround, I ended up relocating the 0805 closer to the MOSFET by scraping off the soldermask on its traces, moving it closer to the MOFSET, and then removing its SMD pads. Cleanup and remove any remaning charring by the via, then cover the area with conformal coating.

[spinella]

dear all,
thanks for your answers ...thanks to them I fixed the instrument ... after reading shakalnokturn answer I checked better the instrument and saw a little spark between the 0805 component ( I measured it and
it is a 20 uH inductor) and the adiacent via, so I moved the 0805 near to the mosfet and the 1520 started to work again ... then I read the ddcc answer and
I was happy to have applied the same solution independently ...
so thanks again to both for your help
Franco

[shakalnokturn]

Thanks for the update.
Does the HV overshoot on your meter too?
Next time I open mine I'll try looking into the overshoot, will also see if it is possible to cut HV tracks to the switch and re-route with teflon wires instead.

[shakalnokturn]

I'm attaching the captures for the H.V. outputs rising on the four different megohmeters I have around today.

Measurements are carried out at 250, 500 and 1kV.

Probing is done with a ÷10 probe and 20Mohm in series so all on screen voltages must be multiplied by 3, the added 20Mohms are not frequency compensated.

This clearly shows that the Fluke 1520 is the worst for overshoot.

[Bucea]

Thanks guys, with your help and a syringe needle and a lot of patience I solved my problem with 1520, all good.

[GGatland]

Hi. I have a Fluke 1520
At some point (it was a while ago) I goofed up and put my probes on a mains voltage when ohms selected. I blew a component (can only see one damaged), but I don't know what it was.
Every other component looks fine. Does anyone have a circuit diagram, manual or comparison to advise how this could be repaired?