Fluke 45 high pitch noise problem.

[zed65]

I have a FLUKE 45 that makes a very high pitch noise (audible).
It's not that loud but I can hear it and it drives me crazy
I've tracked the noise to the T1 on the schematic. Is that a transformer?
I figured that it is driven by the LM3578 at 40kHz (confirmed with oscilloscope).
Humans can only hear up to about 20-25kHz so why can I hear the noise?

Is this normal for an old multimeter or is it some dried capacitors?
Do anyone elese have an FLUKE 45 that makes this noise?

[jumulab]

Hi, zed65

Take a look at these links where you can get the operation and service manual.
The trafo you mentioned are a double coil filter. The noise can be produced by turns not good pressed.
Take to place some glue. Look at the capacitors joined by the filter. Perhaps a hi voltage at input ?
Look in your fix work

see :
http://www.matsolutions.com/Portals/0/Product%20documents/Fluke/45/45%20Service%20Manual.pdf
and
http://www.matsolutions.com/Portals/0/Product%20documents/Fluke/45/45%20Service%20Manual.pdf

regards!


Posted on: October 25, 2015, 06:02:50 18:02 - Automerged
The correct  user manual link : http://exodus.poly.edu/~kurt/manuals/manuals/Fluke/FLUKE%2045%20User.pdf
Searched some in www

[vern]

Hi,

of course you cannot hear the 40kHz, maybe something is broken or the elcos are dry.
If you can measure the 40 kHz you should try to measure if the square wave is constant or if there is a jitter, meaning the sqauare waves vary in length. This would mean that the switching regulator is oscillating with an audible frequency. You could also try to measure the voltage at capacitor C32  10uF / 63V.
If there is a ripple with the frequency (might be about 10kHz) you hear it might be it.
The capacitor might be dry, lost its capacity and the switching regulator with the LM3578 is no longer stable. The compensation network of the LM3578 needs a certain output capacity.
If this is the case just replace the capacitor, that should help.
If it's the capacitor you should buy one that is suitable for a switching regulator, low resistance, high ripple current capable and I would always prfer a 105C tupe, they last longer.

[zed65]

I'm getting lots of jitter out of the lm3578.


This is measured on pin 6.

I've measured voltage att C32, 34v.

When I try to probe pin 1,2,3 on lm3578 the chip halts and "reboot"?
Can it be something with timing capacitor on pin 3, or is the lm3578 bad?

[vern]

What kind of probe are you using? 1MOhm or 10MOhm?
The probe can affect the IC but not as much that it stops working.
Are you sure you have the right GND? What is the Voltage between GND and LM3578 pin 4?
I would check the LM3578 pins if they are correctly soldered.
You should also check the compensation network C28, C29 and of course the timing capacitor C30, but since the basic frequency is all right with 40khz I don't think it's defect.

[perithess]

I can see in your picture that the plastic surrounding on the big el-cap has deformed. In the attached schematic I can find lot of suspect-for-fault el-cap. I suggest to buy good quality high hours nichicon or panasonic caps and replace them all in the power supply section. Try to go up one voltage scale. I can also suggest to check for bad solder joints on the transformers and on vfd.

[vern]

hmmm perithess,

I didn't see any deformed plastic sleeves, which of the el-caps did you mean?
I can see on the lower right hand side two caps where the top safety area looks as if it's ready to blow but they sometimes look like that even when they are new.
I would need another picture from a different angle the see if the are really bad.
But it wouldn't hurt to replace them anyway.

[zed65]

Hi, I have tested with both 1Mohm and 10Mohm, with 1M the signal stops and the display on meter is reset?
With 10M I can hear a difference pitch in the noise.
Voltage between pin4 and GND is 5.20v, solders look OK but I re-soldered them anyway.
I've replaced all the compensation caps, C28 - C30.
So far no luck 

The picture isn't mine.

Although I have replaced some other el-caps but the noise is still there.

[vern]

sorry it didn't help so far.
Do you have the rest of the schematics?
The voltage between pin4 and GND should be 5.0 volt, 5.2 seems a little high. Should be 5.0 volt.
Maybe there is something on the other sheets that could be wrong. There must be some circuitry between VCC 5V and GND that is not on this sheet.

[zed65]

Thanks for the help so far!

Here is the full schematics

[vern]

Could you check the waveforms at pin 13 / 14 of T2? They should be square and without Jitter. What is the frequency?

[perithess]

I was talking about for C42 to C47. You can also check C37 to C40 and see if they have degraded. I agree also with replacing C32. Can you post a photo of your Fluke with the capacitors that you have replaced?

[zac]

You can't hear 40 khz, but you can hear the harmonics.   

[zed65]

Here is pin #14 of T2:



And here is pin #13:



Some photos of the PCB:

[vern]

everything looks perfect to me. Sorry zed65, but I'm running out of ideas.
What I would do if it was my voltmeter: I would replace all the el-caps in that circuit, since those are the only components that can age by loosing their capacity. That would also lead to the symptoms you have because a low capacity would alter the compensation loop and lead to a jitter. You can't tell from the outside, you can only remove them, measure their capacity and replace them if it's to low.
As a final step I would try to replace the LM3578, but I don't think it is the reason for the jitter. In my experience IC's are very rarely defect.
But it's cheap and you might want to try it.
I can see on your picture that you replaced the smd caps, so you know how to solder!

[perithess]

I am also running out of ideas. As far as I can see you have replaced most of the electrolytics. Did you also resolder the transformer pins and any suspect conventional parts? Try to check that the main board and the controller part is not drawing too much current from the power supply. Lastly you can add 100n caps parallel to the filter caps and maybe that solves you problem.

[zed65]

I've sort of solved the problem! I bought a BK Precision 5491B dual display

Thanks for the help guys!

[bigtoy]

Good work. Sometimes you have to cut your losses and move on.