Electronics > Repair

Fluke 732A - ripple on output

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Rax:
Hi all,
Just managed to snatch my first primary-grade voltage standard, and I am seeing some ACV on the output.

I was aware of the potential of finding ripple on the output, so the first thing I did - before powering it up - was to replace the three axial electrolytics. One had clear signs of trouble inside - rubber cap being crumbly dry and bulging - and I replaced all three of those with long life axials (27.5k hrs PEG124RL3150QE4 for C1 on the Pre-Reg and 8k hrs MAL211819471E3 for C3 on the same board and also on the Reg board).

I enclose a scope reading of the output. There's indeed about 70mV RMS at about 293kHz.

I am continuing looking at others caps, and I also plan to grab some batteries today and see if it's happening in floating mode too.

Appreciate input and maybe similar experience and hopefully solutions.

Rax:
Some pics and screenshots.

Rax:
Obviously, trying to cure a 293kHz ripple with repairing the main PS caps was quite a load of wishful thinking - which I was conscious of, but I planned replacing those anyway. I was hoping there would be more of an effect on the issue at hand, but I'm seeing no improvement.

Being AC out, I am still suspicious of the caps in the PS, but I'm running a thin and diminishing hope I'll avoid having to mess with the reference inside the oven - which I was hoping would not be the case.

bdunham7:
Is there any possibility of this being due to external interference?  Can you try shutting anything and everything off other than the 732B and your scope?  I had an issue with the LED lights in my workbench where they had a very large conducted interference at 238kHz.  One of the instruments that was badly affected by this was my 731B.  Are the batteries at all OK in your 732A?  i don't know what is in the unit itself that could possibly cause this.

Rax:

--- Quote from: bdunham7 on February 25, 2024, 11:47:26 pm ---Is there any possibility of this being due to external interference?  Can you try shutting anything and everything off other than the 732B and your scope?  I had an issue with the LED lights in my workbench where they had a very large conducted interference at 238kHz.  One of the instruments that was badly affected by this was my 731B.  Are the batteries at all OK in your 732A?  i don't know what is in the unit itself that could possibly cause this.

--- End quote ---

I think you're right on the money, as this ripple (exact frequency and level) was disclosed prior to the sale. So my assumption prior to committing to the purchase was a "frugal" tech in the seller's shop didn't consider the possibility of EMI from LED circuitry and such, exactly as you posit. Additionally, I couldn't put my finger in anything inside the 732A that could generate this odd garbage on the output. I was willing to bet a decent chunk of money there's nothing wrong with the unit and the seller got the short end of the stick.

But I am seeing all of that at my bench too. Admittedly, the two benches - mine and theirs - may as well be inflicted by having essentially the same LED drivers... I'll look into whether turning off all lights here would suppress this garbage.

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