Fluke 5100B AC voltage issue

[Squarewave]

Good morning.

So I made a thread in the test gear area, before disovering this repair section section.

Origional thread here https://www.eevblog.com/forum/testgear/fluke-5100b-ac-voltage-issue/

Basically to summerise, origional post here

Hello, I recently aquired a 5100B which so far works fine on DC but has some strange AC range problems.

From cold....

1VAC reads <500mV then floats at around 700mV

It will give me the same reading until 2V

When warm after many hours, readings from 1VAC are ok up to 1.2, but reads around 1.2VAC throughout from 1.2VAC to 1.9VAC.

Also

Just discovered 10VAC to 20VAC range......

10VAC is ok, reads a little low up to 15VAC then after 5 mins reads ok, but from 17VAC right now is reading 16.8VAC when set from 17 to 19VAC.

 

Any ideas where to start? Thanks.

A helpful reply here

I wonder if you get a chance to read that page for me as I'm not sure if I've determined a fault or not. I re read it and it's unclear if the manual is suggesting that it should or should not be greater than 17V on the collector, as it's only taking me to further tests in that area if it's not greater than 17V.

Perhaps this part is ok.

The manual suggests it's OK if it's greater than 17V. If it isn't you're supposed to check #11

Anyway, by passing #6, you should have jumped right to #16, so it wasn't required to do #10. Look into the Yes/No column what step is next.
Did you measure and check all output voltages according to table 4-11? If they all are OK, you wouldn't do the troubleshooting sequence anyway.

From your description, I'd locate the fault within "AC BUFFER" (see block diagram fig. 3-4) or "AC CONVERTER (same diagram). These both are part of the control loop for AC but not for DC. Study these diagrams in chapter 3 to get an idea of the different modes.
Then locate the signal "CONTROL" and open the loop - that makes troubleshooting easier. Drive Q106 (as seen in fig 3-4) from an external source (study the schematics to locate the signal and what source will be appropriate). Watch what happens at the output of A1, input and output of AC buffer, input and output of AC converter.

That would be my approach from your description of what doesn't work, beware there alway may be something I missed or you didn't test / describe that would point me somewhere else.
I'd suppose to roughly do all the performance checks to see what ranges / modes are working and what isn't working, then go trough the block diagrams to see which circuit blocks are within the signal path for the failed modes and not for the working modes.

Edit: Often I just don't follow all the troubleshooting tables in the manuals, taking shortcuts like above - but one needs to have a good understanding of how the device works to do so. Luckily it's quite well documented for the 5100 series.

Edit 2: Read your first post again: Check AC oscillator ouput voltage at different frequencies, might be just too low.

Edit 3: Just looked into the performance tests - don't be scared by all the instrumentation that is supposed to be required. Do the test roughly, just use a DMM to verify all the different setpoints, don't check the accuracy to ppm level.

My last post here

So, Q106 is in the Power Amplifier circuit. I'd ideally like to perform the trouble shooting tests, but would really need an extender set for that.

Q106 is a PN4091. I could remove that and test it alone.

I'm still having a think about the best way to drive it and making sense of what to expect.

[Squarewave]

Just had a thought regarding extender cards, which I don't have.

If I use some heat shrink, I could connect wires to the test points, mark them and insulate the ends with heat shrink....

That way, I've extended the test points and should be able to work my way through the troubleshooting on the Power Amplifier section without the use of extender cards.

[capt bullshot]

 

[Squarewave]

Don't understand your post. Unless you mean you can't help any further?

[capt bullshot]

Don't understand your post. Unless you mean you can't help any further?

No, I just left that popcorn here to have that thread on my watch list (show new replies to your posts). So I can follow your progress and maybe answer questions. Otherwise the thread just would vanish from my span of attention. It's common here in this forum to leave such a post if one wants to watch a thread.

BTW: Soldering wires to test points to make them available is a simple way to get around not having the extender boards. I did that also before. But I don't think Q106 is bad. I'd check the relays first.

[FlyingHacker]

I usually just hit the Notify button, and then I get updates to a thread without posting.

Looking forward to hearing the solution, but sadly, I have nothing to contribute either. Sorry.

[Squarewave]

Don't understand your post. Unless you mean you can't help any further?

No, I just left that popcorn here to have that thread on my watch list (show new replies to your posts). So I can follow your progress and maybe answer questions. Otherwise the thread just would vanish from my span of attention. It's common here in this forum to leave such a post if one wants to watch a thread.

BTW: Soldering wires to test points to make them available is a simple way to get around not having the extender boards. I did that also before. But I don't think Q106 is bad. I'd check the relays first.

That's fine, I understand now. I'll keep you updated and thanks for the help so far. I hope to maybe take another look sometime this coming week.

[Squarewave]

Never did look back at this issue, however, I wonder if anyone knows what L. err is?

It's not listed in the manual, it occurs when I switch off my variac. 

[capt bullshot]

Looks and sounds somewhat familiar, I vaguely remember having that seen somewhere ...
But no, my memory ends here, so no idea.

[doktor pyta]

I repaired three 5100B.
One of my cases was unstable AC voltage.
The reason occurred to be faulty C30 (see schematic below).
It is 390nF 'TRW' brand capacitor which became 60ohm resistor after warming up.
That was a surprise for me, however I had previously problems with leaky 'TRW' caps.

Edit: First stage of the repair was replacing all 'droplet -shaped' tantalum caps.


Best Regards BR

[Squarewave]

I'm wondering if it's some spike into the mains that the variac is causing when it's switched off and the 5100B reporting that as Line error? Or Live error?