Beginner fixing Fluke 8840A

[nadona]

Please bear with me. I just started learning electronics, I think.
My Fluke 8840A arrived today. The ebay dealer said it turned on but. Plugged in. No power.
Opened the case, looked inside for possible clue, spent an hour, still no display, hit the transformer with the driver and now I can see the VFD turned on. 
Ran the self test. From number 4 through the end of 21, all analog test failed. Bunch of errors.

I downloaded the manual and I found out this meter's power supply has 5 voltage regulators pumping out 7 different voltages. Two voltages are not ok.
An LM7924 should output -30 because there is a Zener reversing 6.2V. Instead I got -40V. I checked the input, -42V which is coming out of bridge rectifier. supposed to be -24?

So the transformer is wrong?

[Shuggsy]

The datasheet I found for for a LM7924 puts the max input voltage at -35V... looking at the input schematic for the Fluke 8840A (first hit on a google search for 'Fluke 8840A service manual') that zener makes the common reference pin for the LM7924 6.2V higher than the input voltage. So, if you have a -40V input, the input voltage the LM7924 sees is -35.8V or around -36V. Still a bit high...

Is the input voltage selection switch on the right setting for your location? (240V vs. 120V?)? The schematic lists the resistance of the windings from the transformer... maybe check that to see if it's about right. The 7924 may have died with as high an input voltage as it's seeing, particularly if it's outputting -40V.

[nadona]

I checked the Zener diode voltage and it is 1.2V, but it's supposed to be 6.2. So I replaced it with a new one; it still shows 1.2V. I measured the old one, which shows the same number as unused new one.

I flipped the meter and checked the ohms between the secondary voltages. between 7824 and 7924, it shows 64V. middle and 7824 or 7924 both 32V.

[nadona]

Thanks Shuggsy! Yes, I replaced the 7924 and now the new one outputs -30V. I am getting all 7 output voltages out of 5 voltage regulators right.

Now for the real problems: Self-test shows the same result as before. Error from 4 to 9 and 14 to 21.  No change at all. None of the functions work.

VFD shows AUTO, mV, DC,  blinking S, number floats between 1.xxxx to 7.xxx mV. Nothing changed on the display.

[nadona]

According to the manual:
1) Measuring AC voltage or current needs an option card. 
2) To do a self test, the optional IEE488 card should be installed
3) To this and that, the other options are needed.

All the buttons respond. The problems are:
1) Displayed numbers are always changing
2) Number is very meaningless. Example: Should be 5V but shows like around 0.26 V or 10 mV, Resistance value behaviors erratically too.

Getting there!

[Shuggsy]

If you can at least switch modes and the fact that it does seem to perform the self tests, at least the digital bits seem to be functioning as expected. Sounds like the issue lies in the analog portions.

Looking at the previously mentioned service manual, page 130 in the pdf I found (section 6-37. Initial Troubleshooting Procedure) has a good bit of information to help isolate the issue. My guess is that there's one section that's acting up and causing all these errors. There's one sentence in particular that states "If the self-tests display a large number of errors or if readings are noise and/or in error, the problem is usually in the A/D Converter or Track/Hold circuit. (A large number of errors can also be caused by a problem in the Ohms Current Source.)" The manual then goes on into some procedures to help isolate the issue.

It also mentions Tables 6-16 (shows the states of logic lines and switches/relays during normal operation for each function), 6-17 (shows which components/circuits are exercised in each of the analog self-tests), and 6-23 (all power supply levels and acceptable limits). Those three tables should go far toward narrowing down your search for issues. A quick look at Table 6-17 shows that tests 4-9 and 15-21 all (exclusively!) exercise the amplifier in the Track/Hold section, so I might start my testing there. Not sure about test 14. I couldn't find anything that really set it apart from the others to help isolate things, but I bet table 6-16 and/or table 6-23 may help in that regard.

Good luck

[Shuggsy]

In regards to my previous post, section 6-63 of the manual goes over Track/Hold Troubleshooting and has a table for all the power supply voltages for the active components in that circuit.

edit:
Also, just in digging around... for U307 (Track/Hold amplifier), the LF351 that was used is discontinued... a quick google search led me to the TL071 which seems to be a drop-in replacement. If any of those quad analog switch ICs (U301, U302, and U303) are bad, those look like custom Fluke parts. They aren't listed in the replaceable parts list... but then again, neither is U307 which clearly is. You may be able to see if they're Fluke branded or not on the actual IC. I don't have one to look at

[nadona]

Thanks Shuggsy!
I already checked all the Table 2-22 voltages and they are fine.

U101, U301, 302, 303, 402, and 403 have the Fluke name on the ICs.

Thank you very much for guiding me. I think I will have to check ALL the numbers on the manual.

I have no idea about all those TP numbers. The manual doesn't mention most of them or what to expect out of those test points.

[Shuggsy]

Unfortunately one of the (very few) times I've seen the test points mentioned in that service manual was on a couple tests where you could inject voltages. Kinda useful, but if you're already pulling up parts to go around something and inject a voltage why put the test points? Could have been a bit more service friendly.

That said, it's fairly serviceable as-is! Hopefully none of those Fluke parts need replacing. The only avenues I can think of offhand for those would be eBay (either the specific part or another busted unit that might have working ICs) or Fluke itself. No idea on pricing or availability.

[nadona]

Hi everybody,
I worked on my 8840a again.

If I describe the symptom of the meter, it doesn't work at any setting.  Display is fine. When the rate is selected at SLOW, number floats whether I measure the voltage or resistance. Or without any input. If Medium or Fast is selected, it doesn't float. But always random number number.

I checked the Vcc and ground for all the IC and Schematics Voltages. I checked all the resistors. OK.

One thing, I don't understand is:
When VDC is selected AND 200mV, 2V, or 200V is selected AND at Slow Rate, the "HI" input terminal shows  AC of Vpp 3.6xV at 12.xHz on my scope.
If I choose either K ohm or any other button, no AC at the "HI" terminal.

[grenert]

You might try posting your question to the Fluke DMM Yahoo group:
http://tech.groups.yahoo.com/group/Fluke_DMM/?yguid=120617486

There are not a lot of people there, but maybe someone can help you out.

[nadona]

@grenert;
that will help.