Fluke 8840a: is 715680 (U101) bad?

[mlefe]

Hi guys, I'm working on a 8840a I got from ebay.

The display shows nonsense and I could find out that the 7.5V rail gets as low as 5V when the U101 (A/D with "715680" fluke part number) is socketed.

I've desoldered U301, U302 and U303 that also use that rail, but that changes nothing: the rail only gets back to 7.48 if the U101 is not in place.

I've measured the resistance between the 5v and the 7.5v rails and it's only 14 ohms (again, with the U101 placed in its socket)

At this point, looks like it's fried (it doesn't give the 1MHz either, it returns a 8MHz signal on pin 14 so no division by 8 ) but I'm resisting to take the bullet as I'm living in Colombia and everything takes forever to get here...

Do you have any more ideas? What else can I look?

Thanks!

Sent from my Galaxy S8

[Kleinstein]

There is a small chance the voltage regulator part is not working. Especially a bad cap could cause excessive ripple before the regulator. However the chances are slim.

Another part to look at are tantalum caps - they tend to fail short or low impedance.

[mlefe]

There is a small chance the voltage regulator part is not working. Especially a bad cap could cause excessive ripple before the regulator. However the chances are slim.

Another part to look at are tantalum caps - they tend to fail short or low impedance.

Hi Kleinstein, thanks for the response!

That 7.5V rail is obtained from a Zener (that I took and tested ok and comes from a 15V regulator that keeps the voltage fine)
I believe the zener fails to regulate because of the very low impedance with the 5V rail... does that make sense?

What is strange is that if I check the resistance between those two pins inside the U101, I don't get the 14 ohms... [emoji848]

Could it be that the U101 is closing the path somehow but it's not respnsible of shorting it?

Sent from my Galaxy S8

[Kleinstein]

Resistance measurement can be misleading as the chips can act as nonlinear load. A clocked circuit also has a frequency dependent supply current. So the higher clock would cause more current. Without the U101 it might be hard to check the other signals supporting the chip.

In theory the 7.5 V zener could have turned bad (increased series resistance), though this would be an unusual failure.

[mlefe]

I don't think it's the zener, as far as I can tell, it behaves normally (it's normal, I believe that it stops regulating if the current becomes too high)
I do have the U101, so I will socket it again and test a few more things to see if I can be completely 100% confident that it's the culprit...
To me, that low impedance (14 ohm) path between the 5V and 7.5V rails is the reason why I'm getting 5V on the 7.5V rail: the current becomes too high, the zener drops the regulation, and the 5V rail takes over...
I've read somewhere that you could use a 5 digit multimeter to troubleshoot where a short is... have you heard about that technique?

[alm]

The technique you mention is likely searching for shorts by searching for I*R drops. By measuring the voltage drops with a high-resolution voltmeter, you can determine through which paths large amount of current is flowing (larger I*R drops). If a 3 V power rail is shorted to ground, voltage is being dropped as you get closer to ground. The more voltage drop you measure, the closer you are to the short. Ground planes can make this difficult. Pretty sure Dave has showed this technique in one of the Lecroy 9384C repair videos.

[mlefe]

The technique you mention is likely searching for shorts by searching for I*R drops. By measuring the voltage drops with a high-resolution voltmeter, you can determine through which paths large amount of current is flowing (larger I*R drops). If a 3 V power rail is shorted to ground, voltage is being dropped as you get closer to ground. The more voltage drop you measure, the closer you are to the short. Ground planes can make this difficult. Pretty sure Dave has showed this technique in one of the Lecroy 9384C repair videos.

I'll look for the video and review it: thanks!

[gearshredder]

Old post but only one I can find on the topic. Purchased a pair of these 8842a on ebay. I performed the same troubleshooting and have the same results. 7.5v rail is being pulled down. Flir images confirm, U101 is getting hot as well as the zener and regulator for that circuit.
Resistance test with high digit meter as well as voltage drop test says the same thing, but don't know what the result of the repair was back in 2017.
I guess I have a parts unit now unless there's a 715680 laying around.

[bdunham7]

Old post but only one I can find on the topic. Purchased a pair of these 8842a on ebay. I performed the same troubleshooting and have the same results. 7.5v rail is being pulled down. Flir images confirm, U101 is getting hot as well as the zener and regulator for that circuit.
Resistance test with high digit meter as well as voltage drop test says the same thing, but don't know what the result of the repair was back in 2017.
I guess I have a parts unit now unless there's a 715680 laying around.

What does the +15V supply do?  The load on the 7.5V circuit shouldn't affect the temperature of the zener-it should actually run cooler with more load.  And the +7.5V is fed thru 560R from the +15V, so it can only really supply 10mA or so, which I don't think will overheat either the regulator, resistor or U101.  How hot are they getting?

As far as eBay units go, prices for these meters are pretty high and I suspect people are unloading known junk as-is.  It probably isn't a random event that you got two with that issue.

[gearshredder]

Thanks for the reply! Ok I'll check the +15v components. Input to the regulator is 17v or so. The zener gets to around 180F in 10 seconds. Looked closer and the resistor is only hot from being on top.
And.. it's the 8.2v zener. Unlabeled, CR617 derp. It tests ok

The other meter I have works but is intermittent. I haven't inspected it yet, just turned it on. I know.. wrong procedure haha.

[bdunham7]

And.. it's the 8.2v zener. Unlabeled, CR617 derp. It tests ok

OK, that would be the result of an overload on the +7.5V supply.  Bummer.

The other meter I have works but is intermittent. I haven't inspected it yet, just turned it on. I know.. wrong procedure haha.

Do you mean the display blinks on and off without resetting the meter?  The first thing to do is to reflow the PCB power transformer terminals.

[gearshredder]

It is said to be intermittent. Not sure what that means yet. I have a 10v reference and it's there. I'll call that meter#2. It's got to be a different revision because I can't find CR617 zener. Maybe it's on the underside.

Meter1 has a good display as it would occasionally show something for the several seconds I'd turn it on. I reflowed the transformer on meter1, I'll check the joints on #2.

[gearshredder]

Ok, got to use my Jonard ic puller, and yanked the A/D out. Q102, part of the A/D bootstrap is getting to 100F in about 30 seconds. Compared to meter#2 and doesn't show up.
I'll study that circuit.