Fluke 5100B AC voltage issue

[Squarewave]

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.

[Squarewave]

Another quick test...

No more than 1.08VAC at all when switching between 1.1VAC and 1.9VAC

 

[capt bullshot]

Do you have the service manual?

Avaliable e.g. here: http://assets.fluke.com/manuals/5100B___imeng0100.pdf

It has a lot of troubleshooting procedures to narrow the problem, and a lot of diagrams to understand which parts of the circuits are involved in AC generation, e.g. it uses the same output amplifiers as the DC ranges, but a different source and some components switched within the control loop. Maybe the oscillator or the AC leveling circuit doesn't work as intended, there are some internal test points to check that - it's all in the manual.

[Squarewave]

Thank you, I shall take a look.

So far DC is fine, so it may be as you say something like the oscillator or AC leveling.

Thanks.

[Squarewave]

I may need extender cards...

[capt bullshot]

Don't have such a thing.

Managed to repair a Fluke 5100 about three times anyway.
Problems were:
  unstable DC voltages above 20V - one of the relays switching the control loop time constant was stuck
  Found this one by systematic analysis of the schematics, and verification by replacing the relay

  non-working GPIB - a defective logic gate on the GPIB board
  Don't remember excactly how I found this one

  no useful output except resistance - a shorted tantalum cap
  Found by removing modules and measuring the affected voltage until the defective module was located, then locating the shorted tantalum by injecting current into the affected rail, having the module on the desk and not connected to the chassis.

First step is to check all supply voltages, these test points are easily accessible without extenders.

Are the > 20V DC voltages OK? These use the same AC path (such as oscillator and leveling) as the AC output, but with a rectifier inserted - that might give you some hints.
Is the AC voltage behaviour different for different frequencies, or you may check using an external oscillator. This might point to the oscillator circuit.

[Squarewave]

All DC votages are ok so far. The issue appears to be only on AC.

I'm just reading through the book and from the above I'm making the assumption that the power supply regulator is ok. But I'll give that a check.

[Squarewave]

Maybe, just maybe I've found something already in the Power Supply Regulator..

The following is being checked with a Fluke 87V which is has a current in date calibration ticket.

From Page 4-38

(5) Connect the test DVM between TP14 and TP13
(6) Can the voltage be adjusted with R6 to between +14.99 and +15.01? Well, yes it can be. It can be adjusted lower and higher. It was 15V before I started, so I've left it there.

(7) Is the reading more than +15.01? No, I've left it as it were.
(9)Transfer the voltmeter high input lead to the collector of Q2 OK, done.
(10) Is the reading greater than 17V? Yes, it is, it's 20.51V

So, I've got some fiddling to do now...

[Squarewave]

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.

[capt bullshot]

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.

[Squarewave]

Thank you. Having a read of the pages you suggested has also taken me to look at the drawings for the millivolt divider, as the AC buffer is connected to the 2V tap of the divider (figure 7-9, page 7-27, PDF page 302).

[Squarewave]

Millivolt divider circuits are ok.

The millivolt divider is what reduces the output of A1 for AC voltages below 2V and those are problem voltages as well as difficultly with 10-20VAC.

So next I need to focus on the AC buffer as you have suggested, as well as the power amplifier.

[Squarewave]

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]

I didn't realise until now that there is a seperate repair section to the forum, I'll see if we can have the thread moved over...