Fluke 5200A oscillator repair

[Tony_G]

Hi all,

I'm suffering a weird problem I think with the oscillator in my 5200A that you can see here (basically any frequency range above 100Hz generates a pulsating overload annunciation - As always any comments or suggestions greatly appreciated):



I've checked power supplies etc and I'm just working through the troubleshooting table and I noticed it mentioned this:



I have no idea what the MSD switch is in this context as the A9 Oscillator Control Board is out of the unit at this stage and it says that the front panel frequency switches don't work earlier in the table due to that

Can anyone enlighten me?

Thanks,

TonyG

[Dr. Frank]

Hi TonyG,

this failure principally can originate everywhere in the feedback loop.

Most probable is A7, HV-amplifier, the switching which determines 120V or 12V output range, as one hears relays clicking. These may be K1, K2. Read manual page 3-12, the last sentence, maybe this pin 29 shows erratic logic signals.
Does the output produce short pulses >> 10V in your current setting?

So what happens, if you switch to 120V range?

MSD frequency switch is the most significant decade, i.e. set to 0.1000 kHz, as you have done in your video.
I do not understand, why the front panel selector should not be active, even if the A10 is sitting on an extender board.

Good luck
Frank

[Tony_G]

Thanks Dr. Frank - The reason I was saying that the front panel switches wouldn't work because I could have sworn I say the manual say that when the A9 Oscillator Control board is removed you lose access to the front panel frequency switches with this board removed.

I'll take a look at Pin 29 (don't have an extender board so I'll need to see where I can take it off the board with a mini-grabber).

The 5200A is set for 1V output on the 1V range, 100HZ on the 100Hz range, Local Control (no RCU board anyway), Phase Lock Off, Sense Int.

For 1V @ 100Hz:



When I step that up to the 1KHz range I see:



Increasing the volts per division:



TonyG

[Tony_G]

I took a look at A7 Pin 29 - Well by look I meant that I don't have the extender board so I clipped a mini-grabber onto the anode of CR15 and didn't see anything (steady +5V) - If that is 5V then I should see 5V across the coil of K1 and that is what I actually do see.

I think the pulsing is coming from earlier in the chain.

TonyG

[Tony_G]

I found another thread that seems to mention a similar problem:

https://www.eevblog.com/forum/repair/fluke-5200a-ac-voltage-calibrator-teardown-and-repair/msg383454/#msg383454

In the repair notes the author mentions power issues (a lot of AC on the rail). My rails are clean but with a little more digging I can see the voltages on the +/- 30V ones are actually -35v and +25V.

That seems out of tolerance so I'll go see if I can get them back to near 30V.

TonyG

[Tony_G]

I removed all the components that are in the +/- 30V rail section:



Everything with the exception of R16 & R20 was within spec (R16 & R20 were 23 & 25 ohms). The only other thing was that that Q9 was right at the low end of the Hfe range for a D44C5 but it was still in spec.

Given that the rails were basically shifted down 5V it may be a possibility that the circuit really is sensitive to the values of R16 & R20 but it could also be that it is sensitive to the values of R18 & R22 which look like they form a voltage divider for the 30V rails.

Anyway, I don't happen to have 20 ohm resistors here but I do have some 18 and 22 ohm ones so I'll see if I can hand pick something close to 20 and see what comes out.

TonyG

[Tony_G]

I found a pair of 21.99 ohm resistors and used those to replace the ones in R16 & R20. I don't know if they're the issue or if it was simply a dry joint somewhere but now I'm seeing these values on the rails:

Rail values
+30V = +30.256V
-30V = -30.018V
+15V = +15.004V
+5V = +5.002V

All within spec I believe - Now back to checking the oscillator assembly.

[Tony_G]

Just to circle back on this. The oscillator board was fine and I managed to get some working boards for test (Oscillator, Oscillator Control, AC DC Converter) and an extender board.

Discovered it was actually the AC DC converter board and on close inspection I had a "Doh!" moment:



You can see the faint writing of BAD right in the middle of the board - I must have missed that when I was looking at all the boards for blown/broken issues. Anyway, I replaced the board with the working AC DC one and now I have a nice working 5200A (returned the other boards to their owner and he's going to look into fixing the AC DC board as he has a second non-working 5200A - Didn't want to take the chance that while I was diagnosing the fault it would causes something else to fail from the overload pulsing).

Will need to bring it into cal but I'm waiting for my 3458A & 5440B/AF to get back from manufacturer cal (it was expensive but I decided to invest in getting the 3458A under the extended warranty - Good value at $160 a year - and having known starting points for the 5440B/AF).

Anyway, thanks everyone for the comments and help.

Appreciated it.

TonyG