Author Topic: Tek AM503B / A6302 repair?  (Read 7042 times)

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Offline larry42

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Tek AM503B / A6302 repair?
« on: December 23, 2014, 09:56:57 pm »
Bought a current probe system, supposed to be in good condition.
However, in max. sensitivity setting it's noisy - the probe is emitting noise (measured with a spectrum analyzer and H-field probe). Noise gets worse when probe jaws are opened.

I'm in contact with the seller, but if it's a simple fix I'd rather fix than have the hassle of sending it back (I'm out some money in any case due to import costs).

The screenshots below are taken with following settings

AM 503B settings:
Gain: 1mA/div
Coupling: DC
BW Limit: OFF
Degauss + probe closed: OK


Oscilloscope (150MHz):
Vertical Gain: 10mV/div
Probe Setup: x1
Coupling: DC
Termination: 50Ohm
Timebase: 100us/div

Markers show the 100kHz noise pulsed at ~5kHz

The noise drops step-wise as the AM 503B gain is decreased, and the there is no noise when the AM 503B coupling is set to: REF.
Therefore the noise is coming at the  AM 503B input stages / probe.

At the moment I can't open the box until I get an OK from teh seller - in addition the full Service manual for the 503B was never released - at least I can't find any schematics for it.

The Vrms noise is outside of Tek specs (spec: 4mVrms - measured about 6mVrms) - plus the noise hump/spikes just look wrong - I cannot imagine that Tek meant to probe to perform like that.

Any ideas what can cause this (bad caps in the amp unit, damaged probe...?)
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Offline edavid

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Re: Tek AM503B / A6302 repair?
« Reply #1 on: December 23, 2014, 10:28:04 pm »
It's hard to see how the probe could be generating that noise.  Are you sure it's not something in your environment?  CFL or LED lamp?  Try taking the whole setup to a different room.
 

Offline larry42

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Re: Tek AM503B / A6302 repair?
« Reply #2 on: December 23, 2014, 10:32:03 pm »
Tested in two different rooms (about 50m apart basement versus living room in apartment block). Same noise frequency and shape.
Moreover the noise is definitely being emitted by the probe - gets much larger as I move my H-field probe near to it, and is orientation sensitive.
Of course it could be noise on the supply to the probe - but probing the 503B circular connector, I don't find anything that would indicate this.

The system itself appears to be working - both current measurements at DC and at 1MHz appear correct in the 1mA/div setting, just the interference noise is the problem.



« Last Edit: December 23, 2014, 11:39:47 pm by larry42 »
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Online georges80

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Re: Tek AM503B / A6302 repair?
« Reply #3 on: December 24, 2014, 11:43:10 pm »
I have an AM503B/A6302 and see similar noise with the scope at 10mV/div and 1mA/div on the AM503B.

I'm not sure what you are expecting in terms of performance. You're basically looking at levels down in the 1mA range, I have no expectations of a quiet/clean signal at the lowest end of the scale.

I highly doubt you have anything wrong with your equipment and are just expecting more than what it can deliver.

If you are after high resolution you are better off running 10 turns through the jaw and go to the 10mA/div scale on the AM503B.

Returning it to the seller would be a waste of time imho since if you buy another unit it will behave the same way...

cheers,
george.


 

Offline larry42

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Re: Tek AM503B / A6302 repair?
« Reply #4 on: December 24, 2014, 11:49:26 pm »
Thats exactly the kind of data point I need.

All I have had to go on until now has been the manual. The noise test in the old (but identical hardware) P6302A manual doesn't show any impulse noise, and the specs in the 503B manual give <4mVrms typical (I get about 5-6mVrms with the spikes included).

My homemade current probe 30kHz-30MHz, without amp has about the same sensitivity, but no noise (and no DC - that's the hard part ;))

I think it's strange that there is this noise at 100kHz, esp as it is so clearly visible, but perhaps it is functioning as designed. Can anyone else chime in to corroborate? But in thanks George80 for checking and replying.
« Last Edit: December 24, 2014, 11:54:59 pm by larry42 »
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Offline PA4TIM

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Re: Tek AM503B / A6302 repair?
« Reply #5 on: December 25, 2014, 11:21:56 am »
http://www.pa4tim.nl/?p=3135 I have the same probe (ex AM503) but it is connected to a P6042 that was missing several parts including the probe.
To connect them together I had the modify both. One of the mods was removing the Zener diodes. My signal is clean. Also with nothing connected.

This picture is a 40 MHz Tr=500ps squarewave connected to the scopes own 50 Ohm input. The P6042 is at 1mA/div and the scope at 50 mV/div like it should.
The scope is a 350 MHz Hameg
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Offline larry42

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Re: Tek AM503B / A6302 repair?
« Reply #6 on: December 25, 2014, 12:06:23 pm »
Hei PA4TIM

Hmm- the 40MHz signal would not show the 100kHz pulses that I'm experiencing.

My current theory (if there is somerthing wrong with my setup) is that there is noise on the PSU lines, caused by the digital section in my AM503B and possibly a dryish tantalum cap. I don't know the age of my 503B (anyone have a link to tektronix SN decoding?). Of course this is just speculation, but I did see some 500Hz modulated 16MHz being radiated at a low low level from the 503B front panel, when I checked with my H-field probe - a similar spacing to the noise peaks near 100kHz.
As to what is creating the 100kHz - well the manual does mention that the micro controller controls the front panel at a multiplexed 200kHz rate - again, perhaps some cross-contamination to the probe amplifier sections there?
I know all this is speculation, but I really can't imagine that tektronix have left so much noise in the system at 100kHz, which much above the 20kHz Hall crossover (Manual Theory of operation, sec. 3-6)

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Offline edavid

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Re: Tek AM503B / A6302 repair?
« Reply #7 on: December 25, 2014, 05:14:27 pm »
What kind of mainframe do you have the AM503B in?
 

Offline larry42

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Re: Tek AM503B / A6302 repair?
« Reply #8 on: December 25, 2014, 05:18:56 pm »
Mainframe TM502A (two slot, 2nd slot empty)
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Offline guido

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Re: Tek AM503B / A6302 repair?
« Reply #9 on: December 26, 2014, 01:47:10 am »
On my Tek 2430A i don't have the spikes. The noise stays mostly within one division (peak-peak 10-12mV). RMS some 2-3mV. AM503B with A6302 in a TM502A with a FG503 in the other slot, same settings.

Measurements don't show up in the plot for some reason (could be a setting on the scope, don't know).
The noise increases over time, untill you hit degauss.

There is no peak at 100kHz on my SA (7L5). Noise is at ~ -75dBm, but it hasn't been calibrated for a long time.

Scope:





« Last Edit: December 26, 2014, 02:16:58 am by guido »
 

Offline larry42

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Re: Tek AM503B / A6302 repair?
« Reply #10 on: December 26, 2014, 07:10:04 am »
Thanks Guido - that screenshot is much more like what I expect. Does anyone have a service manual with schematics for the 503B?
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Offline PA4TIM

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Re: Tek AM503B / A6302 repair?
« Reply #11 on: December 26, 2014, 11:51:46 am »
Have you tried with the unused slot "shielded" Tek used to sell a sort of frontplates for this purpose. I would try thaty first to be sure.

Mine is clean from DC to the max bandwidth, but my probe is modified and I use a P6042
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Offline larry42

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Re: Tek AM503B / A6302 repair?
« Reply #12 on: December 26, 2014, 11:54:39 am »
Yep, I have the spare "box" to put into it, it makes no difference, nor would I expect it to - interesting to hear that your unit also appears quite quiet - there must be some fault on mine (and georges80 in that case) :/
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Online georges80

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Re: Tek AM503B / A6302 repair?
« Reply #13 on: December 26, 2014, 05:26:39 pm »
I don't have 100KHz correlated noise, but there is 'random' noise when 10mV/div on the scope and 1mA/div on the am503b. The noise disappears if I set coupling to REF on the am503b.

1st capture is the random noise at normal acquisition mode on my agilent msox2024

2nd capture is in 'high resolution' mode.

3rd capture is with acquisition averaging set to 8.

The dc shift is due to not degausing for several minutes AND also depends on the orientation of the probe head (stray magnetic fields are also an issue - including the earth's at this resolution).

cheers,
george.

 

Online georges80

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Re: Tek AM503B / A6302 repair?
« Reply #14 on: December 26, 2014, 05:34:49 pm »
Here's a decent manual (attached) of the a6302 probe and am503b specific info inside, in case you don't have this already.

cheers,
george.
 


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