Author Topic: 1976 vintage AM503 w/ P6303 probe restoration and probe calibration question  (Read 1150 times)

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Offline MarkoAnte

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so I bought a 1976 vintage AM503 w/ P6303 probe. I have done similar stuff to my amp as is described here --->
It had some extra bad caps and a heat sink was missing form 1 transistor. But after that and a whole calibration and verification procedure form the AM503 manual the amp it self runs great now. The probe I'm not so sure.

It seems that the core keeps getting a DC offset. Eg. I mesure 2 amps and it shows 2 amps. Then I turn 2 amps to 0.2 amps and it shows 0.3 - 0.4 A then I turn the current off and see that I have 0.2 amps DC offset. I then degause the probe and level it out again and its ok again.  Sometimes it does this. Sometimes its ok. Sometimes after adjusting the probe (it has 4 pots - dc levelling offset and 3 frequency response & gain adjustments). It looks like even if I adjust it to where it is perfect it sometimes just goes off especially on higher sensitivity.

Is this normal, that this probes are a bit touchy feelly. Like even handling it gives it a dc offset. I know that it can pick up the earths magnetic feald and that the presune on the core and its cover (different air gap)  is critical to the reading. But is this like this for all the probes? Even new ones (the multiple K euro solution). Or did my probe just see better days? I would like to have another one maybe the newer ones (only 20 years old ;D), like the  A6303 to test them if they are the same.
If anybody has any experience with restoring these probes please give me some tips.

Best regards,

Offline radhaz

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The Keysight probe I have at work recommends demagnetizing before each measurement.

Demagnetizing Switch (DEMAG)

This demagnetizes the core if it has been magnetized by switching the power on and off, or
by an excessive input. Always carry out demagnetizing before measurement.
The demagnetizing process takes about one second. During demagnetizing, a demagnetizing
waveform is output. Refer to “Demagnetization and Zero Offset" on page 17
« Last Edit: September 05, 2016, 06:39:26 pm by radhaz »

Offline MarkoAnte

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I do that, on this old one you have to press demag and then manual adjust a pot to center the waveform. But its still a bit iffy.

Offline MF-jockey

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There You can find the calibration instructions of A6303:

Best Regards

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