Need Help Tektronix TDS5000B Series Active Probe x10 Offset

[thorswait]

Hi I've been experiencing a strange behaviour on my TDS5054B on my first input. When I use hook this input with a passive probe the signal is clean and level's working properly. But when I hook up an active probe like a fet the reference signal is boost from 1V to 10V. When I use the same probe on the 3 other channel is behaving properly.

At first when I hook up the active probe on the first channel the relay weren't always cliking so I thought it might be something with the RF relay... I've replace them on the ceramic substrate but the same behaviour as before replacement. Any suggestions are welcome.

Thanks

[ArcticGeek]

It sounds like you might have a problem with the 50-ohm input termination on the problematic channel.  When you attach an active probe the input will automatically change to 50-ohm input termination on the scope.  So I'm wondering if that isn't the problem.

If you manually switch that channel to 50-ohm input does it measure 50 ohms with a DMM?  That would be a quick and easy check to see if the 50 ohm termination is working.  Let us know what you find.

[thorswait]

Thanks for the reply, I did check with a multimeter the 50 ohm input the relays are "clicking" and there is 50 ohm when 50 is selected an 1M when I select the 1M input. Maybe I'm lucky here. But I found out something strange when I hook up an active probe usually I'm not able to modify the impedance but on the non working input I can modify it ?

Maybe there is something wrong with the data reading on this channel (the probe is working properly on other channel). I'll keep investigating.

[ArcticGeek]

Yea, it sounds like there might be a problem with the clk or data on that input channel.  It seems like a pretty big clue that you are able to manually change the 1M/50 ohm selection after you plug in that probe.

Since you are able to change it manually, does the probe work okay if you manually select 50-ohm when you attach your active probe to that channel?

[thorswait]

When I hook a working P6205 on this channel and take a look at the reference 1V 1KHz it display 10V instead of 1V on this channel at 50 ohm. Slighty higher if I change the impedance (because I can on this channel ...)

If I do the same with other channel with the same probe it display the 1V 1KHz ref signal. But without being able to modify the impedance I'll try to track the data and clock using a another scope.

[thorswait]

After the kids were in bed, I took a final cup of tea and I've found it !!!

At first I probe between the ground and the data input on the 3 working channels and there was a famous 0.605V and 0.56V when I revert negative and positive probe on every single port except the #1 port. That lead in the search of the protective dual diode. I finally track the signal to the diode I unsolder it test it but it was fine ... But when I compare all other circuit I had around 12ohm at the dual diode for each port except the defective one.

Then I probe the input resistance leading to the suspicious dual diode and there is a resistor that is label 10ohm but it was around 2.5K in circuit the resistor looks fine no sign of damage or anything it was clean ... I unsolder it anyway in order to test it out of circuit et voilà it really was 2.5K ... which explains why on this particular channel the test diode wasn't successful.

Everything is working fine now the command are properly read by the ATMEGA8535 on the board that control all the RF relay on the 4 input. Now I can read 1V reference signal using first channel as well as the others ! I might be able to close the unit back once and for all ! But for now it's time for a Scotch Whisky ! 

I hope it would help someone ! I'll post picture when I have more time

Thanks ArcticGeek for the support !

[ArcticGeek]

Glad you found the problem - nice work!

So was the defective 10 ohm resistor on the serial data line, is that correct?  I'm just curious is all - thanks.

[thorswait]

Exactly the 10 ohm went defect