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Help with HP 54502a

Question asked by w3kq on Mar 9, 2009
Latest reply on Feb 5, 2010 by tomiwerner
I recently purchased an HP 54502a scope.  I'm really stumped on this one.  Applying a low frequency square wave and selecting AC coupling gives a waveform that looks like it went through a differentiator.  The scope has two input modules - both do the same thing.  I have a "spare" input module - condition unknown- that also does the same thing.  All three input modules exhibit the exact same behavior on Channel 1 or 2 - swapping/exchanging does not "move" the problem - it stays.  I have the schematic on a 54504a, so I've probed around on the 24 pin IDC connector the attenuator plugs into. What I find (doing some circuit tracing and part number cross referencing) is confusing.  I "thought" the input attenuator module output a low freq signal on pin 23 to what appears to be a high speed DG411 analog switch - but that path only seems active in the DC mode - which works fine.  The FET gain feedback signal coming out of the LM317 regulator to IDC pin 5 has the same waveform as displayed on the scope, as does IDC pin 4, which I believe is the error signal  coming out of the module.  Now here's where I'm really confused.  The "service manual" for the 54502 says the low freq amp is external to the attenuator, but from all indications, that's a DC restore path which appears to be working.  I'm starting to think that all 3 attenuator modules I have are bad, but before I go out and buy a replacement I'd like to see if someone else has come across this problem and knows the solution. I've Googled every conceivable combination of words trying to find some feedback/comments on something like this, but I've not found a thing.  This whole loop started because the scope would not hold probe compensation when switching between the AC and DC coupling mode. After a little reading in the service manual, and some input checking, it became clear this problem is worse at low frequencies. It's really depressing to think that all three of my input modules are shot, but that would be par for my luck!  Thanks in advance for any thoughts on this one.  A schematic of the vertical front end of this scope would be really nice - the 54504a front end is quite different than the 54502a, and circuit tracing a multilayer PCB is not much fun!!!

Rick Jordan, W3KQ