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HP 8753 SAMPLER-MIXER PROBLEM

Question asked by broadcastRF on Feb 13, 2010
Latest reply on Jul 18, 2010 by Dr_joel
One the mixer boards in my 8753B has gone bad.  I have traced the problem down to the sample head.  When I input a signal to the sample head, it stops at the next component after the input coupling capacitor.  I am guessing that that component is the buffer amplifier, either a transistor or IC amplifier.  But at the same time, I am not sure because I am wondering where the "50 ohm load" is in this thing.  Dr. joel has given a little info on this circuit in his forum postings, mentioning a 44 ohm resistor connected with the input buffer, so I am guessing that circuit must constitute the 50 ohm load. Looking at the component with a magnifying glass, I can see where it looks like it was burned by an overload, although I am always extremely careful to avoid overloads and ESD.  I will try to confirm that the sample head sampler circuit and IF amp are OK, and I have already confirmed that the main sampler PC board is OK.

     I am hoping that someone can give me some background on any experiences that they have had with this circuit.  I intend to find a work-around repair for that first buffer amp, if that is what it is.  There are a number of small IC amplifiers out there that I may be able to fit into that buffer position. I have got to have this 8753 available when I need it, sometimes for emergencies, and I need to find some solution.  I might buy a parts unit, but I am afraid that the sample boards in it may be bad as well.  I would also like to buy a new or even defective sample board.  

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