Need some Circuit analysis/troubleshooting help

Jim Flanagan · 2011-02-05T19:39:03+00:00

Hi... I'm struggling with a repair issue and could use somehelp. I've placed some schematics/pics at this link:picture Link: The problem that I am having trouble with isa bipolar amp/bandpass filter used on a logamp board from an old HP network analyzer. Thisanalyzer uses two boards, which are identical, forthe reference channel and the A,B channels (muxed).One board functions without issue, the other hasa random noise problem.If you look at the schematic in the link, there is a testpoint, TP 2, which represents the location of the noisebusts shown on the schematic. These noise bursts arerandom in nature and I have verified that they are notdur to mechanical vibration. The problematic circuit card isbeing test/analyzed on the bench using a bench +/-15V supply.The schematic shows the bias point at this TP2 to be -.5V andthe 'good' board has been verified to be true.My analysis shows that this bias point should be about -0.9Vdc,not -0.5Vdc. On initial application of the power supplies,the bias point starts off at about -.8V on the good board andthen over several minutes settles to -.5V and all is happy.On the bad board the voltage starts off about -.7V but settlesto around -.2V or so. The bad board also starts exhibitingthese noise bursts which are causing me problems on the lowend of the dynamic range of the analyzer.Initially, I thought that I was having a leaky npn and or pnpin this circuit. The schematic also indicates in red areaswhere I have removed various parts in an effort to completelyisolate this area from the remaining circuitry. I've also'turned off' many of the other stages (lifting bias resistors, etc.)in an effort to make sure that this stage is the one really at fault.I also shorted out the 100KHz inductor (250uH) to make sure that itwasn't receiving noise bursts from some external means.The normal troubleshooting has been done:All electrolytics have been tested for ESR/CapResistance measurements have been verified.Extra power supply decoupling has...

Feb 05, 2011 22 Replies

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Jim Thompson 15 years ago

Replace all pieces of that section :-)

But the design IS inherently sensitive to power supply noise. ...Jim Thompson

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Fred McKenzie 15 years ago

Jim-

Another possibility is a component lead that carries the circuit from one side of the board to the other, but one side was not soldered.

I think Jamie's freeze-spray suggestion is a good idea for both components and interconnections.

Fred

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TheGlimmerMan 15 years ago

Continuity testing, including for broken vias, etc. is a BASIC diagnostic skill.

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