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8569B: Random spurious noise

Question asked by robj on Feb 21, 2016
Latest reply on Feb 22, 2016 by robj
Hello Forum,

I recently purchased a 8569B. Fixed all the knobs/metal contact wipers as usual with these older units. Cleaned all switches, connectors, contacts, etc. The unit is well calibrated and has been working great for about the past week.

Today I was monitoring some signals in the 900MHz ISM band when I started to notice random spikes that were not coming from the input. I disconnected the antenna and the noise was still there. I changed the Span/Div to 100MHz and notice that this noise was all up-and-down the band. 

I've adjusted every control through its full range. Full attenuation, no attenuation. Full Band sweep, narrow sweep. RBW. Ref Level, Video Filter, bands. Everything. The noise is persistent.  -  Usually, using Max Hold, the trace will be fine for about a minute (no noise), and suddenly the noise will quickly start building up ... sometimes in as little as one sweep. It reminds me of someone throwing a bucket of paint at a canvas, as the noise just "splatters" across the screen, as shown in the attached image. I'm getting the noise in the other bands as well.

I'm wondering if this is more in the DSP section or the RF section. - When I set both 'trace section' buttons to 'Store Blank,' and set the sweep to around 1mS/Div, I can still see the noise on the trace. I have the unit open now and my service manual open. But I must admit I'm stumped at the moment. Any ideas where to begin?