Electronics > Repair
HP8595E YIG Hot-To-Trot 5086-7903
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wa4whl:
Hi Fellow HP analyzer owners.  I have an interesting problem that has me stumped and maybe someone has seen the same problem and come up with an answer.  I am the proud owner of an old HP8595E SA that has worked fine for many years but recently it has been giving me the old refusing to start due to low/no YIG out signal.  Opened up the oscillator to have a look and could see no obvious loose gold wire bonds, burned or cracked tracks and the top circuit board tests fine.  Finally gave up hoping that it wasn't the notorious bad rf transistor and put it into storage (the attic carefully wrapped) and ready for wherever SAs with dead YIGs go.  Hoping maybe some genius will design a cheap YIG replacement (hope springs eternal ;-) ) 

Got bored on a hot rainy afternoon and decided to see if I might decide to part-it-out on ebay given the rest of the unit (ex-YIG) had worked when signal present. Before digging into the unit I thought I'd plug it in and see if power supply still good.  Low and behold it the unit started right up like nothing ever was wrong and continued to operate normally for the rest of the day.  I powered it off at the end of day normally and assumed the unit had somehow undergone a miraculous " self repair ".  Following day I started it up and yes same old death following the initial self checks.  I put back in the hot attic and left it for a while, brought it back to the lab and yes it started right up.  No problem.  So a little testing over next couple hours and a good temperature sensor and the unit has a reproducible pattern:

Unit can't get past initial boot tests as long as unit temperature is 81 degrees F or less.
Unit boots right up if I put the unit into a room at 83 degrees or higher (after equilibrium) and continued to function normally even at "normal" room temperature until I shut down.  Won't come on even if I immediately restart it following a day of operation during which the fan keeps the unit operating normally at a cool 73 degrees.

Before I open up the YIG oscillator and start poking around on what appears to be an otherwise clean unit I thought I'd see if anyone has run across this temperature dependence and can suggest a way to focus on the potential source of the problem?  That unit is so d#@* sensitive and  small to work on.  The yig sphere?  The obsolete transistor?  The silver bonds? 

Any ideas would be appreciated.  WA4WHL
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