8902A 300 MHz Problem

[tpratt]

I have a HP 8902A that has a intermittent problem that I have been unable to find.  above 300MHz it will read the input frequency just fine.  But below 300 MHz it will not read the frequency unless you enter it manualy.  I have alreadycleaned all the edge connectors on the boards the cables have all been cleaned and checked and the adjustments have all been done.  Any ideas

[Broken_Wings]

Also you may just try going straight into A16 at a level of -27 dBm. Your input or one of the hardlines could have a SWR issue. If you have access to Air Force T.O. 33k4-4-283-1 run section 4.2 . Keep the level </= -26 dBm even above 650 MHz.

There is an adjustment you must check out. It is an offset check. Look through the adjustments the value is required to be positive. a negative number can and sometimes will result in non locking. The frequency will flash but will not hold. The step uses an internal voltmeter and the adjustment is in the RF section (looking at top it is in the rear right). Try this. If this doesn't work loosing and tighten all the inside cables. Next proceed to bang on the aluminum standoffs one at a time with the back end of a screwdriver (non conductive part), while looking for the problem to go away.


If you have block diagrams BD2 is my favorite.

edit: my also try is actually my first step. re arranged.
2nd edit afterthought: If you have an old unit with the RF switches AWP it for those. They are now obsolete. NRTS it. I doubt they are the problem now but they will cause problems < 50 MHz later if you do have them.
They were replaced with a higher reliability component set.

[Mr M.]

Previous posting information is good but the number one problem we have seen with intermittent 8902A's are the A26 supply capacitors.  The unregulated supplies may have large amounts of ripple that can vary from time to time thus the TI may work or fail for no apparent reason.  It also seems to effect the lower frequencies more than the upper ones when the ripple is just a bit high.
Easy check, remove the bottom cover check A26 C3(+5V unregulated), C4(-5V unregulated), C6 (+15V unregulated) and C7(-15V unregulated).  DC volts should be well above the regulated supply value and AC volts should be reasonably low.  Since there are + and – supplies you can compare the numbers, they will be reasonably close. 
Also if you have an option 030 (Internal/External LO) the S4 switch may be the issue, although a bad S4 usually will make the TI fail totally it is an easy check.  Exercise special function 23. 1 for external and 23. 0 for internal.  See if things clear up.

[skidaddle skaduski]

I've had the same problem with 8902's not tuning correctly.  They would tune for a while if you manually tuned them but everyonce in a while they would loose their lock when changing IF or RF ranges in TRFL.  We've always sent them out for repair and the repair facility fixed them or said they weren't defective. 

I haven't repaired 8902s but I have aligned them in the past.  I spoke with some friends in the career field and they've said similiar things were happening to thier recievers.  After doing a bit of research, I found that Agilent is aware of this problem and their is a service not for 8902s within a specific serial number range.  I'm too busy to research it now but do a google search for something along the lines of, " HP 8902A not tuning".  I know that is rudimentary but that is how I found it.  I hope it works out for you.