[Premium-Rx] HP 8649B?

[John Perlick]

Hi Hank

I've had several 8640's and you are right to choose one.  It is still an extremely low noise and low distortion generator (especially on HF)  because it uses a great cavity oscillator.  Very high Q = very low noise (incl. phase noise).

The 1024 MHz units have more than a counter option!  They have an internal doubler, with filters, etc, to make the unit go from 512-1024 MHz.  Don't buy one without the doubler installed originally, but then only if you really need the extended range.  There is not much up there, so if you can skip it, you will find a lot more units for sale.  They are on ebay a lot lately because the Mil has been dumping them.  I've sold several...you can usually get a decent one for around $300, which is a great price considering the performance of the oscillator.

Problems: they can drift, so you might want to keep one hand on the knob if you need super stable long term performance.  But the close-in phase noise is exceptional.  The worst problem is a clunker...I've had clunkers iwth just a bad fuse (wow--lucky me!!) but I would never recommend buying them without known output because if the RF output module is bad, it is not repairable and not replaceable.  The story goes like this:  Tektronix (oddly) built the hybrid RF amp output module for HP. There were still a few hundred spares in Tek stock but HP felt that the used market was hurting new generator sales, so they bought all the Tek stock and destroyed it!!!!  So, if the unit has a bad output module, it is probably very very hard to find a replacement.  And, y ou might n ot find the parts to repair it either....so, make sure you get one with good (full) output.  Have them send you a pic with the gen driving a scope to full output (and no distortion).

Another common problem is broken gears.  THe gears on the tuning mechanism are made of nylon and age seems to have caught up with them (or abuse).  If you have a broken plastic gear, it can be very difficult to find a replacement.  Don't even bother calling HP, of course!  But there are junkers out there and you sometimes can salvage a gear (like at the Dayton hamfest).

Other problems are fairly easy to repair.  A little while back I gave the HP 8640B manual to the Prem RX website to post for all the guys here.  It is still a great signal generator and the perfect gen for making low noise receiver tests (better yet, have two of them!).  If you wanted to buy something comparable new, you would be spending $10-20K today!

Good luck with your shopping!  
  ----- Original Message ----- 
  From: Henry Kolesnik 
  To: premium-rx at ml.skirrow.org 
  Sent: Wednesday, August 20, 2003 9:26 AM
  Subject: [Premium-Rx] HP 8649B?


  I've been looking for decent HP 8640B and I need a little help on the options.  From what I understand, Option 2 allows the unit to go to 1024MHz.  Is this an internal feature or external?  The reason I ask is one seller told me that all units go to 1024 MHz but the counter needs the external doubler to get a correct indication.  This sounds bogus to me but since I've never seen a unit with Opt 2, I don't know.  
  In some previous posts I recall someone mentioning that the 8640B has RF leaks that can be bothersome.  How hard is it to  minimize the leaks and once done the unit OK?.  Any advice appreciated.
  tnx
  hank wd5jfr


------------------------------------------------------------------------------


  _______________________________________________

  Premium-Rx Mailing List
  To Post: premium-rx at ml.skirrow.org
  For Info: http://ml.islandnet.com/mailman/listinfo/premium-rx
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mailman.qth.net/pipermail/premium-rx/attachments/20030820/398c990f/attachment.htm