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Monday, September 21, 2020

HP8640B Internal Frequency Counter Fixed (More Repairs Pending)


The HP8640B is a complicated machine.  Above you see just one sub-assembly, and the page from the manual that describes it.  This is what I've been working on.  The little spring "tine" fell out of one of those discs behind the two control knobs.  So I had to open this thing up, find the spot from which the tine had fallen, and glue it back in.   

I used Gorilla Super Glue, followed 24 hours later by a dab of JB Weld "minute weld" dual epoxy. One of the other tines was about to fall out, so I went ahead and gave all the tines in this assembly the glue treatment.  ( I bought some "Weld On" acrylic cement but the warnings on the label were quite sobering.  So I left that can sealed up.) 

This morning I put the thing back together.  This is not easy.  At one point a spring popped and a tiny metal part that is probably irreplaceable seemed to fly away into the black hole that is the shack's carpet.   I had just about given up hope when I found the thing sitting right in front of me on the bench.  TRGHS. 

The HP8640B fired up right away without trouble and the internal frequency counter is working fine. 

As I noted in the last SolderSmoke podcast, a very nice community devoted to the HP8640B has developed around the world.  Here are some of the notable participants: 

Bill at Electronics Revisited is a very nice fellow with lots of experience on the HP8640B.  He offered to sell me a replacement unit for the assembly pictured above.  If you have an ailing HP8640B and are looking for someone to work on it for you, Bill is the guy you should talk to:  http://www.electronicsrevisited.com/  He also very kindly offers to answer any questions you may have about the HP8640B. 

Here is the e-bay page of the fellow in Bangalore who makes the brass gears.  Mine are on the way! 

Marcus VE7CA has a great site devoted to the HP8640B: https://www.ve7ca.net/TstH86.htm

BH1RBG in China has a nice site describing his adventures with the HP8640B: https://sites.google.com/site/linuxdigitallab/home/hp8640b-20v-power-supply-down

K6JCA has a good blog post about fixing the tines and the gears: 

Steve Silverman (who gave me this HP8640B) found a really useful  history of the device: 

And of course special thanks to Dave VE3EAC who alerted me to the falling tine problem and put me on the path to a successful repair.  

The gears should be here in a few weeks, so that will be another opportunity to work on this HP8640B.  Also there are some tines in the attenuator assemby that might reinforce with the glue treatment. 

1 comment:

  1. Many of the chemicals we use in electronics have sobering Safety Data Sheets. The good news is we are usually careful and do not have the vapors causing our workshops and homes to be rapidly disassembled into a large pile of rubble or causing bodily harm.


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