My HP8640 died!

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 My HP8640 died!




This is me catching up on past projects. I did this back in 2022, figured someone would benefit from this information, since the 8640 is still around in great numbers at hams all over the globe.




I bought a nice looking HP8640 at our local hamfest back in 2022.
It was working when I got it home, but a day or two after I got it home, it started acting erratic, with RF ouput dropping out. eventually, there was no output whatsoever.
Did some tests, and got to the conclusion that the cavity oscillator was not oscillating.

Following various clues online, I tracked down the issue to the A3 unit (cavity), and a series resistor, which had drifted in value.
By measuring the series resistance between the + feed and the A3Q1 socket and the - feed and the A3Q1 socket, I could see that the + lead seemed reasonable at 370ohms, whilst the - lead had drifted to 19Kohms, where I was expecting around 900ohms.
The A3Q1 (HP21 pictured below) transistor sits under the small hex cap om the cavity, and can easily be lifted out. Take care no to damage the socket when measuring.
The picture below is from a 1973 HP paper, which shows the idea behind the cavity and oscillating transistor, but I don´t think it´s an actual representation of the reality, schematic-wise, but you get the a




Removal of the cavity was neccessary due to the fact that I had to split the back portion open.
Removal was straight forward following the manual. Take care around the plastic gears, and the small switches.

With the cavity out, the A3 screws were removed, and the lid was carefully opened, and the culprit resistor could be located. I chose to clip it as close to the body as possible, leaving as much lead length I could, to make it easier to fit a new resistor.
Of course, my resistor bins did not contain any 910 ohms resistors, so a made an ugly one, by soldering a 100 and 820ohm 1206 smd resistors in series. Close enought for me. A 1K resistor would probably be just as fine. I guess drifing to 19K does not happen over night?
Installation was succesful, resinstalled the cavity, and the 8640B came back to life again.


Here we see the cavity, the shiny metal tube. A3Q1 is beneath the hex cap, just in front of the clamp.


Cavity removed!



Here, the A3 module is opened and we can get a glimpse of what´s inside.


Here´s the problem. That 910ohm resistor has drifted towards 19Kohms!




Homebrewed resistor in place. Now just to reverse the process.



Success!! 




Good article, if you are an ARRL member is QEX sep/oct 2005, where WB4LNM describes step-by-step, how to disassemble the cavity, and do a similar repair like this.
Good luck with your HP 8640B

//Tommy, SA2CLC

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