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Thursday, September 10, 2020

The Agony of Troubleshooting -- From China


I am troubleshooting my beloved NYC HP8640B Signal Generator (thanks to Steve Silverman and Dave Bamford).  Some of you may wonder why I don't just replace this beast with something small, lighter, cheaper and newer.  Well, I have not found any new sig generators that will do what this beast does:   It goes all the way up to 256 MHz (higher with an extension kit).  It has a great attenuator in it so you can set the output just where you want it.  AM or FM modulation.  Really useful.  So I think I'll fix it. 

My problem is that the internal freq counter stops working above 16 MHz. Using the very extensive  documentation, and without even really opening up the machine, I think I have located the fault.  I think it is in the Counter Time Base Assembly board A8A3.   Now of course, the fun begins.  (Tips, advice, solutions, and words of encouragement would all be appreciated.) 

While planning my assault on A8A3, I came across the web site of BH1RBG.  He too has recently been working on an HP8640B.  His problem was different, but when I read through his site I saw evidence that the agony of troubleshooting is something that is the same all around the world.  Check out his description of the agony: 

I even suspected the LM723 should had something bootstrapping circuit, make sure the Q4 sure start. Because the external reference VR3 is floating too, oh, my godness.

I became hopeless, and ordered several LM732,and waiting delivery for days. This beast frustrate me so deeply, changed the LM723 does not help anything. And i almost desoldering everything in the board!

Oh man, I've been there. Several times while in the throes of a troubleshooting battle I have actually had dreams of removing all the parts from a troublesome PC board.

BH1RBG has a very interesting site with lots of ham radio projects: 



  1. Reading his posts about crystal sets, I think he may have solved a problem about antenna connectivity that stumped me in 1960. Will conduct some experiments and report back.

    1. That's great Keith -- Please let us know. 73 Bill

  2. Wow, I am trying to fix my HP8640 as well! The output level drops off to nearly zero on the lower bands. I think it has something to do with AGC in final amplifier, but I can't find any documentation that explains the AGC. I didn't find the manual very helpfuf.

    1. That is really kind of spooky Grayson. TRGHS. It is kind of scary in there, but not as scary as the Tek 465. My problem appears to be related to the time delay circuitry on the counter. I am HOPING that all I will have to do is re-seat one of the boards (perhaps with giving it a shot of De-Oxit). 73 Bill

  3. I share your discomfort with troubleshooting. I recently had a modern 0.1 uF bypass capacitor purchased from Mouser on an audio amplifier board that intermittently partially-shorted internally which took me weeks to identify. Very glad to finally find that stinker!

  4. I am just not crazy about troubleshooting test gear. But I will make an exception for the HP8640B. When I do troubleshoot, I always hope that the problem will be elusive and hard to find, like Walter's intermittent cap. I don't like it when, after days or weeks of investigating, I find nothing more than a loose connection or a broken wire.


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