[SOLVED] HP 8640B, fixing up an awesome boat anchor - Page 1
Forum section: electronics / repair

author: Cyclotron
author_rank: Unknown
timestamp: June 22, 2025, 02:24:48 am
content: When folks say, "They don't make them like they used to." I will imagine a photograph of the HP 8640B from now on.I picked up this example on the bay for cheap, as it was not functional. It turns out that the 5V power card had blown a fuse. There appears to be a problematic 2N3055. I solved that problem, and the power rails are all in spec now.I found a set of frozen bearings on the range selector housing and could clean and grease those into like-new operation. The problematic cracked gear problem was solved by ordering some fantastic brass gears on ebay. The set was very reasonable given the quality. I'm delighted with that repair.I have started going through the output validation and found something odd. On the 8-16Mhz setting, the frequency out of the BNC port on the back (connected to the HP universal counter) suddenly jumps from about 16.2Mhz to 55Mhz at the top of that range adjustment. Back down a bit, it behaves normally from about 16.2 MHz to below 8 MHz.All other ranges behave as expected, as best I can tell. I just finished making a passthrough 600-ohm terminator so I can start making some adjustments, and I've been reading the manuals' adjustments area. I don't know if I'm reading the RF adjustment area correctly, but it appears to indicate that only the top three ranges have adjustments for the RF ranges.I'm hoping someone has some experience with these beasts on this sort of alignment issue. When I test this range out the front the 8-16mhz range misbehaves some at the top but not like it does out the BNC on the back.Thanks!

author: Cyclotron
author_rank: Unknown
timestamp: June 22, 2025, 04:19:26 pm
content: And there's proof again that all the greatest test equipment is rendered useless by the monkey on the keyboard.I had forgotten to switch the counter to 50-ohm termination for my analysis of frequency ranges.I got a replacement plexiglass cover for it, and I am closing in on completion of validation, so I can put it to work tuning up an old AM radio.

author: marcumr
author_rank: Contributor
timestamp: June 23, 2025, 08:02:32 am
content: Hello.Where did you find the correct procedure for replacing the brass gears?Thank you very much.

author: Cyclotron
author_rank: Unknown
timestamp: June 24, 2025, 03:09:53 am
content: Quote from: marcumr on June 23, 2025, 08:02:32 amI don't know if there is a "correct" procedure. What I did was place both dials in full CCW, take a bunch of photos, noting the location and placement of each set of switches, loosen the coupler and cables (mine has keepers), and then remove the knobs and nuts underneath.Then the whole assembly came out. I took lots of photos and notes. Then removed the four screws, the coupler adapter, and circlip on the shafts.The assembly splits in half nicely, making the gears easy to access. You can see the red marker die on the gears in my photos. I used marks and photos to put the gears back in the same orientation and clocking as they came out. I reassembled them in reverse order, and things were all well and working.I went overboard with being sure that I kept things oriented correctly. The service manual has a section on aligning the range selector properly so you can get it fixed up pretty easily if things are off. I need to start videoing my work. It might be helpful for me and others at times.

author: Cyclotron
author_rank: Unknown
timestamp: August 04, 2025, 03:25:50 pm
content: I've found all the proper fuses and all components are hitting their marks during a calibration review.I'm happy to have put this unit in service as I have some tube radios that it will come in hand for tuning.