HP 8566B - Page 1
Forum section: electronics / repair

author: stfjohn
author_rank: Regular Contributor
timestamp: May 02, 2025, 02:00:42 pm
content: Hello everyone. Suddenly my spectrum analyzer (HP 8566B) has the following problems: the low band signal is about 40 dB lower than it should be. The high band has stopped working: the various bands no longer appear on the screen, but a single band similar to the low band and the signal is completely absent! I strongly suspect that the YTX oscillator is dead or almost dead! Does anyone have any ideas about this? Thanks.

author: MadTux
author_rank: Frequent Contributor
timestamp: May 03, 2025, 10:55:36 am
content: Put power meter or TinySA or something onto 3 to 6.1 ghz @ +9dbm at 1st LO connector, then you know ;-)

author: stfjohn
author_rank: Regular Contributor
timestamp: May 04, 2025, 10:26:01 am
content: Images: HP8566B and output at first LO connector.

author: stfjohn
author_rank: Regular Contributor
timestamp: May 15, 2025, 03:34:53 pm
content: Can anyone tell me which circuit generates the intervals of the background noise? Thanks.

author: MadTux
author_rank: Frequent Contributor
timestamp: May 21, 2025, 10:09:03 pm
content: Not noise, interference between 8566B scan and other HP SA scan.Perhaps put 8566B in zero span mode to get a non sweeping LO??

author: stfjohn
author_rank: Regular Contributor
timestamp: July 22, 2025, 09:23:23 am
content: I replaced the ACLU module and things have definitely improved: the noise intervals have reappeared and the signal placed at the input is visible again on the CRT. in band "A" things are quite good, while the other bands (2 - 22Ghz) show a signal rather variable in amplitude depending on the frequency.... So the fundamental problem was due to the ACLU module apparently...But now a new problem has appeared: a few seconds after turning it on, the screen looks like the picture! What happened?

author: m k
author_rank: Super Contributor
timestamp: July 22, 2025, 10:07:43 am
content: Horizontal shift or signal inversion?

author: stfjohn
author_rank: Regular Contributor
timestamp: August 31, 2025, 02:44:22 pm
content: I solved the problem by replacing the A3A9 board. But now I noticed another problem: when the frequency spams are between 0.1 MHz and 5 MHz, on the A10A2 board the "Look on" LED turns off and the result is what you see in the second photo (the 100 MHz signal from the cal output is applied to the input)! The first photo shows the result with spam = 10 MHz (everything normal).

author: stfjohn
author_rank: Regular Contributor
timestamp: September 02, 2025, 03:27:51 pm
content: I've checked everything, but I can't seem to fix the problem. Can anyone help? Thanks.

author: m k
author_rank: Super Contributor
timestamp: September 02, 2025, 04:53:35 pm
content: I don't understand what I'm seeing.It's like only high side is visible.But if low side is not present shouldn't the rise be faster.Is it constructed from memory?Low side is not flat, so something is still coming out.Is changing SPAN also changing some filtering?That'd be pretty analog after memory.

author: stfjohn
author_rank: Regular Contributor
timestamp: September 03, 2025, 10:58:29 am
content: I've discovered that the problem only occurs with RBW = 30Khz, which is automatically selected by the device. So I think I can calmly isolate the fault. Thanks anyway, Mark.

author: stfjohn
author_rank: Regular Contributor
timestamp: September 10, 2025, 09:47:42 am
content: The fault was in board A4A9: Q10 had an open B-E junction. Thanks everyone for your contributions.