HP 8566B Input Signal gets dropped - Page 1
Forum section: electronics / repair

author: _rushin_or_draggin_
author_rank: Regular Contributor
timestamp: September 23, 2025, 10:46:57 pm
content: Hey all,I have an HP 8566B that I have been in the process of repairing. I had a few PSU issues in the RF section that I believe I have repaired. Before I started the repair, when turning the unit on the IF and RF section would turn on together. Now since repairing the PSU issues upon powering up the INSTR Check I light turns on and the IF unit won't power up at all. I have mains going into the IF and the fan on the unit turns on, but none of the supplies turn on.I'm pretty sure my bus cable is still fine and I didn't mess around with the A15 Board. I have had the front panel off the IF section since then but I don't see why that would prevent it from powering up.The RF unit responds normally to all key presses and all the power supplies for the RF unit are fine, it's just the IF section not powering up.Any ideas?

author: _rushin_or_draggin_
author_rank: Regular Contributor
timestamp: September 23, 2025, 10:59:44 pm
content: What a silly thing to miss...It was the 2.5 amp fuse on the A1A6 Regulator assembly. Not too sure why it blew though...concerning. I replaced it with a 2A because that's all I have currently and it powered right up

author: _rushin_or_draggin_
author_rank: Regular Contributor
timestamp: September 23, 2025, 11:10:42 pm
content: I do have a really weird symptom that I'm not too sure yet how to go about solving though.Whenever I do RECALL 8, I get the signal and I adjust for -10 dBm. However, when I do RECALL 9, I get a flat line all the way at the bottom and no matter how I adjust FREQ ZERO I get nothing. The correction routine does complete without any errors.Also, sometimes I completely lose the calibration signal, and it is somewhat jittery. I am also noticing the PLL 1 Lock indicator light turning off, but it's not showing on the IF screen for some odd reason? I will investigate that, but I am still confused why the Freq Zero isn't working.Any ideas where I might start looking?

author: _rushin_or_draggin_
author_rank: Regular Contributor
timestamp: September 25, 2025, 01:35:03 am
content: I have fixed the RECALL 9 issue. It was a problem within the A4A5 board in the IF section. As I just board swapped I'm not too sure what it is, but there was evidence of flux residue around the 18.4 MHz crystal that is used when tuning the FREQ ZERO, so maybe this was once replaced in an attempted repair? Not sure.As for the PLL "issue", I believe it is normal for the frequency span I noticed it occurring in. Looking at the service manual, PLL2 is not used for certain frequency spans. It still surprises me that the lock indictor LED turns off though, but everywhere PLL 2 is required seems to work fine. And again I don't get any special messages on the CRT.After completing the correction routine, all values when looking at Shift + LINE are well within spec (mostly, there is one that's +1.4dB when it should be within +-1dB. Not bad.I am having a problem with the input gain completely dropping out after the unit sits powered on for awhile. I will be measuring the cal signal and it may shift between -10.11 to -9.98, but then it will randomly drop down to nothing. And it does it for any external source as well. When I hook my scope up the the 21.4 MHz IF output on the IF unit I notice the IF decreases in amplitude during this scenario. So my best guess is that is is within the RF section (I haven't looked to see how the IF BNC on the back of the IF section is routed. If it comes directly from the RF unit then it is definitely in that area, but if it goes through some processing within the IF section I'll need to look at that further). I don't have anything that can go up to 2.5+ GHz to test if it only occurs when the 0.1-2.5 GHz range is selected (signal is routed through a limiter) or not. I do have a parts unit that I could board swap with if needed.So far I'm pretty sure I'm close to getting this thing working.