HP/Agilent 8644B Synthesized Signal Generator bad cap repair - Page 1
Forum section: electronics / testgear

author: Cubdriver
author_rank: Unknown
timestamp: February 16, 2018, 05:10:25 am
content: I spotted this beast on the bay of evil about two months ago. It was listed as something along the lines of "Gets stuck in a boot loop on power up", and the photos showed some random gibberish on the display. I decided to roll the dice on it being power supply related, and made an offer which was accepted. It's a monster; weighs in at almost 70 pounds (30 kg). It arrived in good relatively good physical condition; one of the rear bumpers (bottom left when viewed from the front) was snapped off and the screw is still in the frame, but other than that, a small dent on the top and a few scuffs, it is in excellent physical condition.Based on the serial number, it is from the 1995 production revision, but given its Agilent nameplate and the date codes on the components, it appears to have been built in late 2000 or perhaps early 2001. This is my 'newest' piece of test gear.When powered up, the display and front panel lamps went through some odd gyrations, but it obviously wasn't going to wake up. Onto the bench it went, and after the removal of 48(!!) screws and a set of clamping bars, it was coaxed out of its shell. This particular unit has a high stability time base (option 001), and a "Reduced Leakage Configuration" (option 010). My guess is that the sleeve enclosure with the gazillion screws, rather than the more common removable top, bottom and side panels is part of this low leakage configuration - 1.03 MHz max frequency means lots of screws close together to keep the RF inside. Removing them was great fun as they're Torx head screws, and I didn't have an appropriate bit for my power screwdriver. Hello, carpel tunnel syndrome.Continued in next posts...-Pat

author: Cubdriver
author_rank: Unknown
timestamp: February 16, 2018, 05:11:28 am
content: Once the instrument was open, it became evident that the -15V rail of the power supply was missing or loaded down. They thoughtfully put a nice row of green LEDs on the I/O board at the top rear of the unit to indicate the presence or absence of the various power supply rails - it has eight of them - +/-5V, +/10V, +/-15V, +21V, +25V and finally, +38V.By disconnecting all the ribbon cables feeding power to the modules then sequentially reconnecting them, I was eventually able to isolate the fault to the NSM OSC module (cable 44, roughly in the center of the photo below, connects to it). It took a few iterations of connecting and disconnecting things because at first I missed when the indicator LED went out, as it is on the end and not blatantly obvious when all the others are lit.As I progressed in my plugging and unplugging things, the fault definitely made itself known when the suspect module belched a bit of smoke on power up. This was confirmed a moment later when all but two of the power supply LEDs went dark.Opening the smokey module revealed a barbecued molded tantalum cap.Definitely a bit past its prime...A Digi Key order and a few weeks later I got back to it. The cap came out pretty easily, the soot was cleaned up with a q-tip and some denatured alcohol, and after comfirmation that the polarity was right, the new cap went in easily.Cap extracted:Partially cleaned board (I got the rest of the mess off when I cleaned the flux residue after reflowing the solder on the pads, but was on a roll at that point and put the new cap in without bothering to take another picture):New cap in place:Of course the fried cap appears to be the ONLY tantalum on the board that doesn't have a polarity mark next to it. An ohmmeter check confirmed that it was in correctly, positive to ground and negative to the -15V rail on the board.With that module repaired and reinstalled, the next challenge was to figure out why there were now basically NO power supply rails - all that was lighting were the +21 and +25V indicators, and they're on whenever the unit is connected to power - the AC Mains switch does not affect that part of the supply, it is on at all times. I popped the power supply board out (great fun as it extends across the entire width of the unit, and has several daughter cards and short cables which must be disconnected/gently pried away from it, with limited access) and looked it over; nothing appeared to be cooked, and the fuses on it were good. I'd reassembled it hoping that whatever had tripped the supply was something that would reset (yeah, I know... But hope springs eternal...), but when it was turned on things were, unsurprisingly, still dead.Troubleshooting was further complicated by the lack of documentation - this is new enough that a full service manual was not readily available for less than a small fortune, so I was working with a module level troubleshooting guide downloaded from Keysight. Digging through the manual revealed a combination block diagram/partial schematic of the supply, so I started taking measurements, and, concentrating on the -15V section, soon discovered an open fuse hidden away on one of the vertical daughter cards that's buried inside the rear of the generator. The easiest way to gain access to it was to disconnect the cables and remove the I/O board at the top; I wasn't pulling that damned power supply board out again if I could help it.A trip to the hardware store later (I had no 3A fast blow fuses, and wasn't about to put a different value in for testing as it's such a PITA to get to) and a new fuse later, everything went back together and I had all power supply rails once more. They are apparently electronically interlocked, and if one goes away the rest shut down to prevent damage to the unit. I should have taken pics of the fuse board, but neglected to before reassembling things, but unfortunately didn't.More below.

author: Cubdriver
author_rank: Unknown
timestamp: February 16, 2018, 05:13:39 am
content: With everything re-fused and reconnected, it happily booted up and went through it's POST and self calibration routines.Here it is feeding a 500 kHz signal that is amplitude modulated 100% by a 1.3 kHz sine wave to a scope.I'll definitely need to spend some serious time perusing the operating handbook while playing with it to get a better feel and be able to do more than barely scratch the surface of its capabilities, and also try to do some characterization to see how accurate it is seeing as its calibration is unknown. Knowing HP/Agilent gear, it's probably plenty accurate for anything that I might do with it. It's nice to have my newest piece of equipment up and running.Tomorrow's project will be to get it 'dressed' again and reinstall the four dozen screws that came out to open it.Other pictures with some additional commentary and explanation are in the SmugMug gallery:-Pat

author: VK5RC
author_rank: Unknown
timestamp: February 16, 2018, 10:19:58 pm
content: Great outcome. Thanks for posting, Rob.

author: bitseeker
author_rank: Super Contributor
timestamp: February 16, 2018, 10:58:46 pm
content: That was a bit of an adventure. Sure was good to have those power status LEDs. Once you've gotten comfortable with using it, I'd like to hear your impressions on the generator.

author: Cubdriver
author_rank: Unknown
timestamp: February 17, 2018, 05:52:32 am
content: It was for sure an 'oh shit' moment when that buried fuse blew and the power supplies all went away, seeing as I don't have a CLIP for it. I wasn't looking forward to trying to troubleshoot a dead power supply. I'm glad that fuse was there, even if they did hide it really well.Hopefully I don't have to take that damned cover off again any time soon - it's a PITA to remove and reinstall!-Pat

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 17, 2018, 08:00:32 am
content: why the EF 100mm f/2.8 got stranded over there? poor 100mm. you used it for the above macro shoots? you are nuts. anyway congratulation for the fix.

author: Cubdriver
author_rank: Unknown
timestamp: February 17, 2018, 08:09:34 am
content: Quote from: Mechatrommer on February 17, 2018, 08:00:32 amYeah, I ran out of room to move back when I went to take the overall shot of the unit on the bench, so I had to take the 100 off and put the 24-105 on the camera. And yes, 'nuts' is certainly a descriptive term that fits me! Truth be told, I think something stronger would be more appropriate.-Pat

author: VK5RC
author_rank: Unknown
timestamp: February 17, 2018, 11:19:32 am
content: Every "good" repair has one or more of those "oh s**t" moments. Isn't it called the 'hero's journey' in story telling where the main character heads off nobly then nearly succumbs to ......... etc?I am having a lot of those with a "unique do-er up opportunity' yaesu FT101ham transceiver where some bas***d has dropped it from a height & bent the frame, randomly cut some cables out, several of the modules that remained don't work (I thank fully have a working FT 101 to swap to) but fortunately lots of bit are avail on evilBay for not crazy prices.