Advantest U3661 - ERR 900: NO LOCK DET  - Page 1
Forum section: electronics / repair

author: calin
author_rank: Regular Contributor
timestamp: July 31, 2023, 05:04:14 am
content: Recently I acquired an Advantest U3661 spectrum analyzer; pretty nice unit goes to 26Ghz etc. For the $ I paid on it was a pretty good deal.The machine seems to be mostly happy but under 3.3G it seems to be suffering from something that, based on my searching and reading so far, is a pretty common issue. The 2nd LO PLL is not able to lock anymore; seemingly due to a drifting VCO. So everything which is in the "low band" (under ~3.3GHz) displays the ERR 900 message and everything over seems to be working OK. Also it seems that it starts displaying the error only after it warms up.I checked so far all power and inspected all boards all good; the 30Mhz/-20 dBm CAL signal is there. As much as I could measure and test the high band seems normal; my most capable sig gen goes just up to 8Ghz. Also the low band on the amplitude/atten side looks as being fine when I manage to "catch" a signal which displays it does show the correct amplitude. I say "catch" because the frequency drifts all over the place as the PLL seems to be chasing a lock.I did find a bunch of manuals and notes from other that have fixed other similar Advantest machines (for example R3132) but I wonder if anyone has some prior experience fixing an U3661. Any input or direction will be greatly appreciated before I start dissecting the RF module.

author: Swake
author_rank: Frequent Contributor
timestamp: July 31, 2023, 07:52:25 pm
content: "After it warms up"Do you have enough access room in the machine to cool down some components? Maybe even a thermal camera to see thermal evolution.Have found the maintenance manual but no schematics. Do you have anything? I'm especially curious about what they call the '1st mixer'.I think the upper limit of your issue is 3.2GHz, not 3.3GHz, as this is indicated as frequency 'Band 0'.Is the hole lower band 0 from 9kHz to 3.2GHz bad, or only a part of it?Can you confirm that Band 1 (3.GHz to 7.1GHz) works fine?Same for the lower end of band 2 (6.7GHz to 14.5GHz)?

author: calin
author_rank: Regular Contributor
timestamp: July 31, 2023, 11:33:30 pm
content: Unfortunately there is not much space to cool the machine down - it is really packed. Actually and I have to close it to be able to turn it on. Unless I find out an extender that connects the CPU board to the front board there is no way to have it running open; and I think that extender is very close to being "unobtanium". The only way I manged to test voltages it was to solder fly wires to the test points and have them routed outside of the case on the vent holes. The machine is built to be "ultra portable" ... so everything is crammed in the least amount of space possible.I could try to shoot somehow some cold spray in through some holes but probably blasting that on 26G capable RF connections is not very indicated. When I say ,warm up, I mean the error shows only 10-15 minutes after it is turned on. And I don't have a thermal camera.. yet. This may be a good reason to get one.The RF section does not seem to be connected to the backplane (motherboard) so I may be able to get that out without needing an extender, e.g. be able to get away with some semi-rigid coax routed in some very creative ways to get it out of the case. I have yet to try, but I said to gather more info before I start dissecting.Yes the upper limit is 3.2G as you say, Band 0, and definitely the entire span from 9K to 3.2G suffers from ERR 900. When I tested the other day I wnt step by step .. probably I got bored and jumped over 3.2G. I also tested from 3.2G all the way to 8 and it looks fine. Like I said the fastest sign gen I have goes to 8G max, so that was my limit with a signal applied. Of course I had frequency span set higher than 8G, all the way to 26.5G and there is no error being displayed so I suspect it is perfectly fine even after 8G. Definitely looks like just Band 0 has the issue; the ERR shows up regardless of test signal being present or not.I found the Operation and Maintenance manual and also a bunch of documents and threads on the Groups.io Advantest group, even here on EEVBlog ... see thishttps://www.eevblog.com/forum/repair/advantest-r3132-spectrum-analyzer-repair-information/.On the Groups.io group I found some posts saying that U3661 is same design/or very close as R3132 in the RF section.