R3131 first LO fix - Page 1
Forum section: electronics / testgear

author: Renaud F5ZR
author_rank: Contributor
timestamp: October 11, 2024, 05:11:25 pm
content: Hi,I bought a faulty R3131. I get:I measured the YTO for some center frequencies zero span:3 GHz: around 7084 MHz, should be 7066 18 MHz high2 GHz: around 6116 MHz, should be 6066 50 MHz high1 GHz: around 5146 MHz, should be 5066 80 MHz highAll values drifting very slowly.Any ideas would be welcome!Renaud

author: Renaud F5ZR
author_rank: Contributor
timestamp: October 14, 2024, 06:08:38 am
content: Above I mixed the R3361 and R3131 IF frequencies, sorry. The first IF of the R3131 is 4113.58 MHz, not 4066 MHz which is the R3361's.Hence read:3 GHz: around 7084 MHz, should be 7113,58 around 29 MHz low2 GHz: around 6116 MHz, should be 6113,58 around 3 MHz low1 GHz: around 5146 MHz, should be 5113,58 around 33 MHz highRenaud

author: Bryn
author_rank: Regular Contributor
timestamp: October 14, 2024, 07:24:51 am
content: When you first bought the device, did youknowit was faulty? Or did you discover that upon using it?Anyway, depending onhowfaulty the device is, you may need to recalibrate it which will explain why the readings drift off to what they were originally. Try doing that and let me know if anything else needs to be done.

author: nctnico
author_rank: Super Contributor
timestamp: October 14, 2024, 01:06:27 pm
content: There are old threads about this machine describing repairs for similar problems. Using Google with 'R3131 eevblog' is likely to turn up interesting information

author: Renaud F5ZR
author_rank: Contributor
timestamp: October 15, 2024, 06:23:59 pm
content: Hi, Thanks for your replies;Yes Bryn, I knew and it was very cheap, no complain!Nctnico, I know very well for having read it at least entirely three times the "Advantest R3131A Spectrum Analyzer 9KHz-3GHz" but that one isn't about LO1.I'll follow your advice and look further.I found that the YIG big coil signal they call TUNE is moving correctly the OL from around 4 to 7 GHz. But the FM SWEEP to the little coil is either +50 mA or -50 mA, never between. I suppose, maybe wrongly, that FM SWEEP is also responsible for locking. The sweep does exists on the card, generated by a 12 bits DAC but doesn't reach the FM coil. Not easy to follow as there's a lot of circuits in between.Renaud

author: Renaud F5ZR
author_rank: Contributor
timestamp: October 16, 2024, 05:15:44 am
content: Hi Nctnico,I could not find any reference to LO1 fix on Google. Neither here. Some only about LO2.If you know any I'm interested!Renaud

author: nctnico
author_rank: Super Contributor
timestamp: October 16, 2024, 09:52:18 am
content: It has been a long time since I owned this spectrum analyser. But if your diagnosis is that the signal from the DAC isn't reaching the FM coil then this is a good starting point. I'm afraid this will require good old step-by step analysis. First trying to trace the signal path on the board and determine which components it is likely passing through and / or which components deal with the FM coil. Then check power supply voltages on the chips. If the power supplies are OK, then you'll need to trace the signal itself and see where it stops. This can be a super tedious process if you can't measure on the board itself. A method I have been using is to solder a long wire to a board to make it accessible with an oscilloscope probe and measure while the device is on. Time to put elbow grease on... From what I have gathered, the analog switches used in the R3131 can be prone to failure, so if these are involved in the FM coil path, this is something I'd put high on the list to check.I do seem to have a zip archive with repair information I collected. It has photos, user manual and datasheets. If you PM me your email, I can send it (if you have not found it by yourself already).

author: Renaud F5ZR
author_rank: Contributor
timestamp: October 16, 2024, 05:43:32 pm
content: Hi Nctnico,Some clarifications first!The PS voltages are spot on.I'm sure that no DAC is reaching the FM coilwhen I looked. The FM coil is either + or - 50 mA which seems to indicate an open loop. I tried different span, even 0 of course to no avail.But is the sawtooth going to the FM coil really? This is my supposition only, though it seems logical?Unfortunately the path is really very busy. When you say tedious I agree as I already spent a lot of time there!The access on the board is not a problem as I built an extension for that.Yes I will verify those analog switches, thanks for the advice.BestRenaud