Advantest R4131D spec an - Help needed - Page 1
Forum section: electronics / general

author: sq9nje
author_rank: Contributor
timestamp: July 31, 2012, 07:20:03 pm
content: Hi Guys!I just received a new toy today. It's an Advantest R4131D spectrum analyzer. It's in good physical condition though exhibits some CRT burnout - to be expected on an older unit like this. Contrary to Dave's motto I did turn it on and concluded that I will probably have to take it apart... Here's what happened.The device powers on, passes the self test and generally responds to the keys and knobs. At a first glance it does what it's supposed to. So far so good. To test it I connected the calibration output to the input and after a short while I was able to see the signal. It was actually harder to "find" it than I anticipated so i checked the specs on that signal - it's supposed to be 200MHz @ -30dBm. Hmmm, so why was it so hard to find it down in the grass? What the analyzer shows is actually at -90dBm!!!I checked the cal signal on my Tek 2246. It's only a 100MHz scope, but the signal was a nice sinusoid, about 5mV pk-pk on a 50ohm terminated input. This leads me to believe there is nothing wrong with the calibrator (unfortunately) and the problem lies in the input sensitivity.I know some of you own these analyzers. Do you think it's repairable? Is it perhaps a matter of some simple calibration? Any and all ideas very appreciated. I've got too much mess in my lab at the moment, but should be able to poke around inside during the weekend.

author: w2aew
author_rank: Super Contributor
timestamp: July 31, 2012, 09:33:12 pm
content: I agree - it sounds like the cal signal is OK.A 60dB error is certainly more than a simple calibration, IMHO. Check the level with different (manually adjusted) attenuator settings, to see if the input attenuator is possibly toasted at one (or more settings). The front-ends of spectrum analyzers can be very sensitive (attenuator, and especially first mixer), and prone to damage by ESD, over-voltage, or in many cases, excessive DC. Hopefully you'll find that yours is a simple, inexpensive repair.

author: sq9nje
author_rank: Contributor
timestamp: August 01, 2012, 05:55:57 am
content: Nope, I don"t think it's the attenuatorIt does what it should - attenuates in 10dB steps, the peak still stays at about -86dB.Repairing this thing is going to be tough. The first IF is 4GHz and I don't have any test gear that would go that high. Oh well, guess I will have to come up with some clever ideas... The manual for this thing has a chapter on some test and alignment procedures. Will have to read and follow that, maybe this would help at least pinpoint where the fault lies.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 01, 2012, 09:54:01 am
content: I hope I'm wrong and it's just a loose cable but it looks like the first mixer (2 x diode ND587T) is toast, probably too much RF input.The manual @ KO4BB has the complete schematics.

author: sq9nje
author_rank: Contributor
timestamp: August 01, 2012, 11:23:38 am
content: Thanks for the pointer to KO4BB. I had a copy of the same manual, but his is of much better quality.I too am afraid it's the first mixer, though a loose cable would be a really welcome easy fix. How would I go about testing that mixer? Since these are just two Schottky diodes, would a multimeter diode test be cosher?

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 01, 2012, 11:42:47 am
content: I never had an SA fail like this (fingers crossed), but I can imagine that forward voltage drop or reverse resistance could tell you something, although it would be easier if you had something to compare it against. How easy can you access them?

author: sq9nje
author_rank: Contributor
timestamp: August 01, 2012, 02:22:31 pm
content: Thanks Aurora, you've been a huge help!I bought some of the HSMS-8202 diodes mentioned in the Yahoo thread off eBay (I actually found them earlier and was wondering if they would be suitable.). They should be here in about a week. I'll try to fix this myself, but if it doesn't pan out I'll be sure to contact you about those RF assembliesIt seems that the unit was used for some kind of PMR band monitoring, the center frequency was set to 446MHz. I suspect it must've clocked looong hours displaying the same thing. The CRT is burned out pretty badly, but the display is still readable, however the intensity knob needs to be cranked up pretty much to the top. The amber coating does a great job. I might get one of those eBay replacements in the future, if only to have a spare available.

author: SeanB
author_rank: Super Contributor
timestamp: August 01, 2012, 03:20:43 pm
content: That little CRT looks just like the one I have in a portable telly. Costs $25 brand new, and is pretty popular here. That small a CRT would be easy to post, complete with the yoke. Most likely would work without adjustment straight off with the yoke, as they are very likely to be the same specs, as both run off 12V and use a similar chipset and design. Only thing different is the big focus assembly, the cheep n cheerrful B&W set uses an integrated focus unit, but even the CRT base looks similar ( just a set of connections and a spark gap) to the cheepie.

author: sq9nje
author_rank: Contributor
timestamp: August 01, 2012, 04:47:34 pm
content: All this discussion has been very encouraging, so upon comming home from work I popped the hood, unscrewed the shielding cover and took out the mixer diode. Sure enough, one of the diodes in the structure is shot. Not a dead short though, my ohm meter shows 6.8k in one direction and 7.7k in the other. The other half seems OK.So now I'm waiting for the replacement to come and hopefully the whole thing will be long forgotten in a week or two. In the meantime I think I'll brush up on mixers and try to grab a decent inline attenuatorAurora, your info is indispensible! Thanks again! Looks like this thread is turning into an extensive repair and restoration guide for this analyzer. For the sake of completeness here's the link to theoperating and service manual. IMHO it does a pretty good job of explaining the details of the construction.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 01, 2012, 06:31:05 pm
content: Quote from: sq9nje on August 01, 2012, 04:47:34 pmWell, a little bit more work than a loose cable but with the info Aurora provided it looks like a not-so-difficult fix.Just wondering what the impact on performance could be, but someone in the diode thread seems to have noticed no difference on the ref signal. I think it could give some degradation on the high end if the capacitance of the replacement diodes is higher, but comparing the data sheets it looks like they are both in the .25 pF range, so you are safe.Good luck with the fix!

author: vk6zgo
author_rank: Super Contributor
timestamp: August 02, 2012, 01:13:42 am
content: Yeah,a lot of those small B&W tubes are very similar.It's not directly applicable here,as they are a bigger size,but at the TV studios we replaced the 9"tubes in Panasonics,Teleprompters.&Electrohome monitors with the same tubes from The Electrohome spares shelf.If I remember correctly,they were made by Matsushita.

author: SeanB
author_rank: Super Contributor
timestamp: August 02, 2012, 04:59:40 am
content: CRT's are only made by a few companies, and they supply all. Only holdout is Sony, of course, but when did you last see a new Trinitron set. There are a few specialist manufacturers who do avionics and ruggedised tubes, at a price to match. About the only tube you can post in a single jiffy bag and have it survive.

author: sq9nje
author_rank: Contributor
timestamp: August 07, 2012, 05:23:07 pm
content: Hey Guys!I found a local distributor of the HSMS-8202 diodes. Unfortunately the courier company screwed up and instead of getting them on friday they only came today.Anyway, I installed the new diode, turned the analyzer on and... the cal signal was even 10 dB lower than beforeI took the diode out and turns out it failed, both diodes were shot now. I didn't check this diode before installing it, though. I measured the voltages in the mixer where the diode is supposed to go - nothing notable, about 10 mV. I took another diode, this time checking it was OK and installed it. Lo and behold... failure again. The signal registered at -86 dBm just as with the original diode. The failure mode was also the same as in the original - one diode is shot, the other is OK.I'm out of those diodes for now, the ones from eBay are still in the mail. Any ideas as to what I might check before I blow them all up?

author: sq9nje
author_rank: Contributor
timestamp: August 07, 2012, 07:23:11 pm
content: 1. There is no DC path from the input to the attenuator in my unit - the capacitor is in place and is not shorted.2. I checked for DC again - nothing notable.3. The LO is 4 - 7.5 GHz - I don't have anything to reliably check the levels at these frequencies. I just fried the diodes that were supposed to be able rectify that. I would agree though, that it's unlikely that the LO level is too high.4. The THP117 hybrid looks fine to me.5. Only the diode that is drawn parallel to the 470R resistor fails.6. The cal signal looks OK. I only have a 100MHz scope but I can see about 5mV p-p of a sine with no DC offset on it.I am completely baffled by what is happening. This looks like such a simplistic mixer. I think I need to sleep on it a little. I'll try substituting the diodes tommorow and will keep you posted.

author: sq9nje
author_rank: Contributor
timestamp: August 07, 2012, 08:19:22 pm
content: Jeez, I just noticed the stupidest thing! The input N connector is damaged! The leaves of the center conductor receptacle are all broken off, only a little prong is sticking out of the insulator. I noticed it while poking around with the meter when the center pin of the N to BNC converter wouldn't make contact. I never bothered to look INTO the damn connector when I got this bloody thing!So I guess this would explain the additional 60dB attenuation. In that case I would lean towards the conclusion that the diodes get damaged while being handled by me, perhaps through ESD.

author: sq9nje
author_rank: Contributor
timestamp: August 07, 2012, 09:59:43 pm
content: Yes, it's 50R. I had look through my drawers and found only those crappy Chinese ones. I think I'll grab a few of those Radialls, the price is nice for quality connectors. I'm very curoius to see if that fixes the problem.My iron is not earthed, I'll take care of that next time I handle these diodes.

author: Greenkit
author_rank: Newbie
timestamp: September 07, 2012, 09:30:57 pm
content: I've been reading the comments here with interest, as I have an R4131c which exhibits a similar 'deafness', but at switch-on from cold only. With a known-level input signal (or the -30 dBm CAL signal) the displayed amplitude is initially some 35 dB lower than it should be. After about 2 minutes the display suddenly self-corrects, and the SA behaves normally and correctly thereafter. I need to leave it for about 6 hours before the fault re-appears: switching on at shorter intervals gives correct operation. The problem is not with the input socket / the attenuator / dry joints, as far as I can tell so far. The first mixer is naturally suspect, but I'd be interested to know if this particular fault has shown up on anyone else's analyser.

author: Greenkit
author_rank: Newbie
timestamp: September 07, 2012, 10:24:13 pm
content: Thanks for the fast reply, Fraser. Agreed, it does look thermal; what bugs me is that the speed of correction is really instantaneous, not really what I'd expect from a PSU cap on its way out.... I have checked the PSU rails with a meter, but will 'scope them in more detail in the next day or two. I have the original Instruction Manual for the R4131 series, but I don't see any PSU info in it, so will seek out the service manual. This particular unit hasn't seen a lot of use and is in good shape physically, so I'd like to get it back to full working order. Something to mull over during the longer evenings! Again, thanks for your input.Noel

author: Greenkit
author_rank: Newbie
timestamp: September 19, 2012, 09:48:22 pm
content: Happy to report resolution of my problem: the following might help someone else, though the probablity of the same fault is very slim. As stated a few posts back, I had checked for suspect dry joints previously, but to no avail. All PSU rails were correct and clean when 'scoped. RF-wise and working backwards through the IFs, the next step was to feed 3.58 and 226 MHz CW signal from a sig. gen., at -6 dBm and below. Varying levels gave clean vertical deflections on the CRT. There was no change from the ‘fault’ to ‘no-fault’ condition. Similarly, the injection levels from the first and second LOs were found to be stable. Without a microwave power meter, my old Dymar ‘500 MHz’ unit showed 0.6 mW from the 1st LO and 1.0 mW for the 2nd rear panel connectors, with the SA tuned to 200 MHz: not strictly good practice, but it did work. Attention was therefore directed at the 2nd mixer assembly. There was clear evidence of re-working around the IF-amp transistor stage Q3 and the fault could be induced by loosening/tightening the adjacent pcb fixing screws. External vibration of the RF block had not induced the fault previously, but relatively small thermal changes as the box warmed up had had a definite effect. Closer inspection showed that the cold end of bias resistor R15 (10k) had cracked metallisation / gone dry. Replacement cured the problem: nearby joints were re-made and the board surface flux removed. I was concerned that even a small mechanical movement would affect DR tuning, but the displayed CAL signal is within 500 kHz of where it was previously, so I think I'll leave the tuning screws untouched. I don't know who did what was quite a messy job to the circuit, but they could have been trying to cure a bad R15 all those years ago.....One thing if Fraser reads this: do you happen to know what the difference is between the 1st and 2nd mixer diodes, please? The first mixer is quoted in the manual as a DMJ4317-1 and the second ND587T. I have bought a couple of HSMS-8202 for insurance anyway.