FE-5650A Frequency Standard Repair - Page 1
Forum section: electronics / repair

author: yngndrw
author_rank: Regular Contributor
timestamp: September 13, 2025, 04:54:32 pm
content: I just purchased an untested FE-5650A Frequency Standard but I had an issue while testing it.This unit has the options:12 (Analog Tuning)14 (Heatsink, SMA output)38 (Temperature Operation to -30°C)42 (Temperature Operation to +60°C)This unit also appears to be one of the later ones that all have option 20. (5V supply is internal, you just supply 15V @ 1.8A.)I'm aware of the slow power-up issue (https://febo.com/pipermail/time-nuts/2016-June/098333.html) so I selected 15V and 1.8A on my PSU. I measured around 10MHz on the output before noticing that my PSU was current-limited and the voltage had only risen to ~6V. So I boosted the current much higher, thinking it required a bit more at startup and tried again - It went to 3.6A for a few seconds and then dropped off to almost nothing. The output went dead and there was a ominous smell along with a telltale puff of smoke.Are there any known issues that would cause them to behave in this way? I found it weird that it was outputting a frequency at first.The good news is that I've found the issue quite quickly, the bad news is that I don't know why it failed:Is anybody able to identify the component? It appears to be a Motorola device, R96 2955E.

author: voltsandjolts
author_rank: Unknown
timestamp: September 13, 2025, 05:44:41 pm
content: MMFT2955https://www.futurlec.com/Datasheet/Transistor/MMFT2955.pdfObsolete but similar should be easy enough to find.Not unheard of for fets to fail on their own, so it might be the only fault.I guess it was driving the heater or something?

author: yngndrw
author_rank: Regular Contributor
timestamp: September 13, 2025, 05:57:31 pm
content: Thank you very much, I really appreciate that.I'm not entirely sure what it's driving, I tried to find a schematic but couldn't. It sounds like swapping the FET and giving it a try is the best first step, to see if it behaves the same or not. I'm be sure to order a few FETs in case some further investigation is required!

author: Kean
author_rank: Unknown
timestamp: September 13, 2025, 06:31:36 pm
content: My guess is that P-Channel MOSFET is part of the power supply or heater control circuits, and was operating in linear mode due to the low input voltage and overheated.OnSemi NDT2955 is likely a suitable and commonly available alternative with slightly better specs (or NTF2955).https://www.onsemi.com/pdf/datasheet/ndt2955-d.pdf

author: yngndrw
author_rank: Regular Contributor
timestamp: September 13, 2025, 10:12:32 pm
content: Thank you, I've already ordered some MMFT2955ET1 FETs but I'll see if I can check the heater before I replace it just in case there's anything obvious. The NDT2955 does look to be a little more robust if the VGS is sufficient but I'll try with the original seems as I've managed to find a supplier for them.Edit:Weirdest device I've ever seen under the cover, but I presume Q2 is used as the heater. It seems to check out with a quick in-circuit test so I'll see if it works after the FET is replaced. I've attached some pictures in case anyone's interested in seeing what's under the cover.Edit2:I just noticed that the tantalum capacitor next to the FET has a burn mark and is behaving like a resistor, so that may have been the cause. I think the FET was being used for reverse polarity protection. It also appears that the related LM2941CT is broken as it's outputting the input voltage rather than the calculated 2.55V that it should be. That may just be the lack of output capacitor though - I'll be sure to check it once the other components have been replaced.

author: yngndrw
author_rank: Regular Contributor
timestamp: September 19, 2025, 05:58:28 pm
content: Great news! The repair was successful and I now have a working Rubidium frequency standard. Just in case anyone is wondering about these, they report that they are "locked" long before they get up to temperature. (Mine is currently still warming up, drawing 0.72A and should drop to 0.6A so nearly there and it's currently at about 44.5C so I'd expect it to get to around 50C - Sadly, the target temperature is not in the datasheet.)In case it helps anyone repairing these in the future: In my case it appears that the Tantalum capacitor shorted and blew the reverse polarity protection FET. I replaced one of the regulators but it turns out I didn't need to - I'd just miscalculated the expected voltage for that rail. You can run the option 20 regulator board on its own and there are three test points for the voltages - 5V, ~14V and ~14V. I'm not sure why there are two 14V rails.Now I can get on with my original plan of installing this within a Keysight 53131A - I'll probably install a larger +15V - 15V PSU (Mean Well RT-125C) and then build a small linear regulator for the +12V, -12V and +5V rails that the counter requires.

author: Kwakerman
author_rank: Regular Contributor
timestamp: September 19, 2025, 08:47:00 pm
content: I assume that's going to be a bit of a squeeze getting it in a 53131 especially if you have the Channel 3 option fitted. I made a box (with a PSU and fan) for my 5650A and just feed the 10 MHz into the Ext Ref socket when the extra accuracy is needed. I'd be a bit cautious about the amount of heat the 5650A makes when enclosed in the 53131A

author: yngndrw
author_rank: Regular Contributor
timestamp: September 19, 2025, 09:25:10 pm
content: Eyeballing it, I think I'll just about get away with it but I will have to double check the PSU size. Making a standalone unit is plan B, but having it inside would save a lot of space if I can.The heat is an interesting one, the datasheet isn't very helpful but my understanding is that they require some cooling and thermal management - Evidenced by the fact that two of the options are various styles of heatsink. Given that it only rose to about 50C on my desk, I don't think it will be too unreasonable given that the 53131A has a fan at the back and that it can be bolted to the metal chassis. I'd have to verify this though. Mine has option 42 which specifies a maximum temperature of 60C, so I think that aiming to hold it at 40C or 50C would be a reasonable target.For now, I'm just happy that it's working.

author: Kwakerman
author_rank: Regular Contributor
timestamp: September 19, 2025, 10:20:25 pm
content: I have mine bolted to the case and use heat transfer compound between both but it still gets warn after a couple of hours use. The fan on the 53131A is, if I remember correctly a 40mm one bolted to the PSU itself but doesn't shift that much air. If you can't fit a heatsink of some kind to the 5050A I'd suggest trying it suspended by its cables and run it for a few hours to see what its free air dissipation is like. I notice with mine that it still fine tunes very slightly even after the enable signal switches state for about 10. minutes.Something else you may need to consider is that the standard OXCO options (Medium, High and Ultra high tab) actually produce two 10 MHz outputs (in opposite phase) so you would likely need to replicate that if you are going to use the OXCO ribbon connector as the method of feeding the 5650A signal into the 53131A

author: yngndrw
author_rank: Regular Contributor
timestamp: September 19, 2025, 10:32:32 pm
content: Yes I need to do some longer tests, but from the initial testing it didn't appear to want to go over 50C (24C ambient) and the heat rise had slowed to a crawl. That was with the optional "heatsink" which is just a block of aluminium with no fins, so it could be much better if needed. Yes the standard fan is only 40mm, but I could mount a fan directly to a small heatsink on the oscillator if needed and I think that would be enough. I think you're right that the standard PSU fan is a bit too far away to do much, but I think that would be sufficient to control the temperature within the case.For the output, my plan was to use the LM361M which the normal OCXOs use. I don't think I'd bother with the DAC as I have nothing better to calibrate it to so I'd just keep it simple(ish).

author: Kwakerman
author_rank: Regular Contributor
timestamp: September 20, 2025, 06:45:14 am
content: I'm assuming your plan to use the existing OXCO pcb but with the oscillator itself removed? If so, and you are feeding the 10 MHz from the 5650A via the OXCO pcb's LM361 to the 53131A then the DAC is totally redundant. In the normal OXCO arrangement the DAC is used to adjust the fine tune voltage pin on the oscillator itself during the calibration process so would be totally irrelevant as there is nothing for it to tune.Thinking about it you wouldn't want to use the calibration routine anyway because:1. The 5650A output frequency is non-adjustable (I know it can be adjusted via serial port but you would need a GPSDO or something else to adjust it against).2. You would likely end up with a calibration failure error as the cal routine would be trying to shift the OXCO frequency to match the 10 MHz signal on the Channel 1 input but nothing would be happening.

author: yngndrw
author_rank: Regular Contributor
timestamp: September 20, 2025, 12:50:40 pm
content: I don't have an existing OCXO so I'll need to make something up, but I have the schematics of the original and there are numerous DIY versions which are based off the same output stage - Gerry Sweeney has quite a nice one which I can use to give me a head start. The original DAC is quite expensive so it's handy that I won't be needing it.If I were to turn it into a GPSDO, I'd built it into the unit anyway rather than having it external so there really is no need for me to have the automatic calibration functionality. GPSDO is a project much further down the line though, I really don't need that level of accuracy!

author: Kwakerman
author_rank: Regular Contributor
timestamp: September 20, 2025, 02:14:00 pm
content: FYI I've built a couple of these in the past using the MV89 oxco and they work fine. It also includes the Gerber files to get your own pcbs made up fairly cheaply. The pcb allows for various configurations which could possibly form the basis of what you are trying to do?https://www.circuitvalley.com/2018/07/agilent-53132a-53131a-ocxo-ultra-high-stability-oven-time-base-upgrade-option-open-source.html

author: voltsandjolts
author_rank: Unknown
timestamp: September 20, 2025, 03:58:08 pm
content: FYI I'm selling a UBlox timing grade module with a programmable frequency output which I've found useful for verifying frequency counters. It has too much jitter to use as the reference signal, but as an input signal the jitter doesn't matter much over a few seconds gate time.https://www.eevblog.com/forum/buysellwanted/fs-uk-ublox-lea-6t-dev-board-with-antenna/

author: yngndrw
author_rank: Regular Contributor
timestamp: September 20, 2025, 10:38:37 pm
content: Quote from: Kwakerman on September 20, 2025, 02:14:00 pmThank you, I'll take a look at that. I'll have to consider the packaging as I'd also need to work around the PSU upgrade, but that board could certainly give me a head start.Quote from: voltsandjolts on September 20, 2025, 03:58:08 pmThank you, that's a good price and I could see that as a good option to build into a GPSDO unit. I think I've got quite a way to go before I consider GPS though, as it stands it would probably just sit on my desk for a year so it's better to not have it gathering dust.