SRS PRS-10 Low Output Level - Page 1
Forum section: electronics / general

author: rubidium
author_rank: Regular Contributor
timestamp: July 08, 2020, 04:06:08 pm
content: Hi,I recently acquired a used PRS-10. After a short warm-up time it locks up and there is a nice clean 10MHz sine output. The problem is that the amplitude appears down by nearly an order of magnitude from the published spec. According to the SRS data sheet, the amplitude should be 0.5 Vrms (+/- 10%), i.e. 7dBm into 50R. Unfortunately, I'm seeing -11.7 dBm, i.e. about 58 mVrms. Does anyone have a clue as to why may be? Otherwise, the unit is behaving quite nicely.Thanks,Jim

author: testpoint1
author_rank: Frequent Contributor
timestamp: July 09, 2020, 03:45:21 am
content: I checked Vpp about 3V, not so low.

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: July 09, 2020, 09:28:51 pm
content: Hi,Ohm out the output. It's TX coupled and should read close to zero Ohms.I repaired one once that someone had blown the output winding and I had to rewind it.Cheers,Corby

author: rubidium
author_rank: Regular Contributor
timestamp: July 09, 2020, 11:32:03 pm
content: 80 milliohms - made with a 4-wire measurement, so looks OK!

author: rubidium
author_rank: Regular Contributor
timestamp: July 09, 2020, 11:44:53 pm
content: If you believe the schematic, there should be 1.41 Vpp at the output. There is a 7:2 turns ratio in the output transformer, so there should be about 5Vpp at the primary side. Prior to that there are some passives and then the output of an LM7171 (U101) that is stated to have an 11Vpp output. I thought I'd pop the cover and have a look at these upstream amplitudes, but that's next to impossible to do live, since the board in question bearing the 1xx-series parts is not one of the 3 exposed boards. Bummer!!

author: DaJMasta
author_rank: Super Contributor
timestamp: July 10, 2020, 12:07:27 am
content: Sounds like the output buffer amp is probably bad. They're not too hard to disassemble and deal with, you can probably find the point the output comes from and trace it back a few steps to see if that final buffering stage is dead (it's probably not low out of the OCXO or that would never lock).

author: notfaded1
author_rank: Frequent Contributor
timestamp: July 13, 2020, 05:47:48 pm
content: At this point what do you have to lose? A good working PRS10 is a pretty good Rb from my experience. If it's basically not working now I'm not sure how much worse it could get by opening it up.This might help you... see attached.Bill

author: rubidium
author_rank: Regular Contributor
timestamp: July 15, 2020, 02:58:33 am
content: I took a first crack at it and managed to safely remove the top and 2 side circuit cards without bending or otherwise damaging any of the many thin header pins. Unfortunately, when I unfastened the front assembly (the one bearing the external connector, I found it impossible to remove it for inspection. There are 3 standoff posts apparently press-fit into the forward panel on the package (the face through which the connector passes) which interfere with components on the circuit board. I tried jiggling it every which way but couldn't get around the interference to remove the assembly. As luck would have it, of course, it is this very assembly that bears the 10 MHz output buffer that's potentially the source of the low amplitude output.The only way to get that assembly out would be to remove the entire physics package - which I'm reluctant to do for fear of disturbing some critical alignment of the optics and/or the magnetic field. Arghh...Edit: I also set up communication to the unit with a serial terminal. I noticed that the 10 MHz amplitude is set to 120, and apparently the range of this value is 0-255 (i.e. controlled by an 8-bit DAC). I tried to set the amplitude higher, but such a move is apparently forbidden by the firmware. It is identified as a "factory-only" parameter, and requires some "secret code" to allow modification. So no joy with that approach.

author: DaJMasta
author_rank: Super Contributor
timestamp: July 15, 2020, 07:02:21 am
content: I don't remember where I saw this, maybe in the service manual, maybe a blog post or something... but there are some options set by resistors on one of the side boards (external facing when you remove the metal shield). This includes some output configurations and constants described in the software - it could be that your magic factory only code is actually just a resistor (or the lack of one) in the right place on the board.Can you get a scope probe onto the oscillator pins, even if you can't pull the board? It would at least confirm whether or not the output level from the OCXO is appropriate (and maybe you could just buffer that and bring it out on your own as a hack option to get the signal).

author: notfaded1
author_rank: Frequent Contributor
timestamp: July 16, 2020, 02:57:07 pm
content: Some older version schematics for PRS-10.Bill

author: 5065AGuru
author_rank: Frequent Contributor
timestamp: July 16, 2020, 09:28:27 pm
content: When I mentioned earlier rewinding the transformer, I had removed another output board from another unit but even though it had good output in the first unit it had a bad output in the 2nd unit. The software is indeed set per the original board and I never figured out how to change it therefore rewound the transformer!Don't you just love stuff that you can't repair!!!???Cheers,Corby

author: rubidium
author_rank: Regular Contributor
timestamp: July 16, 2020, 09:56:13 pm
content: I noticed that the output of the LM7171 buffer amp is not looking good, but I can't tell at this point if the chip is bad or of the output is being loaded down by circuit elements ahead of it. The amp output is actually accessible at one of the header pins and is a 500 mVpp sine centered on a DC level of about 1 V. This is supposed to be a 10 Vpp sine per the document (11 Vpp per the schematic). The Vcc applied to the amplifier (also measurable from a header pin) checks out to be 18 V as it should. Unfortunately, disassembly to get at the amplifier won't allow me to examine the inputs to the amplifier with everything in the powered-up state. So opinions: LM7171 amplifier or something else.

author: lowimpedance
author_rank: Super Contributor
timestamp: July 17, 2020, 01:52:05 am
content: Is it feasible to solder some short lengths of wire to suitable points for measuring the inputs for bias and signal level ? when assembled.As I do not have one in front of me I not sure how easy that would be.Is the +5V POT ref on J100 pin 8 there ?.

author: rubidium
author_rank: Regular Contributor
timestamp: July 18, 2020, 03:08:02 pm
content: Well, biting the bullet, this is the extent of disassembly to which one must go to get at the circuit card bearing the output buffer. In the first image, the board in question is to the left of the black tweezers. The rest must be removed for this board to be removable from the enclosure. I was super careful with removing the Rb lamp assembly, shown just below the top cover. The resonant cavity remains untouched within the enclosure.The bottom image shows the board that needs examination/repair. The TIP107 and LM340-12 evidently provide the heat to the OCXO.Further disassembly beyond this is required to get at the RF output buffer amp.

author: rubidium
author_rank: Regular Contributor
timestamp: July 18, 2020, 03:31:09 pm
content: One has to remove the heating elements from the OCXO to get at the output buffer amp and related circuitry. Once I did that, there were several things that I noticed. First of all, things were pretty "dirty" in there (see 2nd image for example). There were a lot of solder balls and metal filings all about, and these were embedded in flux. Secondly, there was some kluge in place for what was supposed to be an MMBV609LT (D100 in the schematic) dual varactor diode (see 3rd image). There were 2 SOT-23 packages piggy-backed atop one another, with 2 pins of the upper package solder-bridged to corresponding pins on the package below and a 3rd pin left floating. I don't know what to make of that.I checked out the passives and found no shorted or open MLCCs or resistors. My inclination - since I have gone this far - is to simply replace the LM7171 (U101), and perhaps the 2 NPN transistors (Q100 and Q101). I already know that the voltages (+18 and +10.00 regulated) going into this subcircuit are spot on. I don't know what to make of the varactor, other than to perhaps just leave it alone and see if the LM7171 is really at fault.Any thoughts?

author: rubidium
author_rank: Regular Contributor
timestamp: July 18, 2020, 05:37:36 pm
content: More insight on the weird varactor diode piggy back arrangement. Both 3-pin packages have SMD codes 5L, which indicates that both are MMBV609. So the arrangement on the board is to have 3 varactor diodes in parallel, despite the schematic indicating 2 diodes in parallel. Perhaps adding a second package and using only 1 of the 2 diodes contained within was a tuning tweek. So I'm not going to worry about it and just leave it alone. I have no trouble whatsoever tuning the output frequency within the specified tuning range (0.02Hz) by applying an external 0-5V level to the frequency adjust pin, so there's no issue with any of this. It just initially looked like a hack job to me, but probably each of these units has some degree of personalized hand tuning in places.I'm still appalled by all the solder balls and fine metal shavings embedded in flux. That's more the style of a cheezy Chinese power supply than something I would expect from SRS.Needs a careful swabbing out with IPA. I have some op amps ordered from Mouser and, once they arrive, I'll see if that was the culprit. Difficult to diagnose a problem when you can't power up something and probe around with a DMM or scope.

author: rubidium
author_rank: Regular Contributor
timestamp: July 18, 2020, 10:04:59 pm
content: Hmmm ... now that the board is out on the bench, and taking another look at the schematic, I decided to just put 18V into the board. No heater, no anything else. Surprisingly, at the RF output on J100 I'm seeing a 1.05Vpp 10MHz sine into 50 Ohms!Sure, it's not temperature controlled or disciplined by Rb, but it's *about* 10MHz, a nice clean sine, and I'm shocked that the amplitude of the output is more than acceptable. Seeing this, I can hardly blame the LT7171 or any other component on this board. So what did I do, other than strip everything down and clean up the board bearing the OCXO with IPA. Could I have removed some crud that was previously attenuating the output? Do I dare go through the saga of reassembling everything and hope that problem has been solved? I guess that's what I need to do ...Update: Success!!I put everything back together and we have the proper RF output. Now that the microcontroller is back to being involved, the output amplitude actually increased to something close to spec (i.e. 1.41 Vpp) So the original problem must have been a partial short inside the OCXO assembly. After I meticulously picked out all the solder balls and metal shavings under the microscope, and swabbed everything with IPA, I apparently removed it. How that stuff got in there is beyond me, but the bottom line is that i have a working PRS-10 now.Thanks to everyone for the comments and suggestions ...Jim

author: edpalmer42
author_rank: Super Contributor
timestamp: July 18, 2020, 11:19:44 pm
content: I'd guess that there was a bad solder joint that looked like a high impedance in series with the signal. When you took everything apart and put it back together, you resoldered the connection. Considering how toasty the inside of a Rb standard is, I'm surprised that this type of problem doesn't occur more often.Regarding the flux residue, it's almost inert and doesn't usually cause a problem. Exceptions are in high impedance circuits and possibly under high humidity conditions. However, there are an endless number of secret recipes for flux so you have to be careful. Combine the wrong type of flux with poor or no cleaning and you could have a real problem on your hands.I've seen many messages that suggest that flux residue indicates sloppy workmanship. If you're a manufacturer, you spend time and money on things that make a difference. Things that 'look pretty' aren't even considered.

author: rubidium
author_rank: Regular Contributor
timestamp: July 18, 2020, 11:52:39 pm
content: I really wasn't overly-concerned with the flux or with the solder balls. I've encountered that on many PCBs, including my own, and the combination is generally harmless. I was surprised to find fine metal shavings - presumably aluminum - some of which were a millimeter or 2 long while others resembled "dust." My suspicion is that this OCXO was opened up at some point in its (presumably long) telecom history, and the shavings got introduced then. Given the radius of curvature of some of the longer shavings, I'm guessing that they came from the 2 tapped holes in the aluminum block that are used to secure the 2 TO-220 packages used to heat the block. That would support the suspicion that this block was opened up since the unit initially left the factory - perhaps more than once. Perhaps under humid conditions and amid vibration these shavings found their way into the wrong place(s) and became embedded in the flux residue. At that point, someone probably called it an unsat unit and it made it's way to eBay. Who knows the details in the sordid lives of these ex-telecom components? In any event, I now have a stable working PRS-10 and I'm pleased with that.

author: rubidium
author_rank: Regular Contributor
timestamp: July 20, 2020, 03:03:56 pm
content: Now that I have the PRS-10 working sensibly, I thought of playing around with disciplining with an external 1pps from GPS. I've read a lot here about the pros and cons of doing so, but at this point I simply want to explore the process. As I go about it, I have a more fundamental question that I've not seen raised or answered in any of the previous posts: There are internal resistors that configure the device for external 1pps, since the pin in question (pin 5) is labeled as both an input and output. I checked these and the device is indeed configured for 1pps input. However, as the output of the Rbmon program indicates, there is this red box next to the TO variable, and I am unable to change it's value. When I captured this screen, I had connected the 1pps output to the 1pps input as suggested in the documentation (on page 32) and, as the documentation suggests, I should be able to issue the command TO-1835 in order to shift TT to 0. However, TO appears to be locked and I cannot change its value.Any thoughts from someone who's been down this path before?Thanks,Jim

author: mikepi
author_rank: Newbie
timestamp: July 20, 2020, 04:26:28 pm
content: Jim, Congrats on getting the PRS10 to work. I enjoyed reading about your path to work it thru. Page 33 of the printed manual and page 35 of the PDF manual Version 1.5, at the second line from the top contains a NOTE: Firmware revisions prior to Rev 3.23 don't allow user TO! commands. I found this when I tried to perform the same procedure as you. Luckily I had a spare with Ver 3.24 and it worked o.k..I did get my PRS10 to discipline to a HP 58540A GPSDO 1PPS signal and tracked it for 2 weeks with Lady Heather and I could watch it smoothing out on the graph. I'm a new to this and enjoy playing with the OCXOs and Rubidium standards. You guys are way ahead of me in this area and I'm always learning something new.Thanks, Mike W7GHZ

author: notfaded1
author_rank: Frequent Contributor
timestamp: July 20, 2020, 05:38:23 pm
content: Congrats on the fix!!! The SRS PRS10 is pretty nice Rb from my experience. Even when running in free run it's not a bad reference. As you're finding out some firmware versions are probably preferable. The units aren't quite as accessible as the say Efratom FRS-K or M-100 with all through hole parts but they're still able to be worked on. It still amazes me everytime I look at one of these that we can do all that in such a small space. Now we've got Microsemi SA.35m starting to hit the used market... talk about small! It's getting so small it makes me wonder how they can last (with Rb reserve) and I have doubts whether you could even work on these newer units.Bill

author: rubidium
author_rank: Regular Contributor
timestamp: July 20, 2020, 05:41:51 pm
content: Mike: Yup, I read that but interpreted it verbatim as not allowing "TO!" commands - i.e. the command used to write the current value of the time offset to the unit’s EEPROM. I didn't think it discounted doing a simple TO<value> command (although the red box next to TO in the status page wasn't encouraging), and I thought I could at least get the change to endure for the duration the unit was powered up. Apparently not. Oh well.I, like you, am starting to enjoy playing with OCXOs and Rubidiums. I actually had this PRS-10 (an eBay find at a low price) sitting around for about 3 years, powered it up once or twice, but never advanced much beyond that. Just in the last couple of weeks my inspiration to get back into these things was rekindled. I'm not looking for time-nuts level of accomplishments, but I would like a nice stable, low phase noise, reference for my home lab. It would finally be nice to get my 53132A, 8560EC, and 8664A to all sing to the same sheet of music - particularly after repairing the 2 step attenuators in the 8664A and getting the PRS-10 RF amplitude back out of the grave to where it should be. GPS disciplining the PRS-10 is just a "nice to have" for me.Thanks,Jim