Experience with Trimble 57963 GPSDOs bought off eBay. - Page 1
Forum section: electronics / general

author: WPXS472
author_rank: Regular Contributor
timestamp: June 04, 2020, 09:07:48 pm
content: There has been some activity recently regarding these inexpensive, small board GPSDOs. I already had the Symmetricom version of this I obtained some years ago. I wasn't impressed by its performance compared to my trusty Z3801. Since, the Z3801 has died, and I wanted to take a second look. In reading about the Trimble version, usually identified as 57963, or 57964, it seems that these are prone to having components missing, and other problems as well. I ordered one from seller mingxing200808 back in late March. It arrived just over a week ago. Upon inspection under a microscope, I found 2 inductors missing, and an op amp cracked. When I hooked it up, there was no 10 MHz output even after having been left on for hours. I initiated a return request from eBay. The seller responded a few days later saying that the contacts were probably dirty and to clean them. After I responded that there were no contacts, and the 10 MHZ was missing, they responded that I should just use the oscillator. After I told them that i wouldn't do that and wanted a refund, they replied that they couldn't give me a prepaid return label. After I contacted eBay customer service, they said I should mail it back using USPS. I checked the price for a small flat rate box to China, and found it would be $36.45USD. I relayed this to the seller and told them that if they would send me that amount, I would mail it back. Again, they said they were unable to give me a prepaid return label, and I should send it USPS. I contacted eBay customer service again and was told that it was the seller's responsibility to send a prepaid return label or else eBay would refund my money and I didn't have to return the item. Late yesterday, I received $5.00 from them via PayPal. This morning, I received a message saying to mail it to their US address, or, I could accept $25 and keep it. During all this, I ordered another unit from seller zyt_gps. It arrived yesterday. Examination didn't reveal any missing or broken components. I hooked it up and about twenty minutes later the 10 MHz output seemed to have "locked" to GPS. There was still some slip compared to my RFTG-u which has been on for over a year. I left it for maybe an hour, and when I checked it again, things seemed more settled, though there was still a fair amount of slip. 100nS every 2 minutes or so. Suddenly, it just jumped about 30 nS. I thought that was strange. I had never seen the output of a GPSDO suddenly just jump. As I was watching, both LEDS flashed red twice, them back to one green only. This was accompanied by another sudden jump. It does this every 30 seconds or so. Not good! I noticed that there are differences between the two boards, even though they have the same part number. The LEDs are on bottom on one, and on top on the other, and there are differences in some of the ICs. The LEDs act differently to what they do with the Symmetricom model. I don't know if I will be able to return this one because, in the very strict sense, it does "work". Just not correctly. I will message the seller about it. At this time, I can't recommend buying one of these devices. There just seem to be too many bad ones being passed off as good. The sudden phase jumps are, I think due to a problem with the OCXO. I can't see the control circuitry causing a sudden jump. The flashing red LEDs are a mystery. If anyone has a clue, I would like to know.

author: tkamiya
author_rank: Super Contributor
timestamp: June 09, 2020, 07:48:45 pm
content: If anything with OCXO was sitting in warehouse not turned on for long, that would explain some of what you are seeing.I've gone through dozens of OCXO so far, and some takes up to 2 months to settle down. Basically, you'll have to treat it as new and "bake it in." Perhaps GPS is yanking the EFC and the oscillator is not settled enough to maintain stable frequency between "yanks".GPSDO jumps sometimes when a satellite goes out of view and new one comes into view. Short time stability of GPSDO in only in order of 10E-9 or something.

author: WPXS472
author_rank: Regular Contributor
timestamp: June 13, 2020, 01:20:28 pm
content: I don't think it is an issue with the OCXO. The LEDs both blink red, like it was resetting, every 30 seconds or so. The seller, zyt_gps, read the above post and messaged me through the eBay messaging system saying that this was not a problem. Well, yes, it is. It is causing sudden phase jumps. I haven't decided yet if I am going to file a return request with eBay. The other unit, eBay gave me a refund of the total amount. I told the seller, after receiving the refund, that I would still return the item if they sent me a prepaid label, even though I didn't have to. They replied that they were just unable to do that. It seems strange that someone would set up to sell a large volume of stuff, especially surplus stuff, and not have a way to pay for returns, which are bound to happen. I am going to replace the two inductors and op amp, but I am doubtful that it will fix the unit. In the end, I have two useless units. Which is why I think it is not a good time to buy one of these units. I think they have been culled through and the good ones already sold. What we are seeing is what was left.