Symmetricom TrueTime XL-DC GPSDO

[Colin1256]

Hey all,

I've gotten myself a Symmetricom TrueTime XL-DC GPSDO (Model 151-602, with the large backlit time display) that I plan to use as my lab Frequency Standard. The Seller (eBay) indicated that it has 1 MHz, 5MHz and 10 MHZ Frequency outputs in addition to the 1PPS-Pulse and an IRIG-B Output; so it's got everything I want to have in terms of outputs and also looks cool.

I've cracked it open yesterday and had a look inside. Very clean design, I think. I also made sure it powers up properly. I'm still waiting for the 12V-rated antenna, so I can't test the GPS-Side of it just yet.
There's a number of BNC connectors in the back, three are fixed (labeled Antenna, PPS and IRIG-B), whereas the other five are unlabeled. Thesy are connected to the 87-6XX Mainboad using jumper-wires and it would appear that they can be configured for various things as per personal preference.
Unfortunately, there is little labeling on the boards; just the regular Component-Designators.

I'm planning to change a few things on this particular unit:
For instance, it currently relies on an internal TCXO. Being in an urban area with plenty of higher buildings and no roof-access, I'm not sure if it will be able to see enough satelites for a reliable GPS-lock at all times; so I'd like to increase the precision of the unit when GPS is not available.
It seems like Symmetricom used to manufacture add-on cards with higher accuracy (OCXO / Rubidium) frequency standards, but these seem to be very hard to come by today. Fortunately, it supports a configuration where it can be tied to an external clock that I'd like to use with an external OCXO; assuming I can wire and jumper it correctly inside.
Furthermore, I'm going to add some buffer amplifiers which will allow me to connect it to a number of devices in the lab to the unit in parallel. My plan is to install these in the internal bays and just add BNC-Connectors to the currently blank slot-covers.

I do have a manual for the unit, (http://glacier.lbl.gov/gtp/DOM/Support/xl-dc-manual.pdf) which refers to a "87-6XX configuration drawing". This supposedly identifies all the jumper settings and connection points of the main-PCB.

Does anyone have this particular drawing or even better, a service manual for the unit that would help me get started?

Thanks a lot in advance!

Cheers,

Colin

[ZigmundRat]

A little late on the reply here, but hopefully this is still of value.

A more complete manual is available here: https://ics-web.sns.ornl.gov/timing/151-652-893man.pdf and has the 87-6xx description/diagram. However, it's still not a complete manual. There are no schematics or service manual available.

The TrueTime XL-DC although old, is still a decent unit. I have four of them - two rubidium and two with OCXO. One is a 12v unit that WAS configured for a downconverter. I modified that unit back to GPS (not for the faint of heart, believe me). Besides that conversion, I have also switched 12v units to 5v (and back), replaced the rubidium oscillator, etc.

A few observations:
The jumpers indicated in the above referenced doc regarding the 87-6xx configuration are not all pin jumpers. Some are 0 ohm resistors.

Consider that you'll probably wind up with more than one GPS or GPSDO and need a way to feed them from one antenna. Get a DirecTV 4 way splitter ('2-2150 MHz Splitter SWM 4 Way') or something very close. This is a satellite splitter that has one DC passthrough so you can power the antenna. Put your more modern device that provides the right power to the antenna on that port. On another port (that is DC blocked) connect an inline satellite antenna amplifier and your 12v XL-DC to that. The DBS inline amps need more than 5v to work right and they work great with the XL-DC. DONT go looking for the maximum gain amp - just a regular one will do. Although the XL-DC GPS is usually a little deaf you don't want to swamp it either - the performance will go DOWN. More is not always better.

You can convert from 12v to 5v antenna without too much trouble - but you may not want to. OR at least it's not a big deal. But 5v antennas are easier to come by.

Firmware is in ROM. Different revisions XL-DC have different firmware of course, and the rubidium appears different than the OCXO. The DAC slopes are different between the rubidium and the OCXO. There are several different GPS engines used in these, but all Trimble I think. Haven't seen anything else. I believe they started with the SV6, some have SV6-CM3, maybe SV8, and I have one with an ACE (I forget whether I, II, or III) which uses a daughter card with FPGA to do sentence translation making it backward compatible.

As for other cards, they are sometimes available. You just have to watch for them.

I don't know if it's possible to easily get a unit to work with an external OCXO. This is labeled and 'option' and I don't know what's required to enable it. It could be as simple as making the right jumper options (as on the 87-6XX document) but it's note entirely clear what those are. But these units also have NV storage, and I suspect that there are flags in there which determine the enabled options. Don't know for sure - just guessing. If you have figured out how to use an external oscillator that would be good information to have.

Hope this helps. Please update the thread with any progress you may make or additional XL-DC info. I'll try to do the same.

Ziggy

[pacAir]

Hi guys!  I am working towards a similar conversion on my TrueTime XL-DC (w/ onboard up-converter).  I'll share a few quick scans out of one of my XL-DC manuals not only showing the jumper options but also the top and bottom artwork locations showing where all these jumpers are located.  I included a short document describing a few of the initial changes I did to the circuitry towards this end (based on information in these scans and actual circuit measurements).

If the system will allow me to post this way (they are not all small enough to post here):


TrueTime XL-DC T/F board Bottom Jumper Locations

TrueTime XL-DC T/F board Top Jumper Locations

TrueTime XL-DC T/F board Option Jumpers

TrueTime XL-DC Antenna Configuration Changes


Hopefully these links will work and that this info might help.


Steve
WN7VHF

[pacAir]

While I am thinking about it, I scanned a few additional pages for you.  Sorry about the quality but this is the best I can do without disassembling the manual! 

Two pages relate to the TrueTime 142-602 downconverting bullet antenna that works with the XL-DC receivers with on-board upconverters:

TrueTime 142-602 Down-Converting Bullet Antenna #1

TrueTime 142-602 Down-Converting Bullet Antenna #2

(Please note that this is NOT the same up/down conversion system used by the later 142-6150 external upconverter box and the 142-615 Downconverting antenna.  These are using different L.O. and I.F. frequencies than those used with these internal upconverter units and the bullet antenna shown!)


The final scan shows the header pin locations that the unmarked rear BNC jacks can be connected to for the various output options (Note: Some of these BNC connections require a matching option board.):

TrueTime XL-DC Rear BNC Configuration Header Connections


Steve
WN7VHF

[ZigmundRat]

Just to provide some information on what was involved in the conversion of a down converter unit to one that uses a standard GPS antenna, I've written up some of what I had to go through. I believe it is mostly complete. In order to do the conversion I had to source a replacement GPS receiver. I found that at least some of the Placer 400 GPS units have these SV6 modules inside - and they usually go pretty cheap.

For reference, I've attached a pic of the down converter receiver board - a modified Trimble SVeeSix (SV6). Notice the depopulated sections and how there are pins in place of the DB9 connector of a 'normal' SV6. In the bottom view, notice how the oscillator signal is routed from the MCX connector to the C93/R15 pins via twisted pair. Also how the left side has essentially no power supply components populated. Power is taken from a 3 pin connector near C136 (removed in this picture). One of these pins is battery backup.

In the top view, i have temporarily mounted a MCX connector (this is usually on the bottom for connection to the CPU/main board) so ignore that. Notice the lack of oscillator and components in the upper left. On the right middle you can see the three holes for the power/battery connections to he CPU/main board.

Doing the conversion of the standard SV6 to a version that can be used in the XL-DC involved switching the serial port configuration and line drivers, depopulating the power supply and oscillator section (the XL-DC has it on the CPU/main board), reprogramming the data stream from TAIP to TSIP, and ultimately transplanting the serial EEPROM (below the chip labeled 22880-85-B8) from one board to the other. The DB9 is missing for the second port as is the RS232 driver at U81. Instead of U81, U82 is populated and different passives are there (or not). There is also the change on the CPU/main board where a capacitor must be added and a resistor removed. This is documented in the jumpers setting PDF that was uploaded by pacAir.

Once I am in a position to take my unit down fro a while, I'll try to get some picture of the replacement SV6 as modified and update the post.

[pacAir]

As is often the case with different generations of TrueTime gear, my up-converting receiver board looks ENTIRELY different than this!

Steve
WN7VHF

[ZigmundRat]

Can you post a pic? I haven't pulled my converted unit yet (so no pics yet) as I'm STILL cleaning up my pigpen of a lab

[jeffp8]

Greetings.
I am new to this forum. In fact I joined specifically because of this thread.
I have a TrueTime model XL-DC that I acquired awhile back in non-working condition.
I fixed a power supply problem and replaced a bad lithium battery and the unit "seems" to be working. I say "seems" to be, because It will never GPS lock or PLL lock.
It tracks satellites and displays the correct time and position but the time display always shows the letter "A" on the far right and "unlock" on the far left.
I noticed while going through the menu, the DAC is maxed at 32767. This only happens when satellites are acquired. I figured the the oscillator may have aged out since it was 20 years old, so I sent it in to Wenzel Associates for evaluation. The said it was bad and not economically repairable. I purchased a new oscillator, but the symptoms remain.
Can anyone familiar with this model suggest a course of action?

Thanks,
jeffp8

[ZigmundRat]

Sorry to be blunt but:

The first question is whether you are capable of doing component level repair and troubleshooting. If so there could be a further conversation.

If not then this is most likely unrepairable due to cost and even finding a suitable person/shop to do the work. Your best course of action would then be to resell it as-is and try to recover some of your expense, then replace it.

[jeffp8]

I am capable, I just have zero experience with GPSDOs. Until I obtained this unit, I had never heard of these. I've been reading all I can find on the subject and all I can find on the truetime product.
It seems to me, since I have a new oscillator on board, that the problem lies either in the "sampled" oscillator signal, or the GPS "reference" signal accurately getting back to the microprocessor. The DAC output is reasonable at startup, around 2650. Once satellites are acquired, the DAC output goes full scale, 32767.
My understanding is that schematics are unavailable for this product so I'm trying to trace this out.
Any tips anyone can provide would be appreciated.
Thanks

[ZigmundRat]

Ok, that's great to hear.  At its simplest the GPSDO typically compares the 1 pulse per second from the GPS to a 1 PPS derived from the OCXO and steers it via a PLL with a long time time constant. The processor does that via the DAC output which is applied to the OCXO , either directly or indirectly.

Since your XL-DC reports a DAC output change the next thing would be to make sure the DAC voltage is actually changing. You can check that directly on the OCXO EFC pin. There is also a jumper on the board that sets the DAC range to either -5 to +5 or 0-10. You should probably make sure it's set correctly. Check the earlier posts in the thread for links to docs.

You mentioned that you replaced the OCXO, and I seem to recall it was from the OEM with the same model. If so, great. But if not an identical replacement the EFC polarity, slope, or range could be wrong which will of course prevent its being steered correctly by the processor.

[jeffp8]

I had a little time to look at this today.
Confirmed jumper setting for -5/+5 control voltage. The voltage / DAC value increments about every 50 seconds for 3 intervals, then waits 90 seconds and increments again. This repeats until the voltage hits +5vdc and the DAC is at 32767.
I do see a 1pps square wave on the 86-357-9 pcb.
Am I correct in assuming that there should be a 1pps signal from the Trimble SV6 board on what would be pin 9 of the non-exsitant DB-9 connector?
I am measuring a constant +5vdc at that point.
I haven't had a chance to trace out where the 1pps signal originates yet. Looks like that trace is buried on an internal layer.
Do you know where it originates?

Thanks for your help.
Jeff

[ZigmundRat]

I assume you know whether or not the -5/+5 is correct for your OCXO. The unit I have with an OCXO has it mounted on a small board behind the big LCD. As I recall it's an Oscilloquartz, the exact part number I would have to check. I will try to remember to open that unit up and check the OCXO part and the DAC jumper setting.

You don't mention if you have any other option boards installed but if you do, pull them and make sure the problem persists.

Which exact GPS receiver do you have? They used several - all Trimble AFAIK. At the least, there have been the SV6 and the ACE III on an adapter card with FPGA.

The PPS should appear on pin 9 of the GPS receiver. However it is very short, about 10uS. You will most likely need a digital oscilloscope to see it. You won't see it on a meter and probably not on an analog scope.

As for the PPS on the controller board, I have not tried to trace it so I can't help much there.

[jeffp8]

Yes, -5/+5 is correct for my OCXO.
No other option boards are installed.
GPS receiver is TNL 22880-B. Googling that indicates it is an early Trimble SV6.
Pin 9 is at a constant +5vdc. Using the Oscilloscope, I see no activity on this pin.

Questions:
1) Is it possible to measure your pin 9? Is it at +5vdc and pulses low, or at 0vdc and pulses high?

2) Does the "A" in the left most position of your LCD go away as soon as the satellite is acquired? Mine remains regardless of how many satellite have been acquired.

Thanks again for your help,
Jeffp8

[ZigmundRat]

I just powered up my #3 XL-DC. This has OCXO, one labeled TrueTime OCXO 8666-C2S, which is really an Oscilloquartz 8666. Which btw, has a 0-10v EFC. This is attached to the little PCB behind the large LCD at the front (see pic). The other OCXO is the low phase noise option.

The GPS receiver in this is an Ace III with a converter board, but it will still put out PPS on the same pin of course. I have the same SveeSix in two other units - they work.

Second pic shows the 10uS negative going PPS pulse on pin 9. What kind of scope do you have? Again, you will need a storage or digital scope to see this pulse.

 On this particular power on, once the display went to Mode: Time 3D the A changed to flashing *, then turned off. Unlock also turned off at this time. Sometimes the timing is a little different. The A is 'acquisition' so should go off after it sees enough satellites. It may need to acquire an almanac before it goes off, not sure. Unlock should go off after it is tracking, has a good fix, and has PPS within tolerance. Note that it wants to take 90000 fixes to set its position. It is possible to have no A or * and still have unlock.

I know you feel it's correct but I'm  skeptical of the -5/+5 EFC you say you have. If the DAC is railed it could just be the range is wrong. What was the exact model of the original OCXO?

I've updated this post to include a pic of the PPS on the SVeeSix GPS (TNL22880-B like yours) in my XL-DC #4. It's not as clean, and it's not as wide either. It's < 2uS compared to the 10uS of the other unit. Another point of reference I guess.

[Colin1256]

Hello everyone!

I'm sorry for having been away for a while.

ZigmundRat: Thank you very much for the link to the manual, this one contains a lot more information than the one I have been able to find.

My XL-DC is running happily, except that it has once lost PLL Lock and couldn't recover from that. I have since changed the internal battery and the problem hasn't returned. I think it's unlikely that this was the cause, but hey, whatever I did; it seems to have helped. :-)

I did get the original Symmetricom 12V Antenna with the unit, so at this time I'm not really interested in converting my unit down to 5V, though it is good to know that it can be done this easily.

Other than that, I'm still hoping that one of these days I will come across a kit or schematic that will enable me to upgrade my unit from the standard simple TCXO to at least an OCXO.

Cheers everyone!


Colin

[gpsdo_nut]

Ran across this topic today and it piqued my interest.  Though older, these XL-DC servers are solid and fun units to work with.

I also have a small arsenal of XL-DC units that I swap into action from time to time - XL-DC with the standard TCXO, one with the high-stability OCXO and finally one with a rubidium.  While I'm not especially versed in the technical side of these units - I've found that it appears the only real way to convert these units between different oscillators is to swap out, or reprogram the PROM chip (and I don't have expertise in PROM reprogramming).  Specifically it's the chip with the software label on it on the CPU-XL board.

That chip apparently holds the software for several key functionality:
- Ability to communicate with the specific type of GPS receiver you have (I believe the SVEE-Six and ACEII/III speak the same language, but requires a different daughterboard to convert to the proper frequency for the internal GPS timebase)

- Timing constants for your time base - for example, my XL-DC examples have the following 'Response Time':
TCXO: 15 seconds max
OCXO: 200 seconds max.
Rubidium: 2000 seconds max.

- Volts/Hz DAC stepping to properly lock-in the oscillator

As a fun test, I took out my TCXO XL-DC's software PROM chip, as well as my SVEE-Six + daughterboard and transplanted it directly into the CPU-XL of my OCXO unit.  While the unit does perform lock, I was getting super-noisy oscillator phasing due to the low response times of 15 seconds (remember OCXO's were configured with 200s).  At one point during initial locking, most likely due to the incorrect Volts/Hz stepping figure, It was having a hard time keeping the OCXO under control with the DAC wildly moving out and then back in, back out, and back in....  Eventually after locking, the DAC was being moved around like it was trying to discipline a 'noisy' TCXO...

So those are the things to look out for if you're still thinking about converting timebases, a large hurdle will be the software itself.  To me, probably not cost effective, and look toward eBay for a OCXO-capable unit.

[ZigmundRat]

All in all, I think these XL-DC units are pretty good. I wish they were more tunable (ala Trimble Thunderbolt) but for all practical purposes they do fine.

The firmware seems to be picky. I have low phase noise options in two of my units and I can't swap those cards between boxes. One is 'sys ver 020', 'GPS-XL V1.036' & 182-6200v002 with the other 'sys ver 030', 'GPS XL V1.047' & 182-6534v003 so apparently not the same at all. The 1.047/182-6534 unit has an ACE III GPS receiver with daughter card in it. Maybe we should start grabbing some ROM images and sending them to KO4BB. Then maybe some field upgrades from TCXO might be possible. AFAICT the Rb and OCXO both are just tacked onto the backplane. So with the proper firmware it could be possible.

Some additional information for your time constants:
TCXO with LPN: 2.121e+03 seconds
OCXO with LPN: 2.121e+03 seconds

So... I really don't know which oscillator is getting steered or for that matter which one is output on the CPU card. The LPN card has its own outputs - but since both TCXO and OCXO have the same time constant, I would think they are both steering the LPN OCXO - not the TCXO or 'normal' OCXO. If I get ambitious I'll pull the LPN card and see if/what the value changes to.

As I recall, my Rb unit has a time constant of 5000 seconds but since I'm not sure I turned it on and will update with the actual number.

** Confirm 5.000e+03 for Rb

[gpsdo_nut]

Yeah, definitely very decent units.  With additional option cards like N.1, N.8, FTMIII, external oscillator disciplining (getting close to Thunderbolt tunability) and the LPN would be an XL-DC that does it all.    I'm also curious as to what happens when the LPN card is pulled... One of my units indicate a LPN status in Function+73, so I'm assuming this may be a case where it may apply new oscillator characteristics if LPN is present (and may be the only FW that supports LPN).

My current house standard is my Thunderbolt which has been humming along for the last month (by the way, as of 7/19/2016 LEAP: PENDING! is present!).  Because of it's tunability, I've been flirting with the idea of replacing the standard OCXO with a different unit...  However, it currently ain't broke so I feel less inclined to mess with it.

And I'm pretty certain you're right - the Rb standard has a TC of 5000s, not 2000s.  It's been a while since I've fired mine up too.

[ZigmundRat]

I kind of forgot about it until I went to pull the cards, but one LPN module is 5MHz and the other one is 10MHz. The 5MHz unit is in the chassis with the 10MHz OCXO behind the front panel. The 10MHz unit is/was presumably in a plain TCXO unit (more on this).

I pulled the LPN module on both units. The 'TCXO' unit with the 10MHz LPN module removed does not show the clock in the big display, so the LPN module would seem to be the clock source in that unit - not the TCXO. The other one (OCXO) with the 5MHz LPN module removed still functions normally and goes to 2.121e+03 as before. I will also swap the CPU modules to see what happens when I take the one from the chassis with no display and put it in the one with the OCXO. I bet it will come up. I will also compare both processor modules and all the 'jumpers' to see if there's any difference.

All of my units have an 'auxiliary' 10MHz oscillator: either a Rb, an OCXO, or a 10MHz LPN module. So I don't know if there is a XL-DC that has ONLY the CPU card with a TCXO. My CPU modules seem to require an external 10MHz.

@gpsdo_nut : Do you have an EPROM reader/burner? Is there any chance I could get ROM images? I'd be happy to trade and as I mentioned before we could post them up to KO4BB as well. I really kind of interested as none of my units have anything in Function 73 regarding LPN status (and I have two units with LPN). Do any of yours have Function 07 (external oscillator control) enabled? I have a couple of Austron 1250A that would be interesting to discipline.

[Colin1256]

Very interesting indeed.

My XL-DC is a very "bland" one; it just has the standard 87-610 card with the GPS receiver, the other bays are empty.

@ZigmundRat: you're mentioning an 'auxiliary' 10 MHz Oscillator. Could you tell me a little more about that? How did you connect it? Maybe this would be a way forward for me; use one of the two unused bays for a custom board with an OXCO and feed that back into the unit as external 10 MHz Source?

Does anyone have any information on the pin-headers towards the rear of the unit? A lot of them (but not all of them) have black/white twisted-pair wires installed on them that lead to the BNC connectors (my unit is wired for 10 MHz, 5 MHz and 1 MHz Outputs, along with 1PPS Pulse). I'm curious as to what the unused pins can be used for. Any idea?

Thanks!


Colin

[gpsdo_nut]

The 'TCXO' unit with the 10MHz LPN module removed does not show the clock in the big display, so the LPN module would seem to be the clock source in that unit - not the TCXO.

Huh.  Well that's interesting...  Does this particular TCXO GPS-XL board have two oscillators - or just the 16Mhz TCXO?  The 10MHz TCXO would have been next to the big Xilinx chip.  If you don't have it, your GPS-XL is indeed externally clocked.

The other one (OCXO) with the 5MHz LPN module removed still functions normally and goes to 2.121e+03 as before.

This one baffles me too - on paper I'd assume the GPS-XL would have been disciplining the 'main' 8666-CS2 osc. while the LPN (or GPS-XL) disciplines the Wenzel against the main...

So I don't know if there is a XL-DC that has ONLY the CPU card with a TCXO. My CPU modules seem to require an external 10MHz.

That's because you have nicely optioned XL-DCs.    My TCXO-only unit is exactly as you described - and it has that 10Mhz TCXO next to the Xilinx chip.  I suspect yours just has the part outlined and is not fitted with the TCXO.

@gpsdo_nut : Do you have an EPROM reader/burner? Is there any chance I could get ROM images? I'd be happy to trade and as I mentioned before we could post them up to KO4BB as well. I really kind of interested as none of my units have anything in Function 73 regarding LPN status (and I have two units with LPN). Do any of yours have Function 07 (external oscillator control) enabled? I have a couple of Austron 1250A that would be interesting to discipline.

Unfortunately I don't - I've kind of wanted a reader/burner for a while for other projects but those never took off either.  Also I lied - Function 73 LPN is available on a XLi (TCXO-unit), not XL-DC.  I really meant Function 72 where I have a "LPN OK" fault flag.  I'm pretty sure yours has that too.

None of my units have the external osc. control ability - that would have been neat.

[pacAir]

Does anyone have any information on the pin-headers towards the rear of the unit? A lot of them (but not all of them) have black/white twisted-pair wires installed on them that lead to the BNC connectors (my unit is wired for 10 MHz, 5 MHz and 1 MHz Outputs, along with 1PPS Pulse). I'm curious as to what the unused pins can be used for. Any idea?

I posted a link to a page out of one of my XL-DC manuals that describes those headers in a post nearer the top of this forum page.  The link is:

TrueTime XL-DC Rear BNC Configuration Header Connections

Just be aware that some of those options are only valid if the supporting hardware plug-in is installed.


Steve

[pacAir]

Judging from the current threads it is obvious that there is a hunger for more info regarding TrueTime products!  Perhaps we may be able to answer some questions by "pooling" our resources.  Namely, if everyone could document the following pieces of information regarding their unit perhaps we can learn something from comparison?

This list is my starting point.  I recently started documenting my own TrueTime "pile".  This includes GPS-DC, XL-DC & XL-AK receivers, plus TCU-800 Time Code generators/translators, which have their own plug-in family (some models even have their own built-in GPS receiver) and a few of the LED Time Displays (RD-1, RD-2). 

For the XL-DC family, my initial documentation list includes:

1.   Model Number
2.   Warranty Expiration (if labeled, provides a rough date of deployment)
3.   TrueTime XL System Version (Shown at power on)
4.   GPS-XL Version (Shown at boot-up)
5.   EPROM Part Number (TrueTime p/n for programmed firmware chip, appended on LCD screen to #4 above)
6.   Antenna Type & Voltage setting (Antenna is either Standard or Down-Convert... Down Convert units have aluminum label next to Antenna Jack & the GPS Receiver board)
7.   GPS-XL CPU Board (Model, Firmware & Version marks)
8.   GPS Carrier Board (Model, Firmware & Version marks)
9.   GPS Receiver Daughter Board (Model, Firmware, Chip & Version marks)
10. OCXO Option Board (Model & Revision) (If installed)
11. OCXO Oscillator (Manufacturer and all data) (If installed)
12. Type of power supply (AC, DC or both)
13. List of all installed option boards (Name, Model, Firmware & Version marks)
14. List of all installed options (Oscillator type, Plug-in Card Names, LCD Backlight, Power Supply, etc.)
15. Revision History
16. Internal & External Photos

Item 15 above is based on the multiple Assembly part numbers each particular unit went through before reaching its final configuration.  Each time the unit reached a milestone it had an assembly label placed on the bottom or side of the chassis (with part number and revision).  If the unit was grabbed off the shelf and modified or added to,  other labels showing these steps were added.  Some of my units have 6 or more of these small labels!

One thing I learned about the XL-DC and XL-AK receivers is that if the Model Number has a "-1" appended to it, then that unit is equipped to use the Trimble "bullet" antenna with the built-in down-converter circuitry that is NOT compatible with the later TrueTime down-converting antenna with the separate up-converter box that it is mated to.  Finding these older down-converting antennas is VERY difficult (and usually quite expensive) as they were usually left on the roof when the receivers were decommissioned and were surplussed "as-is" without them.

Examples:

Standard Receiver (my #1 XL-DC): Model 151-602-817-12
Down-Convert Antenna Unit: (my #2 XL-DC): Model 151-602-586-1

Every unit I have seen (on eBay & elsewhere) that has a "-1" at the end of its model number also has the "Down Converter Required" label next to the antenna jack and on the GPS receiver Carrier Board internally.  IMHO, unless you already have the correct antenna, these are best left for spare parts or avoided altogether.


Steve
 

[ZigmundRat]

First, let me clarify what I said about the two LPN modules. They are both 10MHz output, but the one with the Wenzel 5MHz OCXO has an X2 MiniCircuits frequency multiplier. So both are 10MHz output.

@gpsdo_nut - To answer your questions:
1) Both GPS-XL modules have the VCTCXO but appear not to use it. In both chassis, neither GPS-XL init the big clock if only the GPS-XL module is plugged in (and OCXO unplugged). 
2) The behavior is slightly different between the OCXO unit with it's Wenzel LPN module, and the non-OCXO unit with its Oscilloquartz LPN module.
    - Both GPS-XL modules work in either chassis
    - In the OCXO chassis, neither GPS-XL modules init the big clock with the OCXO unplugged from the backplane
    - In both chassis, both GPS-XL init the big clock if the Oscilloquartz LPN module is plugged in
    - In both chassis, neither GPS-XL init the big clock if the Wenzel LPN is plugged in
I also have a LPN status in F72 - I never spent much time in the fault screens since everything works 

As for EPROM burner/reader I have found the TL-866 to be a good value (OEM page: http://www.autoelectric.cn/en/TL866_main.html - Amazon,eBay, etc.) and has worked for everything so far. You might need to get more adapters, but they aren't that expensive.

@Colin1256: By 'auxiliary oscillator' I mean there is an oscillator not on the main cpu board. For example, the TCXO units apparently only have a VCTCXO can on the CPU board. No other oscillator. All of my units have an additional oscillator - an OCXO (mounted behind the big clock display), a LPN module, or a Rubidium.

@pacAir: That's all good info to collect. I don't know how but maybe someone could create a shared spreadsheet or something where we can compile that information. Google docs or something? I dunno.

The more I look into these the more it looks like the firmware is customized for each unit built. (Could be wrong of course.) One of the reasons I say this is because my units won't operate alone with their own VCTCXO. You'd think they would at least do that. Also, the CPU from the Rb units will not recognize the LPN modules. I am guessing that if we start collecting the info from the labels (151-...) and F18 we'll find a lot of variation.