HP55300A GPS Time Reference Source

[trobbins]

Hi, on the lookout for detailed description/service documentation for a HP 55300A rack-mount time reference with GPS input, and 10811-60159 OCXO.  This is a GPS disciplined 10MHz oscillator for TELCO applications and includes various timing outputs and monitoring/alarm functions and uses 48Vdc powering.

I can only locate the 55300A GPS TELECOM PRS USER’S GUIDE (Doc ID 097-55300-01) by Symmetricom from 2000, and the 10811 OCXO.  The other doc links on http://etoysbox.jp/1_TestEquipment/12_F_std/GPS/GPS.htm#55310A are no longer valid, and were not archived by the Wayback Machine.

I will work through identifying some simple powering tests, as the unit had a fault of some kind, but it would be great to get some more insight.

The main pcb assembly is HP 55300-20001, and the dc/dc pcb assembly is HP 58503-6003.

Ciao, Tim

[Wallace Gasiewicz]

From :  https://groups.io/g/HP-Agilent-Keysight-equipment/search?p=created%2C0%2C%2C1%2C2%2C0%2C0&q=HP+55300A+


"I have found the Manuals for this Instrument and will upload them to KO4BB. Gary "
So try KO4BB

BTW, I think this unit uses NEG 48 volt telephone supply. I think the telephone company 48 volt supplies are isolated though. They are pretty inexpensive. I picked one off the ground at a hamfest after the seller left it. It was half broken, now working well.

[trobbins]

Thanks for the direction to KO4BB - that site did have the remaining docs about the 55300A.

With a bit more searching it appears that the 55300A uses internal modules from the Z3801A GPS receiver, and in particular the dc-dc pcb module, which I will now go through as a first step:
https://support.ntp.org/Support/Z3801AReceiverModifications
https://www.realhamradio.com/GPS_Frequency_Standard.htm

Ciao, Tim

[trobbins]

Partial success so far.
 
The power supply pcb can be powered in isolation by removing its output socket, and applying power through either of the main redundant 4-terminal inputs, or via clips to the 96-pin connector (Fig.6-3), as those options include series diode polarity protection.  If the power supply for 48Vdc has any kind of foldback then the pcb may not power up due to monitoring of under-voltage - that happened on my normal benchtop supply.  All the test-point/control voltages can be confirmed using the info from realhamradio.com, and a schematic for most of the power supply pcb assembly (A2 Dc-DC power supply pcb assembly 58503-6003 RevD) was located on a bulletin board with a bit of googling (see attached) so any pcb issues can be deduced if needed.

With the power supply pcb reconnected, the self-test went well with no immediate alarms/faults.  An OCXO fault can be confirmed by disconnecting its control plug, as a RED 5 LED will enunciate.  Without a suitable active antenna connected then Red 5 LED and a front panel major alarm comes up after about a minute.

So far I can communicate to the unit using the Symmetricom SatStat app on PC/Win using the software in the link, but at the moment only through the Time-Of-Day DB9 port, via a x-over cable and through a USB-RS232 adaptor.  I'll have to recheck why I can't get comms through the Port 1 DB9, as that should be a direct connect to the adaptor.
http://www.neurotica.com/misc/58503_SatStat50.zip

I only have a cheapy active GPS antenna to hand, and with its SMA plug on a 3-meter lead and then jury-rigged to 2m of coax to the N connector, SatStat is not showing any sign of GPS acquisition success.  I may be able to avoid the coax portion, but it may be that the antenna gain and signal amp quality is not sufficient - so WIP on that.

It looks like the rest of the unit is operating ok, as on power up the SatStat status soon shows no Health Monitor errors, and shows that the OCXO is either warming up or ok, but it seems that after some time and failing to get any GPS signal it then shows error status on Self Test, OCXO, and GPS Rec.

A quick check of the 10MHz output indicates it is within a Hz or so of a vintage Systron Donner 6250A counter that I tweeked last year for 10MHz from a GPSDO.