[time-nuts] Trimble Thunderbolt GPSDO Troubleshooting

Clint Jay cjaysharp at gmail.com
Fri Mar 25 15:09:11 EDT 2016


That 232 chip should run at near as damnit ambient so I'd say it's highly
likely to be dead.

Replacement should be a doddle
On 25 Mar 2016 19:02, "Ryan Stasel" <rstasel at uoregon.edu> wrote:

> All,
>
> I recently got an (obviously used) Trimble Thunderbolt (case says Rev B).
> Hooking it up to my bench PS, I got the unit fired up, and the OCXO
> relatively rapidly settled down to just over 26hz over 10Mhz (According to
> my Racal-Dana 1992, which I’ve found to be EXTREMELY accurate compared to
> an Rb and other GPSDO). I thought that meant the PLL had locked, but I’ve
> since reconsidered since it didn’t take that long (looks like it takes
> quite a bit longer than that for the GPS to do a survey). Sadly, I couldn’t
> check the serial connection because I left my adapter at work, so I left it
> for the night.
>
> Next day, I hooked a serial connection up, and got nothing at all.
> Checking the TX pin of the unit with my scope, I could see bursts of data,
> but with less than 1V deviation from the idle voltage level.
>
> So, I opened the unit up, and a quick inspection showed me pints 78 and 79
> (according to data sheet, those are TDO and +5V) on the Xilinx XC5202 were
> bridged with solder. That’s odd. Also had obviously been reworked at some
> point due to the heavy amounts of flux residue around. After finally
> finding a good image of this area online (
> http://www.thegleam.com/ke5fx/tbolt/conns.gif), I determined that they
> shouldn’t be bridged, and fixed that. Powered the unit back up, and still
> nothing. Also noting that the 5V rail was drawing over 600mA. Finger
> testing around the board quickly found the RS232 chip (232IBE, now called
> an ICL232) to be HOT! That it gets so almost immediately after power on.
> Checking the pins, the +5V and GND for that chip show about 54ohms
> resistance. I checked the support tantalums and ceramic caps around it, but
> they all show higher resistance (Keithley 6.5digit meter for the win).
>
> So, my questions to all: pull the IC and see if there’s any data showing
> up on pin 11 or 10 (haven’t traced out which is being used), just replace
> the chip (I _MIGHT_ have an extra (ICL232, MAX232, etc) in a drawer
> somewhere), or should I look somewhere else for an issue (shorted Tantalum,
> etc)? Guess just pulling the chip and seeing if the power draw on the 5V
> goes down might not be a bad bet either (I do have a hot air rework “gun”).
> Or just clip/lift it’s 5V pin and see if that resolves the issue. The unit
> should still lock eventually without that chip.
>
> Anything else I might want to check?
>
> Thanks all, and thanks Tom Van Baak for the encouragement to test more
> before posting to the list. =)
>
> -Ryan Stasel
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