There aren't enough resistors for a high resolution R-2R DAC so its likely to
be something like 4 (or more) x 4 bit (R, 2R, 4R, 8R) sub dacs with weighted
resistive summing of the sub DAC outputs.
Bruce
On Tuesday, 29 March 2016 6:09 PM, Ryan Stasel <rsta...@uoregon.edu> wrote:
All,
Yay progress (one step forward into the next wall). I got the MAX232 chip
replaced with a MAX202 (pin compatible, better ESD and Latchup protection).
That got the serial working, and I was able to watch the unit do a survey, and
then confirm it saw that the Osc was off by ~ -2700ppb, and the PPS. I did a
factory reset as well.
After it did a full survey, it came back and confirmed that the oscillator
adjustment was at a rail. It read the DAC 4.999990. But the output of the
reference was still -0.6V. So I’m guessing that chip is toast. I’ve got one on
order. Also confusing is the quad op-amp seems to be saturated at the negative
rail. I can see this Opamp feeds the adjust pin on the OCXO, but I’m not sure
what feeds it. Guessing the FPGA? I still can’t find the DAC… all I see are
these pictures http://www.leapsecond.com/pages/tbolt/photos.htm but the
pictures aren’t of the DAC, unless the DAC is a resistor ladder (hadn’t thought
of that until now). Now I’m starting to realize this is probably the case. =P
So, any input on where I might want to look? Does it seem likely the 5V
reference is dead, or just being down below ground? Tracing the board out is
proving rather beyond my ability. I don’t really want to just lift the output
of the 5V reference and leave whatever it feeds floating.
Any help would be greatly appreciated.
Thanks!
-Ryan Stasel
On Mar 27, 2016, at 4:59 PM, Ryan Stasel
<rsta...@uoregon.edu<mailto:rsta...@uoregon.edu>> wrote:
All,
So, ended up pulling the 232 chip, and it seems to test okay as far as
resistance between pins, and the board still only shows 65ohms or so between
+5v and ground around that chips pads (support caps, pads, etc). I’m not sure
if that’s “normal”, but it doesn’t seem like it is to me. Nothing seems to be
getting abnormally warm. =/ I’m half curious if my meter (Keithley 196, and a
Fluke 189) is turning on a transistor partially. =/
I do see what appears to be TTL level serial communication on the inputs to
that chip. The replacement should be here on Monday, but at this point I'm
wondering if my meter might be turning on some transistor (it shouldn't),
or...? I would think a shorted cap would itself heat up.
There's a thread online relating to MAX232 type chips suffering from latchup
under certain conditions
(https://e2e.ti.com/support/interface/etc_interface/f/392/t/233847). But the
low resistance between power and ground rails has me a bit concerned. :/
I did leave the unit running for about an hour, and the 10Mhz never came down
from its 26hz high. Seeing that, I looked at the adjust pin on the OCXO, and
it’s pegged at -10.25V. That also appears on all the outputs from the LT1014.
=/ Also in that area is the 5V reference, and checking the output of that shows
-0.6V (should be 5V). I’m… really starting to worry this whole thing is toast.
All the rails appear fine, so I can’t help but think the previous owner hooked
up the power wrong, and either -12V or +12V got applied to the 5V parts. =/ I’m
fine replacing these two other LT parts, I’m just starting to worry that
something not as easy to replace could be damaged (the FPGA, the Trimble chip,
etc). Where is the actual DAC (is that in the FPGA)? Though I suppose if the 5V
reference is dead, that would probably prevent the DAC from functioning. I
guess the question would be if something is pulling the ref down (below
ground), or is it just borked?
Anyway, hopefully I’ll at least be able to hook up serial to it tomorrow
evening and see what Tboltmon says.
Thanks again for the help. Let me know if you have any other thoughts for
things I can check.
Ryan Stasel
IT Operations Manager, SOJC
University of Oregon
Sent from my iPhone
On Mar 27, 2016, at 8:53 AM, Robert LaJeunesse
<lajeune...@mail.com<mailto:lajeune...@mail.com><mailto:lajeune...@mail.com>>
wrote:
Ryan,
The cap across 1-3 is the one that boosts +5V to +10V, with pin 2 being the
+10V result. Thus the pins on this "flying" cap swing 5V p-p (pin 3 is 0 to 5,
pin 1 is 5 to 10). The cap across 4-5 is the one that inverts +10V to -10V,
with pin 6 being the -10V result. Pin 6 should NOT be at ground. The pins on
this second flying cap swing 10V p-p (pin 4 is 10 to 0, pin 5 is 0 to -10).
Inputs to the transmitter stages (10 & 11) should be at valid logic levels, for
the ICL232 these are standard TTL levels. Be aware that other part types that
do this same function may not have the same logic levels (Vih and Vil) so be
careful when substituting.
I did not mean to suggest lifting pins 10 or 11, although you might consider
that last, but pins 7 and 14 should idle near -10V since the transmitter inputs
have pullups and the drivers invert. What I was suggesting is to make sure pins
8 & 13 were disconnected [just pull off any RS-232 cable] and look for pins 9
and 12 to be at a good high level, say above 4V or so.
FWIW it's just as difficult to get -12V from +12V via an inverter chip as it is
to simply use the ICL232 (or MAX232 or similar) part. Add to that the fact that
older high-voltage RS-232 parts tend to be power hungry, and require separate
packages for transmitter and receiver. Why use 3 parts when 1 works?
Good luck with scoping out the real problem.
Bob LaJeunesse
Sent: Friday, March 25, 2016 at 5:33 PM
From: "Ryan Stasel"
<rsta...@uoregon.edu<mailto:rsta...@uoregon.edu><mailto:rsta...@uoregon.edu>>
To: "Discussion of precise time and frequency measurement"
<time-nuts@febo.com<mailto:time-nuts@febo.com><mailto:time-nuts@febo.com>>
Subject: Re: [time-nuts] Trimble Thunderbolt GPSDO Troubleshooting
Hi Bob,
I think I’m going to take a look, since I couldn’t find a spare, so I’ve got a
couple on order (and a few days to wait).
So, looking at the datasheet
(http://www.intersil.com/content/dam/Intersil/documents/icl2/icl232.pdf),
you’re saying check the two caps across pins 1 and 3, and 4 and 5, and make
sure they’re swinging. And also check to see if Pin 6 is at “ground”
(basically). +V should be at around 10V? Likewise, I’d assume the input to that
stage (wherever it comes from) should be a nice logic level input.
And then lastly, you’re saying lift/cut pins 10 (and/or 11, depending), and
then look at 7 or 14 and see if they’re sitting at 5V (they wouldn’t sit at
+10V)? It’s actually kind of funny that they used this chip, since it’s whole
point is to eliminate the need for ±12V… except this board specifically has
±12V! =P
Anyway, I’ll check all this as best I can, or just get frustrated lifting pins,
and remove the whole darn thing (like Clint suggested). =P
Thanks all!
-Ryan Stasel
On Mar 25, 2016, at 12:52 , Robert LaJeunesse
<lajeune...@mail.com<mailto:lajeune...@mail.com><mailto:lajeune...@mail.com>>
wrote:
Personally I'd take a few minutes to look closer at the '232 chip with the
scope. Are the charge pump caps swinging 5V p-p and 10V p-p? if not swinging at
all, trash the chip. Is -V pin voltage about equal to the negative of the +V
pin? If so, that's OK. If not might have an output shorted, or a bad chip. Is
the +V pin about +10V? If so, that's OK. If not might have an output shorted,
or a bad chip. With no RS232 driving source connected does the receiver output
look like a valid 5V logic high level? If not, look for a short on this line.
If all these pass and the chip gets hot, replace the chip.
Bob LaJeunesse
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