Tektronix TDS644A question

[sherlock]

Hi everyone.  I'm trying to repair a TDS644A oscilloscope.  I'm in the process of cleaning the PC boards and replacing all of the surface mount capacitors.  I have taken some resistance readings from one of the power connectors (J700) on the A10 aquisition board to ground and have come up with a reading that I think may indicate a further problem.  I have the board on my bench with no cables attached and all of the surface mount capacitors removed.  The board has been cleaned and visually inspected for any obvious issues such as open or shorted vias and/or traces.  When I measure the 5VDC power pins (J700 - pins 35-40) to ground I'm getting a reading of 24.5 ohms.  The probe polarity makes no difference.  That reading seems too low to me for an unpowered and not connected board.  Has anyone encountered a problem like this?  I'm guessing a component has failed but there's a lot of parts on this board and I'd like some indication of where to begin.  If someone there has a working TDS5XX or TDS6XX scope that could make this measurement and post it here I would be very grateful.  (The J700 cable should be removed from the board to do this).  Any additional J700 resistance measurements to ground would be helpful too as it would give me a reference to work with.  I have some schematics for the TDS544A which is similar but not exact and I can see some component differences beyond a different board shape and layout.  (Better than nothing).     Any help would be appreciated.

sherlock

[nctnico]

IIRC it should be OK. You are probably measuring some termination resistors or a divider.
You should be aware that the TDS644A does not have peak detect so it's use is actually quite limited because you'll be missing narrow pulses in a signal. Just a heads up...

[skennedy]

I have measured something similar on a TDS544A between 5V and GND. Makes it pretty hard to work out if there is an issue when debugging.

[sherlock]

Thanks for the replies.  I managed to get a schematic of the aquisition board and traced the 5V line.  There are indeed several resistor combinations from 5V to ground, all in parallel but in different parts of the circuitry.  I calculated the resulting value of what I found and it came to about 30 ohms so I figured what I measured on my board was probably OK.  I have removed all of the surface mount caps on all of the boards that have them and cleaned everything.  I replaced the caps with leaded radial types (A LOT of them!) and the scope now passes all of the internal diagnostics and seems to work.  I've only had time to test the 4 inputs with the calibration signal but they all showed the proper square wave and amplitude.  As far as the peak detect is concerned, I read this somewhere after I had already gotten this scope.  I thought I saw peak detect (or maybe MAX AMPLITUDE) on the measurement choices menu, but I just verified that the menu was there.  :-)  I'll look again later.  It's most likely not going to be a big factor for me as I'll probably use it mainly for Ham Radio work.  It does seem like a nice scope now that it's working though.  Better than my old 465 or my TDS380.   

[nctnico]

Peak detect is an acquisition mode. If you use the scope to look at high frequency signals you'll probably be OK. I used to have a TDS644A but it turned out to be completely useless for general purpose work (look at vsync signal on a relatively slow time base for example) due to lack of peak detect. So out it went.