Hello all,
When I bought this TDS 620 it was failing:
++Processor
++Acquisition
++Attn/Acq Interface
Opening it revealed the usual leaky SMD electrolytics, so it went through the usual process of removing the old caps by twisting them off, cleaning pads, dishwasher with soap, oven, fitting new caps, cleaning and repairing tracks in spots that had heavier corrosion.
There may still be cut tracks but the corrosion did not look as serious on this scope as on the two previous TDS (500) I had worked on.
I also dumped and socketed the DS1245Y NVRAM. (File attached.)
Since the recap I have absolutely nothing working any better, bedsides the previous fails I also now get a fail most of the time on:
++Cal Initialisation
I got hold of the A10 board schematics on Håkan's resource page:
http://www.hakanh.com/dl/docs/hardtofind/TDS644A_A10(Acq).pdf(Thanks Håkan!)
Some probing around last night revealed that the power supply values mostly look good, I measured across all the SMD electrolytics with a DMM.
Voltages may have been a little low at C590, C1710, C1603, C1693, C1016. I have not yet checked whether there was a good reason for this. (Low value series resistor?)
On the screen both traces give what looks like noise, not exactly the same on both though.
The V/Div and offset controls have no effect whatsoever, also the "TRIG'D" LED never comes on.
The offset control not working got me to start probing at the AD667 DAC. (It is outputting the various control voltages and they move around when adjusting V/Div and offset. I'm not sure the AD667 reference is spot-on, I measured 9.988V at TP901, however this should not stop the scope from working, possibly make it fail cal.
I followed through the offset controls to the U1400 preamp where they look OK. (Approx +/-1.7V maximum.)
The input gets through the FE attenuators (I'm not saying they work perfectly yet, just trying to get a trace on screen...) then to the preamps U1300, U1400.
The preamp differential outputs still carry the signal to the "FISO" CCD (U850), then the output to the ADC is just plain garbage, similar enough to the crap displayed on the screen.
This is my first experience with a CCD based scope and I'm a little lost at this point as to what to expect on different pins of U850, to check if it is defective or just missing some clock/control line.
I'll get a couple of photos up later.