Re: TDS694C 50 Ohm Overload


Two things:

- I've checked a TDS694C board here and there are two paths going into the analog preamp/mux (the row after it are the ADCs). The scope selects one or the other path according to vertical settings. On the first path it's a direct feed, the other path is routed via a 50 Ohm load network for the larger signals. The latter path is selected by default when powered off, so it will read 50 Ohm. But only starting after the second relay in the chain, as the first relay will be open to the BNC input. Relay schematic is printed on top of them. On the first path, low signal, first contact point from the analog preamp/mux, I measure 57.2 Ohm to GND, and on the second path with resistive network I get 26.7 Ohm to GND, with the scope totally switched off. The relays should be easy to check when powered off, isolation is high open and closed it's a very low resistance. Any other values mean trouble. You could also compare measurements between your channel and compare reading of the components. But my guess would also be on a blown analog ASIC.

- BUT: I read you are facing another problem with the NVRAM/cal constants as well. You have ExtCal errors which means it sees a problem with the constants in the EEPROMs on the acquisition board. It will have loaded defaults, so you calibration will be nonexistent. In case you can't correct the defect due to the overload, the question is whether new calibration would be able to compensate for it. Probably not in case it's really off, but you never know. One thing to do for sure is to make a dump of the NVRAM/EEPROM values, using for example the GPIB tools or the floppy dump scripts (see my reference post on EEVblog or user @ragge 's GitHub). Or read them directly from the EEPROMs if you are suspecting an interface problem. It may safer. It could be the EEPROM values are still "repairable" or can be saved, you never know.

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