THS720/Repairs
Repair Issues
The service manual shows how to remove the calibration protection jumper. The DMM calibration step requires a good calibrator capable of doing 850 VDC and 600 VAC at 500 Hz. Scope calibration is much simpler requiring only a square wave pulser and a DC voltage source. Note that after a cal fail, there is an error log in a nearby menu indexed by power on count.
When confronted by scope calibration failures that say "Cal error, DC balance" and there are persistent DC offsets that increase with the attenuator settings try replacing U45 and U46. They are HP CNR201A analog optoisolators, now made by Avago as the HCNR201-300E. They send a low pass filtered version of the input signal across the isolation barrier to give the DC levels that can not cross the high frequency transformer isolation barrier.
The battery pack is, of course, custom. It appears to be 4 C cell NiCd pack with the positive terminal moved to a ring around the bottom (most negative) cell. If you are not concerned with interchanging packs or using the separate charger, the cells can simply be soldered into the lower housing.
See THS700 for the manuals.