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Tektronix THS720 Std probe compensation

The Tektronix THS720 is a portable 100 MHz, 500 MS/s (each channel) dual-channel digital oscilloscope with isolated inputs. There were two variants, the THS720 STD and the THS720A. The primary difference is that the THS720A will trigger from the DMM input.

Key Specifications

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Due to the isolated inputs, the probe ground must be connected even when compensating the probes with the "PROBE COMP" output. It can be used with any BNC probe but the scale factor pin found on many Tektronix probes is not used. The vertical menu has a scale factor entry (1×, 10×, 100×, 1000×) which can also be used for direct BNC inputs.

Recommended probes are the P5102 and the P6113B, both are 10× probes with protected grounds for isolated measurements. The P5102 has the best isolation, allowing 600V between reference ground and earth ground.


Primary microprocessor is the Motorola/Freescale MC68331CFC16. An Intel E28F008SA120 Flash Memory chip is used for setup, cal and waveform storage. The switching power supply is run by a TL1451A, a TI two channel boost/buck controller. The DMM is a 3¾-digit Maxim MAX134CQH.

The analog scope section starts with an attenuator hybrid driven by four channels of HP (now Avago) optoisolators and NEC relays. The hybrid contains the AC coupling capacitor and many laser trimmed resistors along with a custom Tek 165-2545-00 BGA. The signal then passes through a single-ended to differential transformer and goes to a custom labeled Maxim two-channel high speed A/D converter 234-1503-20. Note that the differential signal layout for channel 2 is not as good as channel 1 since one side of channel 2 goes through 2 vias not found on channel 1.

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.