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See [[THS700]]. | [[Image:Ths720std-comp.jpg|thumb|400px|right|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. | |||
{{MissingSpecs}} | |||
==Operation== | |||
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. | |||
==Internals== | |||
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 V<sub>DC</sub> | |||
and 600 V<sub>AC</sub> 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. | |||
==Pictures== | |||
<gallery> | |||
ths720std-comp.jpg|THS720 Std displaying probe compensation waveform | |||
ths720std-toppcb.JPG|THS720 std top of main PCB | |||
ths720std-mcudmm.JPG|THS720 std main board MCU and DMM | |||
ths720std-powerbnc.JPG|THS720 std main board BNC inputs and power supply | |||
ths720std-backpcb.jpg|THS720 std back of main PCB | |||
ths720std-attenuator.JPG|THS720 std attenuator with cover removed | |||
ths720std-scopeanalog.JPG|THS720 std analog scope section | |||
ths720std-runningboard.jpg|THS720 std operation without case | |||
Tek ths720a 1.jpg|THS720A | |||
Tek ths720a 2.jpg|THS720A | |||
Tek ths720a 3.jpg|THS720A | |||
Tek ths720a 4.jpg|THS720A | |||
Tek ths720a 5.jpg|THS720A | |||
Tek ths720a 6.jpg|THS720A | |||
Tek ths720a 7.jpg|THS720A | |||
Tek ths720a 8.jpg|THS720A | |||
</gallery> | |||
[[Category:THS series scopes]] |