Tek THS720A thinks there are 110V on the input but there is just a 50 Ohm R - Page 1
Forum section: electronics / repair

author: Zucca
author_rank: Unknown
timestamp: August 04, 2014, 02:38:04 am
content: Just got a THS720A from ebay, sold as "used, need calibration". I got some money back since I told the buyer I am 99% sure the unit is broken.Anyway this is the situation, two 50 Ohm terminations on the inputs:Here the funny CH1 500V/Div DC:110.6V mean...do we really think a calibration will compensate that ??Then I found this:http://www.tek.com/support/faqs/my-scope-has-developed-dc-offset-how-can-i-remove-itI tried that, it pass the SPC (like the self test on startup)but the offset is still there.here the 500V/Div GND:According to my investigation this offset is there also in the other rangesand here the Probe Comp signal (of course without the 50R termination):.The good news is that CH2 looks reasonable:so I "just" have to compare the CH1 circuit with the CH2 and find the bad guy.Do you think I will be able to fix it? Any suggestions?

author: tautech
author_rank: Super Contributor
timestamp: August 04, 2014, 10:21:22 am
content: As it seems like the offset is present with differing input attenuation settings.I suspect that a portion of a resistor divider on the input has drifted or been damaged.It should not be hard to check this with a good DMM as you have a good Ch 2 to compare to.Check all attenuation settings for offset to be sure before you rip it apart.

author: Zucca
author_rank: Unknown
timestamp: August 11, 2014, 02:49:01 am
content: Thankstautech, what you says sound reasonable. Yes, the offset is there in all the attenuations. So I tear it down the beast, see here:https://www.eevblog.com/forum/testgear/tek-ths720a-portable-scope-teardowndiscussion/No hope to find the schematics in the entire www.EDIT 17 Feb 2016: Look for THS7XXAPDiag.pdf and you will find itI found a service manual supplement which looks interesting:QuoteSo my attach plan is to check the power supply voltages, found a good ground and start to check the Attenuators and the Sampler Driver if I can find them.I really hope it is not simply a calibration thing.

author: simingx
author_rank: Contributor
timestamp: August 11, 2014, 05:10:57 am
content: I had a THS720 with this problem once, no amount of SPC would get rid of it. A calibration at Tek solved it but it came back eventually...

author: qno
author_rank: Frequent Contributor
timestamp: February 14, 2016, 08:19:49 pm
content: Sorry for the late post.I had the same problem. A calibration does the trick.You can do a calibration by yourself.You can find the calibration manual online.This problem occurs when you replace the battery before switching off.

author: macboy
author_rank: Super Contributor
timestamp: February 15, 2016, 12:15:03 am
content: You can't complain about 3.2 V offset on a 500V/div range! Even if it uses a 10 bit ADC, that isone ADC count, remember that full scale is +/- 2000 volt! In other words, there is no offset, you are imagining a problem, at least for that one range. Does the SPC procedure reduce the offset in all other ranges to less than 1/10 of a division? (e.g. 5 V/div range, offset should be < +/-0.5 V). If so, then you are doing fine.

author: Zucca
author_rank: Unknown
timestamp: February 15, 2016, 10:33:07 am
content: Quote from: qno on February 14, 2016, 08:19:49 pmAre you sure? I looked at the input stage of the Scope, I was thinking that the optoisolator for the DC Signals (for the AC components you have the little signal trafo) was the bad guy here. It can well drift with the time.My next step is also to do a calibration, and I was probably confident it would have solve the problem. Thanks for the confirmation.My unit is still in a box... shame on my I have no f... time.Quote from: macboy on February 15, 2016, 12:15:03 amI have just 50 Ohm connected to the BNC, and the scope is telling me 110V. It can´t be right.

author: amyk
author_rank: Super Contributor
timestamp: February 15, 2016, 11:03:41 am
content: 110.6V11.35V1.115V119.2mVI suspect something closer to the ADC than the input stage... the offset seems to be constant at the ADC's input. If you can check the data coming out of the ADC, that might also determine if the problem is on the digital (stuck bit?) or analogue side.

author: Zucca
author_rank: Unknown
timestamp: February 15, 2016, 12:23:06 pm
content: Quote from: amyk on February 15, 2016, 11:03:41 amthat's correct, but according to the schematics before the ADC and after the range selection circuit there is the DC opto-isolator. The input circuit I ment is not the hybrid PCB right after the BNC CH input.Anyway congrats for fixing the problem!

author: qno
author_rank: Frequent Contributor
timestamp: February 16, 2016, 07:37:57 pm
content: I did this a couple of years ago so i cannot remember the details.I had a calibration manual from the net.You have to open the THS and remove or place a jumper. (not shure)Then you need a few DC voltages you can make with a dc power supply (or 2)I think it needed 60 volt or so so I put two in series and adjust them with your DVM to required voltage.You also need a 1 kHz 300 mV square wave generator with a rise time < 3 ns or so (it is in the manual)I made one from some fast gates and a resistor divider resulting in a 50 ohm load.It did the trick. (I had another 1 G scope to verify this.)I think the THS writes the offsets to an EEPROM or so during power down and reads it @ start-up.When removing the Battery while still on this data is damaged and some how the software offset calibration or gain restores to a factory default that is beyond the range of the offset compensation allowed by the SPC.Remember that the scope can be calibrated without a a screwdriver so some clever switching of gain and bandwidth compensation is done by the software.After this mine worked ok. I compared it with a 1 GHz Wavesurfer. But that was a decade ago....

author: mk_
author_rank: Regular Contributor
timestamp: February 16, 2016, 09:06:26 pm
content: Quote from: zucca on August 11, 2014, 02:49:01 amSearch for THS7XXAPDiag.pdf and you will find it.Anyway, PM sent as the file is to large (1.7MB) for attachment.

author: Zucca
author_rank: Unknown
timestamp: February 17, 2016, 07:38:08 am
content: Thanksmk_I already got them, my previous post is now updated.

author: walter.aranda
author_rank: Contributor
timestamp: March 26, 2023, 01:13:50 am
content: I had the same problem, I solved it by calibrating it, maybe it's not the optocouplers or anything like that, maybe it's just calibrating the oscilloscope, note: during the calibration make sure that the channel input is X1