THS720A baseline offset problem - Page 1
Forum section: electronics / repair

author: mwbarth36
author_rank: Regular Contributor
timestamp: February 08, 2020, 09:40:30 pm
content: I recently acquired a THS720A scope with large voltage offsets on channel 2, channel 1 had small offset of 20 mV or so but did not trouble me. I know that large offsets are indicative of a bad Opto, so I replaced both optos as well as the JFet amplifier driving one of the optos in channel 2. That eliminated the large offset in that channel. Offset now is 50 mV or less. I felt that the self calibration would probably take care of the rest of the offset. Thing is, I cannot get it to pass the second step (no signal) of calibration. Always get the error message (DC Balance) and it, of course, terminates calibration. The rest of the scope now works fine, and the first step of calibration always passes (that is the test using a 300 mV P-P 950 Hz square wave). Also passes a very slow square wave, period of two seconds with total accuracy. So the voltage offset in the base line has nothing to do with the signal path I am guessing, fast path, or slow.The other factor, don't know what it means, is that the offset appears to be related to the vertical setting. For example, For 100mV, 1V, 10V, 100V it is .4 Div., for the 20mV, 2V.....settings it is a full division, and then at 500mV, 5V.......it is only .2 Div. At the lowest setting 50mV per div. it is once again at a full division. I have no idea as to how this scope attempts to do calibration, and the error message does not tell even which channel is the problem. I do have full schematics for the unit, and have looked at the dc balance signals for channel 1 and 2, and they look normal.I am wondering if anyone has any ideas as to why this scope cannot deal with these relatively small dc balance issues? Any help will be greatly appreciated!

author: Zucca
author_rank: Unknown
timestamp: February 10, 2020, 09:46:09 am
content: Did you tried a full calibration and not a just a self calibration?More info hier:https://www.eevblog.com/forum/testgear/tek-ths720a-portable-scope-teardowndiscussion/

author: mwbarth36
author_rank: Regular Contributor
timestamp: February 12, 2020, 03:42:25 am
content: There is no 'self calibration' with the THS720, I do the total calibration. It gets through the 300mv square wave test, tries the 0 volt input test, and fails. Gives a DC balance error.

author: Zucca
author_rank: Unknown
timestamp: February 12, 2020, 10:02:20 am
content: I meant the signal compenstation feature (SFC? something like this).Ok it you are doing the total cal (the one which require the jumper moved on the board) then it is a component level error. I guess.Take the schematics out, measure the board and report back your findings.

author: mwbarth36
author_rank: Regular Contributor
timestamp: February 12, 2020, 10:42:10 pm
content: Zucca, I do appreciate your thoughts. Here is the situation. The scope, on start up, passes all tests except Calibration, and that is a 'Fail'. When the SPC compensation is done, that is a PASS every time. The 'Factory DMM' is a PASS. The 'Factory Scope' is a FAIL. The manual then calls for a Calibration with the jumper removed. So, I do the calibration, the first test is with a 300 mV p-p fast square wave, the second test is with both channels tied together and a 50 Ohm termination on one channel. When I do the test, the first test (sq wave) always passes. I know it passes because if a test does not pass, the calibration process terminates with a failed message, and the old calibration parameters are restored. The error message is always "BALANCE ERROR'. There is no way to tell anything else about what went wrong since only a complete successful calibration will preserve the new parameters.Now comes the really strange part. I originally had replaced both optos in channel 2, they were bad. After replaced them the base line offset was varying with the vertical setting a total of two divisions. Channel one was not nearly as bad with only a few tenths of a division, and I was willing to live with that. Yes, I should have run SPC after I changed the optos in 2, you reminded me of that. thanks! So, after running SPC, the base line offset on channel 2 varies only a few tenths of a division, not right, but livable. But, channel one now has a variation of up to two divisions depending on the vertical setting. Full calibration still fails at the same step, with the same error. This is strange since when the SPC fails, it is a signal to do a full calibration as SPC cannot fully compensate for such errors. I have the situation where SPC passes, and calibration fails.Any further thoughts will be appreciated.

author: Zucca
author_rank: Unknown
timestamp: February 13, 2020, 08:31:36 am
content: I have two THS720A, one of them is sick like yours:https://www.eevblog.com/forum/repair/tek-ths720a-thinks-there-are-110v-on-the-input-but-there-is-just-a-50-ohm-r/SPC passes but I see an offset there. I did not try the full calibration.I am now relocating my lab and I have unfortunately time to work on this. What I would do is:1) Open the unit2) short both "ghost" offset and working channel input.3) measure the signal path to the ADCSince the two channels are 1:1 the same circuit it should be easy to find a broken component in the input circuit.

author: mwbarth36
author_rank: Regular Contributor
timestamp: February 13, 2020, 06:26:56 pm
content: Problem is before the SPC channel 2 was the bad channel, and channel 1 was good. After running SPC channel 2 is the good channel, and channel 1 is the one with offset. The fact that the offset is constant in terms of divisions with the different settings should tell me something about where the problem is, I just cannot determine what. The 500 Volt, 50 Volt, 5 Volt and 500 mV settings all have the same offset of 1 Division on the vertical scale. The 200 Volt........200mV are the same and at 0.2 divisions. The 100 Volt, 10 V...........100mV settings are again the same, but at 0.6 divisions. The only setting that is different is the 50 mV setting which is greater than any of the others, at 2 Vertical divisions.I have the unit apart and can find no differences in reading between the two channel circuits, which makes sense since I started with a scope with channel two bad as far as vertical offset with no input and, after SPC, wound up with a scope with channel 2 OK and channel 1 the bad channel.One thing I did find out is that SPC appears to have a big effect on this offset. Do not know what the calibration process tries to do during the time the inputs are at 0 Volts with a 50 Ohm termination. I had assume it tried to find a setting that would give a zero offset at the minimum vertical voltage setting, but that does not seem to be the case. Again, the error is 'DC Balance Error'.After tomorrow, I will not have access to the Internet for several days, will respond to any postings after Feb. 23.

author: Zucca
author_rank: Unknown
timestamp: February 13, 2020, 11:53:49 pm
content: mmmm complicated,Assuming the circuit is ok, then we need to suppose a digital problem.What about reflash the FW 1.16?