Tektronix THS720A Voltage offset

[mwbarth36]

I have a THS720A which appears to operate normally in every respect, except that there is a Voltage offset that changes with the vertical gain setting.  I have read where others have performed a Calibration and corrected this in many cases.  My problem is that I have performed the calibration according to the steps listed in the Service Manual for this scope, and it never gets past the first step, inputting a fast rise time square wave, frequency 500Hz to 950Hz, amplitude 250 mV p/p to 300 mV p/p.  Always ends up with a 'Fail' and the following error messages in the log.

Signal Source
Timeout
CVR Source Gain

I suspect that for some reason it does not like the signal being presented, but can't determine why.  Can anyone help to shed light on what I am doing wrong, or even what precisely this sequence of error messages might mean, the manual is useless in determining this.  The scope appears normal in every other respect.

[mwbarth36]

After switching pulse generators the first calibration step passes every time, strange too, as this generator appears to have a marginal rise time compared to the other.  Anyway, the next step in the calibration procedure is with zero input, just a 50 Ohm termination, and, of course, the scope will not pass that test, ever.  The scope measures the offset as follows:

Range Setting                 Ch 1          Ch 2

100V, 200V, 500V           -340 V        -220 V

10V, 20V, 50V                 -34 V         -22 V

1 V,  2 V,  5 V                 -3.4 V         - 2.2 V

500mV, 200 mV, 100mV   -340 mV     - 220 mV

I know the scaling of the offset like it does should give me a clue as to the defective circuit, I am guessing the DAC.  Anyone care to make a guess.  Thing is, the DAC appears to be part of a large integrated chip that would be nasty to replace even if I could get one.  And since this is the only problem with this scope, the ac signal measure perfectly and look normal, it would have to be a failure unique to the DC level adjust.  I was hoping for a reference voltage failure somewhere.  Any ideal will be greatly appreciated!

[Zucca]

Here you can find many info and the holy schematics.

[zxase258]

The THS700 Series uses a linear optocoupler to transmit DC signals to isolated channels. These optocouplers will drift differently with the two photodiodes inside. As long as the HCNR201-300E near the channel module needs to be replaced, the problem should be solved.

[mwbarth36]

Thanks, I have ordered new opto's so I can replace them.  Just seems odd that both channels would fail in a similar manner.  There are four opto's involved, guess there is no way to determine which are causing the problem?  When you say 'closest to channel module', what do you mean?  Are you trying to tell me which two may be causing the problem?  Sure would like to only replace two!

[mwbarth36]

Wow, took the scope apart.  There is no way to replace those opto's without removing the shield can around the input circuits.  That is going to be an unpleasant task!  Hope this solves the problem.  Once cans are off I will just replace all four.