HP 1720A Oscilloscope vertical instability - Page 1
Forum section: electronics / testgear

author: dzseki
author_rank: Frequent Contributor
timestamp: August 02, 2013, 05:57:57 pm
content: Hello,First time poster here! I bought a HP 1720A oscilloscope because I needed a high speed oscillocope at the lowest price, and this was the best I could get. Of course the oscilloscope had issues, I was able to repair all of them except the most important/annoying. The problem is with the vertical amplifier: even with grounded input the trace jumps on the screen intermittently. Sometimes it good for days, sometimes it produces the problem very often, the rate of the jumping is usually less than 1 div, but the frequency and the extent always varies. I was able to track down that the vertical output amplifier generates the problem, as I removed its input from the delay line, and put a short on the input, yet it jumped.Now I resoldered the whole board, measured all components they all seem to be good (when I measured them at least). I have an other oscilloscope as well so I was able to take some measurements on that board. For the sake of simplicity I've attached the circuit diagram of the vertical amplifier. I veryfied that the U2 ic already gets the false signal, the jumping looks similar to adjusting the DC balance very fast, but the potentiometer works good, I also measured all the supply rails and they are all clear on this board. U1 is in socket, I've pulled out a few times to clean the contacts somewhat. All my efforts made the problem appear more seldom, but it isn't disappeared completely yet.Any thoughts will be appreciated.

author: ModemHead
author_rank: Frequent Contributor
timestamp: August 02, 2013, 06:47:46 pm
content: I had a problem very much like this in a Tek scope, in the vertical pre-amp section. After much hair-pulling, it turned out to be two "dogbone" caps touching each other. (The ends are just painted wire.)

author: edpalmer42
author_rank: Super Contributor
timestamp: August 03, 2013, 03:31:19 am
content: How hard is it to get near the board while it's in operation? Have you tried heating (soldering iron close by) and cooling (freeze spray) components individually to see if one of them has a thermal intermittent?Ed

author: dzseki
author_rank: Frequent Contributor
timestamp: August 03, 2013, 07:30:28 am
content: Thank you Guys! The vertical board is well accessible during operation so I'll check your suggestions. I think I've already checked the components that are touching each other, but haven't tried the heating method yet.

author: peterpan
author_rank: Newbie
timestamp: August 03, 2013, 02:46:32 pm
content: NTK resistor (RT1) Is not faulty? (only idea ). The yellow tantalum capacitor original?Anyone who owns a similar detailed drawings?

author: dzseki
author_rank: Frequent Contributor
timestamp: August 03, 2013, 03:19:34 pm
content: Quote from: peterpan on August 03, 2013, 02:46:32 pmThe NTK seems to be fine, the tantallum cap has been replaced already (not by me)

author: ddavidebor
author_rank: Super Contributor
timestamp: August 03, 2013, 03:24:29 pm
content: Quote from: dzseki on August 02, 2013, 05:57:57 pmTry to freeze it piece by piece with a air spray. Is a good way to simply find intermittent issue.

author: dzseki
author_rank: Frequent Contributor
timestamp: August 27, 2014, 04:28:56 pm
content: This thread got actual again, unfortunately... Figure this, I never really found out what was wrong with it, I sprayed a half can of freeze spray on the board without even narrowing down the problem, the scope however just worked a whole year without a single problem since that, now the problem is back again and is driving me nuts.I rechecked once again that the problem is really happening on the vertical output board. I've spent two days of troubleshooting on the bench, then the problem usualy emerged when the machine was cold started, the instability was present in the first few minutes (or less) then stabilized somewhat. Then I closed the covers and put back in it's place after that the scope immediately began to produce the problem very hardly and for half an hour at least... So the scope is again on the bench and "jumping" only from cold start.In the meantime I've done a few things.-I suspected the socket for U1 to have intermittent contact problems, as had other sockets in this scope, now U1 is soldered, but the problem is still present.-As of now I've never seen the instability when I shorted the two input pins of U2 (while maintaining the DC level), the output is dead stable then. Without the short there is always a little (random) swing on the output, order of 5-10mV (AC), but that is not visible on the screen.-The above mentioned random swing is also present in the +53V line, which is only used by this board, yet I don't have any evidence if there is any intermittent problems with this supply, nor it would cause a jumping effect on the screen.I also found an other manual where U1 and U2 ics are showed in a more detailed manner.One thing what I should try is to short together pin 4-5 of U1, that would shut out the whoule compensation circuit as a suspect.Again, the problem is that the trace jumps in vertical direction (only) and seemingly it doesn't affect the gain or bandwidth.