Oscilloscope Y axis drifting. - Page 1
Forum section: electronics / general

author: elliott
author_rank: Regular Contributor
timestamp: September 01, 2011, 10:49:49 pm
content: I picked up a BK Precision 2120 the other day as my first oscilloscope. It seems to work fine, for the most part, except the signals will drift. Channel 2 seems to be the worst, it will eventually go off screen and the vertical position knob can't bring it back. I downloaded the service manual and it said VR301 (CH1) and VR302 (CH2) will center the signals, which they do, but it will still drift.I attached the service manual, there are full schematics, VR302 is in the left diagram on page 9 of the PDF, near the bottom in the center, being fed by -12V. I checked that supply and it is stable and doesn't change when the line drifts.I didn't see any capacitors leaking, but I haven't gone through and checked them all with my meter yet.Any ideas?

author: Computeruser
author_rank: Regular Contributor
timestamp: September 01, 2011, 11:48:32 pm
content: Look in the maintenance / calibration section for DC Balance adjustment. DC Balance in several of my Tektronix vertical amplifiers has to be adjusted for minimum trace shift (vertically) as you change input polarity and attenuation.If the DC balance circuit (assuming you have one) is faulty, I could see the effect you get as a consequence of that.... C

author: elliott
author_rank: Regular Contributor
timestamp: September 01, 2011, 11:55:37 pm
content: I think you're onto something. When set to 5mV on CH1 and turning the VAR control from CAL to full the line barely moves, the manual says 1 minor division or less is normal. When I do it on CH2, it shoots off the bottom of the screen.Thanks, I've got some probing to do.

author: Computeruser
author_rank: Regular Contributor
timestamp: September 02, 2011, 12:17:56 am
content: >>> When set to 5mV on CH1 and turning the VAR control from CAL to full the line barely movesYes - That is the kind of description my Tektronix manuals give under the calibration section.So let us know after you have checked Channel 2. ... C

author: elliott
author_rank: Regular Contributor
timestamp: September 02, 2011, 12:36:28 am
content: Turning VC201 and VC202 to adjust the DC balance of channel 2 seems to have no effect at all on the trace.

author: Computeruser
author_rank: Regular Contributor
timestamp: September 02, 2011, 01:03:11 am
content: So then there may be a component in the DC Balance circuitry that is drifting with heat (from normal operation). An adjustment would not cure such a drift. ... C

author: elliott
author_rank: Regular Contributor
timestamp: September 02, 2011, 01:12:34 am
content: It may be VC202 itself that is the problem. It is a 6pF trimmer cap according to my parts list, I measured VC101 (CH1) and it seemed to react properly to adjustment, VC202 seemed to be stuck between 4.53 and 4.59pF, VC101 is set to 4.85pF.EDIT:Guess not, swapping them had no effect.EDIT2: I Swapped them back, set them exactly where they were, CH1 was still acting normal when doing the DC balance test. After letting it run for a while CH1 started acting the same when doing the DC balance test.

author: Fraser
author_rank: Super Contributor
timestamp: September 02, 2011, 11:00:55 pm
content: When looking for drift in a CRT deflection circuit I would start at the low voltage stage looking for stability in the low voltage supply rails to the predriver stages. If all was as it should be with stable rails and constant voltage offsets from the predriver stage, I would check the supply rails to the driver stage that actually changes the voltages on the CRT deflection plates. I wouldn't be surprised to hear that these rails are drifting as the unit warms up due to a PSU age related fault. If there is no drift in the rails then monitor the output of the plate deflection drivers and see if there is drift in the voltages around that stage.Fortunately, as it is effectively a plate bias voltage fault, it should be a voltage level drift somewhere in the Y deflection predriver/driver stages and this should be relatively easy to pin down with a multimeter but make sure your multimeter and leads are safe for working on a CRT circuit. The plate voltage supplies in scopes are often around 200Vdc and the electron gun acceleration p.d. ranges from 1200V to over 12kV, depending upon design. (Stay away from the electron gun acceleration voltage as it has little to do with your problem.)

author: elliott
author_rank: Regular Contributor
timestamp: September 03, 2011, 12:41:41 am
content: I checked that first, -12V and 5V are dead on and stable, which is all that is going to the vertical pre-amp stuff. It is stable on the rails and at the UA733 video amplifier IC.The vertical pre-amp is on its own board, I'm going through the list of components now and making a shopping list for capacitors. I think the heat is getting to some of them. I found 2 bad ones on channel 2 and one on channel 1, those are bad when cold, I didn't retest with it warm. I figure it can't hurt to replace them all, it is a pain to get that board out and get the RF shield off, I'd prefer to only do it once.