CockEyed Osilloscope ! - Page 1
Forum section: electronics / repair

author: Randall
author_rank: Newbie
timestamp: September 20, 2022, 02:58:11 pm
content: Help !I've done a lot of research via youtube and forums and found nothing that points the way to solving the problem with my Elenco MO-1252 Oscilloscope gifted to me.It's Cockeyed ! It appears fine while cold but once it warms up for about 10minutest it goes off kilter.Maybe it's the vertical amp ?Service Manual is attached below..see photos.

author: bdunham7
author_rank: Super Contributor
timestamp: September 20, 2022, 03:07:03 pm
content: Try adjusting the trace rotator both cold and warmed up. Does the adjustment have an effect cold but none when hot? If so, the circuit supplying power to the rotation yoke is probably failing when warm. Do you have a schematic?

author: Gyro
author_rank: Super Contributor
timestamp: September 20, 2022, 03:26:00 pm
content: The trace rotate circuit is very simple, just varying the current through the trace rotate coil (wrapped around the tube neck) in respose to the trace rotated pot setting. It could just be a leaky transistor (Q14 or Q15 on the 'Power Block Unit' schematic), but my bet would be on the 1uF capacitor (C??) on the pot wiper, it's probably a failing Tantalum bead.P.S. Also check that the +12V and -5V rails aren't drifting as this might also cause the problem.

author: Randall
author_rank: Newbie
timestamp: September 20, 2022, 03:40:11 pm
content: Yes, the rotation knob has a great effect when turned while cold and not so much when warmed up.It changes the angle some but not enough to correct the trace angle.The manual is attached to this post and the circuit diagram is at the back.I've RTFM'ed as we said in the AirForce but I'm new to O'Scopes.Thx for the Response !

author: bdunham7
author_rank: Super Contributor
timestamp: September 20, 2022, 04:40:50 pm
content: That schematic is hard on the eyes, but I've attached an excerpt with the trace rotation yoke circled. The simple circuit there will adjust the constant current through the coil that gives it a corrective magnetic field. You can simply measure the voltages at various points with a DMM and you should see something failing as it warms up. You can try freezing or cooling the transistors to see if they come back to life.

author: Gyro
author_rank: Super Contributor
timestamp: September 20, 2022, 05:54:47 pm
content: I forgot to ask, is it a sudden rotation shift when the scope warms up, or does it slowly and smoothly rotate during that time?If it's a sudden shift then the posibility of a dry PCB joint or dodgy connector (P4?) becomes much more likely too, in addition to the 1uF cap and transistors. BTW looking at the schematic again, it looks as if the negative rail might be -8V rather than -5V. It doesn't matter anyway, as long as it isn't changing or going away.

author: james_s
author_rank: Super Contributor
timestamp: September 20, 2022, 08:12:30 pm
content: Try using freeze spray, a can of air duster held upside down works too. Let it warm up until it goes crooked, then give areas of the PCB a quick blast of freeze spray and see if you can find out what part responds.

author: Gyro
author_rank: Super Contributor
timestamp: September 20, 2022, 09:54:28 pm
content: Alternatively just nudge the relevant parts and connections with a non-conductive probe / stick... again, depending on your answer (?) of whether it is a slow rotation (thermal drift) or a snap change (dry joint or failing device).

author: Randall
author_rank: Newbie
timestamp: September 21, 2022, 02:42:50 pm
content: Thx, all for the suggested diagnosis technics.Once I go to the city I'll purchase some freeze spray and a non-conductive probe.I'll pop da top and have a look around and locate the circuits involved.This sounds like, hopefully, an easy fix !

author: andy3055
author_rank: Super Contributor
timestamp: September 21, 2022, 04:49:04 pm
content: Non conductive probe = plastic chopstick.

author: bdunham7
author_rank: Super Contributor
timestamp: September 21, 2022, 07:03:07 pm
content: Quote from: Randall on September 21, 2022, 02:42:50 pmI'd start by putting a voltmeter on the +12V, -8V supplies and the junction between the emitters of those transistors. Let it warm up in each case and observe the voltage to see how it changes when the scope goes crooked. I'd start with the junction...

author: Randall
author_rank: Newbie
timestamp: September 21, 2022, 07:19:44 pm
content: THx !As I wait for the opportunity to pick up supplies I thought to look again at the display as it warmed up.It's getting a little more jittery and when it goes CockEyed it takes less than a second to go off kilter.This is in response to a question above about the display.I'm cleaning off my bench to avoid clutter and give proper respect to the high voltage I will encounter.

author: Gyro
author_rank: Super Contributor
timestamp: September 22, 2022, 04:53:00 pm
content: That's good, it sounds like a nice thermal expansion induced intermittent rather than a pesky thermal drift issue then. Much easier to find.

author: Randall
author_rank: Newbie
timestamp: October 06, 2022, 03:04:10 pm
content: Well after Playing with it I found a connector that is questionable.I cleaned the connector in the photo below, the associated connectors, and all controls on the front panel. I tested the scope without a problem all day. But the next time I turn it on it went off kilter when I bumped the bench hard.What is the recommended method of re-tensioning these connector types? see photo.Red arrows point to the male and female ends of the connector. I believe it goes directly to the front panel rotation control for the trace.I don't want to be HAM HANDED f--k it up !

author: wasedadoc
author_rank: Super Contributor
timestamp: October 06, 2022, 03:31:17 pm
content: Typically each metal contact has a barb that stops it coming back out of the plastic shell. If you carefully depress that, you should be able to pull the wire and contact out.Before reinserting you may need to carefully bend the barb back outwards a little.Having said all that, are you really sure that is the problem?

author: MikeK
author_rank: Super Contributor
timestamp: October 06, 2022, 04:17:07 pm
content: Is the issue recreated when fiddling with that connector? If not, move to something else. Hard to believe a connector that has seen only one or two cycles needs to be "retensioned". Dirty or oxidized contacts maybe.

author: Gyro
author_rank: Super Contributor
timestamp: October 06, 2022, 04:46:02 pm
content: Don't rule out a bad solder joint on one of the header pins. That would tie in better with the previous thermal behaviour and still respond to connector wiggling.

author: Randall
author_rank: Newbie
timestamp: October 12, 2022, 03:50:04 pm
content: CocckEyed Oscope Videosee Video here:https://youtu.be/LBljcNHVAhQI had retensioned the plug connecter mentioned above on Oct 6th and the tilted traces didn't return until I made this video.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 13, 2022, 12:06:17 pm
content: I'm with Gyro here. some connectors have problems where they are soldered to their board. sometimes you can see a "bullseye" in the solder around around the soldered pin.Sometimes this is hard to resolder correctly, Corrosion, I think.

author: EPAIII
author_rank: Super Contributor
timestamp: October 13, 2022, 11:24:20 pm
content: Re: Connector Repair1. Remove the header from the PCB. Clean the pins. Check the pins for proper wetting by the solder. If needed, remove all solder and clean with pencil eraser or fine abrasive (220 grit or higher) or wire brush. Coat with flux and re-tin. Check the foil pads for proper wetting by the solder: if needed, clean and re-tin as above. Replace header on PCB insuring good solder joints.2. Get the proper tool for removing the contacts (pins) from the body of the female connector. Check for proper crimps by pulling on each wire, one at a time. If any wire comes out of the crimp or even moves by just a tiny bit, the crimp is bad. Either replace any bad crimps with NEW pins or try to solder the existing wire in the existing pin: this may or may not work well. Look for the solder flowing into and wetting the crimped area (at the wire crimp area, not at the insulation part of the crimp). If the solder beads up, and more flux and heat does not cure this, then you must replace that pin.If you need to replace a pin, either buy one that is intended to be soldered or get the PROPER CRIMPING TOOL. I have seen more problems due to the wrong crimping tool or the improper use of a crimping tool than I can count.For a proper crimp both the wire and the metal of the connector MUST flow under enough pressure to produce SPOT WELDS at multiple points inside the joint. Creating such a condition requires the correct tool and the proper usage of that tool. The tool must be matched to BOTH the connector and the wire size. BOTH!!!That's how I would repair a bad connector of this variety.Quote from: Randall on October 06, 2022, 03:04:10 pm