Tex 465 Sweep Issue - Page 1
Forum section: electronics / repair

author: litmister
author_rank: Newbie
timestamp: May 11, 2016, 12:15:10 pm
content: A few years back I picked up a Tex 465 in perfect working condition. I used it a fair bit for a year or two, but it then sat unused for a while. The other day I went back to use it, and operation wasmostlyas expected. There is a strange fault present, though, and I haven't been able to diagnose it. The ends of the trace appear to be "reflected" back onto the main trace. The reflections occur about 1cm from the edge of the screen's edges and remain at the same points no matter how the horizontal position is set. Adjusting horizontal position such that the edge of the trace does not cross the "reflection point" stops the reflection. Any ideas on what this could possibly be?

author: tggzzz
author_rank: Super Contributor
timestamp: May 11, 2016, 12:43:11 pm
content: Sometimes switches have a buildup of crud that can be removed by operating them several times. Don't forget the beam-finder control.The next thing to check are that all the power rails have the right voltage and that the ripple is not excessive. It sounds as if you could use the 465 to measure its own internal ripple voltages.Take great care with the higher voltages, i.e. anything from 100V to 300V to 2kV to 17kV. Avoid the Duning-Kruger effectThere are free online manuals available, and google can be very helpful. There's also the Yahoo TekScopes group, but it would be polite to do homework and simple sanity checks first!

author: MrSlack
author_rank: Frequent Contributor
timestamp: May 11, 2016, 12:50:45 pm
content: +1 for checking the power rails. The 55v line was off on mine and did that.Also leave it at least 30 minutes to warm up before you assume it's broken - I've been caught by that one a couple of times nowEdit: my main 465B's trace unblanking doesn't work and the trace intensity is all over the place for about 20 minutes if it's been left somewhere cold for a couple of days. can I be bothered to find the issue? nope.

author: tggzzz
author_rank: Super Contributor
timestamp: May 11, 2016, 01:41:26 pm
content: Quote from: MrSlack on May 11, 2016, 12:50:45 pmI would guess an electrolytic takes 20 minutes to re-form inside. My 485 took 60s to turn on, then was fine, and worked if turned off for minutes hours. Next morning that repeated.My 465 couldn't be dimmed and was a good approximation to a torch. I eventually isolated the probleminsidethe CRT, but before that I found out more than I wanted about scope Z waveformsIf the trace brightness can be changed normally and only the unblanking is affected, then I would bet the problem is not in the high voltage section, but is probably in what is driving Q1466 in the Z amplifier in the "CRT Circuit". (N.B. that's from the file tek-465-late.pdf available at several places around the web).

author: MrSlack
author_rank: Frequent Contributor
timestamp: May 11, 2016, 04:15:27 pm
content: That's what I thought initially but all the smoothing caps have been replaced with nice new Nichicon ones on this unit.If you stick it in X-Y mode you get a clear easy to focus dot right away in the middle of the screen so this is down to one of the amps for sure.

author: litmister
author_rank: Newbie
timestamp: May 11, 2016, 06:01:17 pm
content: Quote from: MrSlack on May 11, 2016, 12:50:45 pmJust gave her about 15 to warm up and the issue is gone. Great to know and certainly makes repair a lower priority, but something still must be wrong, correct? I'll check power rails next time the problem occurs.

author: MrSlack
author_rank: Frequent Contributor
timestamp: May 11, 2016, 06:21:48 pm
content: Unfortunately it makes it more difficult to diagnose the issue

author: litmister
author_rank: Newbie
timestamp: May 11, 2016, 06:31:57 pm
content: Update: a few minutes off and the fault is back. 95v rail is at 73 volts. I have the service manual around somewhere, I'll dig that out and have a peekSent from my Moto G 2014 using Tapatalk

author: AF6LJ
author_rank: Unknown
timestamp: May 11, 2016, 06:42:35 pm
content: Good deal, happy hunting. Let us know what you find.

author: litmister
author_rank: Newbie
timestamp: May 11, 2016, 10:17:53 pm
content: So I may have found the culprit, but now there's a new issue... I found a capacitor I suspected to be the issue. Upon replacement, the trace was way off the top of the screen and could only be moved onscreen by turning the vertical control all the way to the end. It was also quite fat. I figured there must be a connection issue with the vertical module, which I had to remove to access the capacitor to replace. After removing, checking, and reseating the vertical board (and associated connectors such as the channel 2 coax and vertical deflection wires) there's a high frequency buzz that sounds like the high voltage power supply. This was not there before and I'm not sure of the cause, or if turning the scope on for long could damage something. Anybody come across something like this or have ideas of what to check?

author: tggzzz
author_rank: Super Contributor
timestamp: May 11, 2016, 11:16:01 pm
content: Quote from: MrSlack on May 11, 2016, 04:15:27 pmNot all electrolytics are across the power supply rails. Ive had two that werent., one in a 485, one in a 465.

author: MrSlack
author_rank: Frequent Contributor
timestamp: May 12, 2016, 05:38:06 am
content: I've done all those as well. All the tants and the other electrolytics.Quote from: litmister on May 11, 2016, 10:17:53 pmBuzz could be the HV supply. Worth checking that out. I've not seen/heard that one yet

author: litmister
author_rank: Newbie
timestamp: May 12, 2016, 12:27:25 pm
content: Quote from: MrSlack on May 12, 2016, 05:38:06 amThat's what I was thinking too. Really strange though how I didn't touch anything affecting the HV supply and the issue suddenly came about.

author: MrSlack
author_rank: Frequent Contributor
timestamp: May 12, 2016, 01:42:57 pm
content: Old analogue oscilloscopes are sneaky and like to annoy you. One way they do this is the "ninja attack", a spontaneous persistent fault just when you need them the most.