Help repairing a Textronix 465 scope - Page 1
Forum section: electronics / repair

author: rt160001
author_rank: Newbie
timestamp: January 09, 2014, 04:27:08 pm
content: This is my first post to this forum so a big HELLO to all. I'm a long time tech looking for some help on a tektronix 465 scope as it is kicking my butt. This is my first scope repair. The visual symptom is no horizontal trace. The horizontal position needs to be full left in order to see the dot on the screen and can't be centered. The A channel position appears ok, trace moves from top to bottom of display. The B channel will not go above the center line and as you move though the volts\div the dot zips off screen in some positions and zips back in other positions. A signal applied to A or B results in vertical deflection. In X/Y position a signal applied to A results in horizontal deflection and B vertical. The power supply rails are ok. I do not have any signals from the sweep generators, and it appears that the 55volt supply breaks out into DCPL-A, and DCPL-B and both are being pulled down, .093 and .3 respectively. According to the schematic these should be appox 12.3 volts seen at J1 pin 3 (A channel) and J2 pin 3 (B channel). I feel I'm chasing my tail so any help is greatly appreciated.

author: PaulAm
author_rank: Frequent Contributor
timestamp: January 09, 2014, 05:49:31 pm
content: Was this scope working before, or did you get it in the current condition?Have you measured the PS ripple? What are the actual PS voltages? Measure the ripple, not just the supply voltages. A common failure is open filter caps and large amounts of ripple mess everything up. It should be in the mv range.In X/Y with no input, can you position the beam anywhere on the screen using the positioning controls? If the answer is no, that gives you a clue on what to focus on.Is your trigger mode set to Auto? There will be nothing out of the sweep generator if there is no signal input and the mode is not in Auto. It may be disabled in X/Y mode as well.Have you exercised the switches? They can get dirty if not used for a long period.Depending on the year of manufacturer, there may be a problem with the transistor sockets. They don't look like sockets but they are and oxidation can cause significant problems. I've fixed more than one scope by pulling and reseating the transistors.There could be more than 1 problem. You need to narrow things down a bit.

author: rt160001
author_rank: Newbie
timestamp: January 09, 2014, 06:57:40 pm
content: Thanks for viewing !! The scope was found in it's current condition. It's been sitting around a long time. The first thing done was exercise all switches. Power Supply measures: +110V, 107.4, 4mv-- +5V, 4.99, 1mv-- +55V, 54.8, 1mv--, +15V, 14.97, 1mv--, -8V, 7.97, 1mv. All ripple measured with DMV set to AC. (best i got)In X/Y mode I have limited horizontal and vertical movement. The horizontal position needs to be full left to display beam. Display 2 grads from right side, won't go to center. Vertical position control goes from center (full CCW) to way of screen (full CW)Trigger set to Auto, and the transistor sockets are crap! blah! Easy replacement but I can't believe they used these things!You hit the nail on the head with the narrow things down a bit, there is alot going on in a scope, triggers, generators, etc. I realized I needed some help.

author: PaulAm
author_rank: Frequent Contributor
timestamp: January 09, 2014, 07:56:29 pm
content: Do you have access to another scope? Even a crappy one? It's almost impossible to fix these without one.I'd start with the horizontal and get that working right. It's a differential amp and you should be able to follow voltages through until you find the stage where they go wonky. Try to start with the H position control centered. Could be a bad transistor, shorted cap, drifted value, shorted/open diode etc.If all you have is a dmm to check transistors, get some replacements and do substitution. It won't show if they're leaky, have low Beta or get wonky under operating conditions. I just had a problem where a transistor looked perfectly good, even on a curve tracer, but the Beta had drifted lower causing the circuit to lock up under operating conditions. Sometimes specs mean something.Once you get the H amp sorted, go on to the sweep and leave the vertical for last.

author: N2IXK
author_rank: Frequent Contributor
timestamp: January 09, 2014, 07:59:03 pm
content: Do you get a centered spot if you press the "Beam Finder" button? While holding it in, do the H and V centering pots do anything?Does the spot respond to signals on the vertical input?

author: rt160001
author_rank: Newbie
timestamp: January 09, 2014, 10:09:45 pm
content: I can get a centered dot using the beam finer, the position controls are at extremes but it will center. The horizontal controls moves the dot. From center to full CCW, from center to full CW it has no response, Channel 1 position cntl moves the dot from almost top to almost bottom of screen and the channel 2 goes from center of the screen to almost the bottom. both channels respond to signals, although channel 2 volts/division is weird as described previously.I have a crappy scope, i can tell if there is signal but i can't use it for real measurements, I start by looking more into the horizontal position cntl.1 thing I've discovered today is: if the mix button is pressed or released I'll get a single trace across the screen, if signal is applied I get trace of signal, no signal I get flat line trace. It is only with the mix button being pressed or released. I'm not sure if that helps me or just complicates things further.

author: lowimpedance
author_rank: Super Contributor
timestamp: January 09, 2014, 10:31:21 pm
content: One thing to check is tantalum caps for short circuit. (I cannot remember if there are tant's in the 465 but I would be surprised if it did not since a lot of TEK gear around this vintage has them and they are a prime source of trouble on distributed power rails!!. (ie go short and pull rails down --as you have mentioned you have in your 465!)