Help with Tektronix 465 scope - Page 1
Forum section: electronics / projects

author: fwd
author_rank: Newbie
timestamp: February 06, 2013, 05:44:56 pm
content: I have a Tektronix 465 that has developed a problem. The screen lights up like a solid green raster when triggered, but no input. You can move the green raster up and down with the vertical position control. I have checked the power supplies. The 120V unreg. measures 165V with about .35V ripple, but all of the low voltage supplies are right on with the worst having 3mv ripple. I connected another scope to the vertical preamps output, and connected the 465 to the calibrator and I could see a nice square wave on both pre-amp output test points. Any ideas on what to check next? I do have the manual, and the S/N is B260xxx. It only has a couple of ICs, almost everything is transistors.Thanks,Frank---KA9J

author: ModemHead
author_rank: Frequent Contributor
timestamp: February 06, 2013, 09:37:33 pm
content: What happens if you run the sweep speed knob back to X-Y mode? Dot, vertical line, screen washout, etc.Also does the beam-finder, or intensity and focus controls have any affect?

author: PaulAm
author_rank: Frequent Contributor
timestamp: February 06, 2013, 10:03:19 pm
content: Sounds like the beam may be off to one side of the tube.Did you check the 110V and -8V? The 120 being 45V high is not right. Did you put a scope on that or just a meter?Also, be sure you check the correct +55V used by the V amp. I was misled for a bit while working on one of these by looking at the wrong +55V testpoint.

author: fwd
author_rank: Newbie
timestamp: February 06, 2013, 10:50:13 pm
content: Modemhead & PaulAm---thanks for your help.I ran the sweep knob full CCW to X-Y mode, and I still have a green raster filling the screen. Yes, intensity appears to work normally, as does focus, because when I push the beam finder I can focus the dots. Beam finder gives me 2 dots on the left edge of the screen, about in the center. They flash a couple of times when you push the button, and then flash randomly every second or so.Yes, I checked the 110V (116.2V no ripple) and the -8V (OK). I measured the voltages with a Fluke DVM, but no I looked at the 120V with the scope. It measures the same. I see that there are two 55V supplies. Both measure 54.8V with no ripple. By both I mean one @ TP1536 and the other at the anode of VR1515. Since I wrote my first message, I've connected the scope to the vertical deflection plates, and the voltage there depends on where the vertical position control is, but both seem to be be about 35 volts max. and will go down to about 15V when I turn the vert. position. I didn't read the scope exactly, but they both seemed to be OK. The manual has 34V printed in blue.What does the 120V power? Both capacitors have about 82 volts across them. If it's not supposed to be up at 165 volts, I would think that maybe it doesn't have much load on it?

author: ModemHead
author_rank: Frequent Contributor
timestamp: February 06, 2013, 11:30:33 pm
content: If you get dots with the beam-finder and a glowing screen without, I'll go with PaulAm's idea, the beam is off to the side and reflecting a spray back on the screen. Presumably the left side.Set the horizontal position to midrange and check for a severe imbalance in the horizontal deflection amp. The schematic should have nominal voltages for the key points.Testing my memory, but I think the 120V is mostly for the horizontal deflection output stage and the Z-axis amp. And if I recall correctly, the unregulated side of those supplies is quite a bit higher than the regulated output.

author: grumpydoc
author_rank: Super Contributor
timestamp: February 07, 2013, 08:23:10 am
content: Quote+1 for the "beam off to the left" idea.120V is unregulated so 165 is possibly OK.If it's still off to the left in X-Y mode probably check the final X amplifier.Some Tek scopes have separate connectors for the plates on the tube (on the neck rather than the back connector) and (one of) the wires can fall off. Not sure if that applies to the 465, or if the beam finder would pull it back but worth checking that the CRT connections are OK.It's worth joining TekScopes on Yahoo and looking through the message archive.

author: fwd
author_rank: Newbie
timestamp: February 07, 2013, 03:54:55 pm
content: It's fixed!!!!!! The wires to the horizontal deflection plates were attached properly (they do come out the bottom of the crt) but with the sweep set to X-Y and the horiz position control centered, I had 49V on one, and 114V on the other. I started checking and found an open 10K resistor. I replaced it and it's mate in the other plates circuit with a 10% composition resistor that I matched as close as possible with the ones in my stock. Now I realize that these resistors should be matched to keep everything linear, but I will have to find some precision 10K resistors (actually there are 2 10K resistors in series in each plate circuit, so I could replace them with 20K precision resistors). I fired it up and I had relatively equal and lower voltage on both plates, and I had a dot on the screen. Now I've got a normal display, and except for noisy switches and pots. it seems to work fine. Thanks everybody for your help, I learned a lot about oscilloscopes.Now I've got a Telequipment (I think this is the name that Tektronix uses in the U.K.) 20Mhz. scope with triggering problems that I'm going to work on after I put the 465 back together.Frank D.

author: ModemHead
author_rank: Frequent Contributor
timestamp: February 07, 2013, 04:49:43 pm
content: Excellent! Good job on the repair.

author: w2aew
author_rank: Super Contributor
timestamp: February 07, 2013, 04:53:51 pm
content: Excellent! The 465 is a great analog scope. My 465B only gave me trouble once, about 20yrs ago, with a faulty resistor in a power supply circuit. Been fine ever since.