After buying and refurbishing some older Fluke DMMs for use on my workbench during the past couple years, I recently decided to try something a little more complicated. So I found a crusty old "parts or repair" Tektronix 465 on eBay. It looked physically intact and the pix showed that it at least made a dot when the beam-finder button was pushed. Good enough, so I set about to win the auction.
The scope arrived safe and sound since it was well-packed. And yep, it was a bit dirty and pretty much dead. Looks as if it has spent the last 20 yrs in someone's garage.
Looking inside: The sweep/trigger board, with the timebase board behind it. Power supply transformer and filter caps are visible towards the rear.
The vertical amp board is on the other side.
The power supply, high-voltage, horizontal deflection and calibrator circuit are on the bottom board:
The vertical deflection driver is nestled in beside the CRT neck. Lots of flying leads and point-to-point construction on the output. I won't be touching this unless absolutely necessary.
Powering it on, I got a dot when I pressed the beam-finder, and that was it. Checked all the power supplies, all of them are pretty much spot-on with no ripple to speak of. All of the transistors are socketed, so since there were some signs of oxidation, I re-seated every transistor I could get my fingers on. Horizontal sweep appears! Looks linear and perfectly normal. But no vertical deflection at all, including the position controls. And the intensity control has to be turned all the way up to see anything. Which leads to the trace disappearing at higher sweep speeds.
The Tek 465 has a "trigger view" switch which basically disconnects both vertical amps and instead connects the vertical deflection driver to a trigger signal sample from the sweep/trigger board. It's a cam and leaf arrangement built right onto the PC board. Punching this switch vigorously showed some noisy vertical deflection. Examining the switch I can see some crud in it.
So I cleaned it by soaking a paper strip in IPA and moving it back and forth between the contacts.
Vertical deflection now looks pretty good, so on to the intensity problem. Selecting either the "Mix" or "A-intensified" horizontal modes should result in the delayed section of the sweep being very bright. Instead, it seems this section is about the only thing readily visible. The regular part of the trace is just barely visible with the intensity turned to max. Not normal, but somewhat promising.
The high voltage supply is supposed to be -2450V. Checking it shows -2400V, so I tweaked the HV adjustment up a bit. The manual says the CRT grid bias should be adjusted for a dim dot in X-Y mode when the intensity control is set for a 20V output on the Z-axis (intensity) amp. Interesting, the Z-axis amp output will only go to 15V with the control at max. The Z-axis amp consists of 4 discrete transistors, so I check through it and can't find anything wrong. The inputs to this amp are several diode-ORed signals from various places. Blanking inputs pull up, intensifying inputs pull down. So I start checking the blanking inputs, and when I get to the "Chopped Blanking" input, it looks fishy. Pulling the cable for this input results in a very bright trace!
Working backwards, the chopped blanking signal comes from a common-emitter transistor amp, which is fed from a small toroidal coupling transformer. It's supposed to provide beam blanking while the beam traverses from one channel to the other in chop mode. The transistor tests good, but is never turning on because it appears to have no DC bias. This results in the collector resistor pulling up the Z-axis input really hard. Found it! The secondary winding on the toroid is broken right in the middle. It appears to have taken a serious blow, perhaps from the case of the scope.
As a temporary fix, I scraped some enamel off of each broken end and made a little solder splice. I'll make a more permanent repair after locating some enameled wire to rewind the secondary.
Success... It's alive!
Further checkout reveals no other major problem that I can find. Both vertical and horizontal calibration is a little off, but not too much. The front panel area needs serious cleaning, and of course all the vertical attenuator and timebase switch contacts as well. I'm pleased and somewhat surprised that the scope is operating this well without having to replace a single component (except for the bypass cap I broke with a pair of needle-nose pliers.)
Next up, some disassembly and cleaning...