Picked up another toy today. Spent the late afternoon cleaning it up and started fault finding. It's a "Tektronix 502 Dual Beam Oscilloscope", originally introduced in 1958. Its one of Tek's low-end models from the time and doesn't accept plug-in vertical amplifier(s). It's a little smaller than the plug-in 500 series mainframes, but similarly still has a 5" diameter CRT.
The (identical) vertical amplifiers have differential inputs and 200uV per division/cm sensitivity, though bandwidth is dependent on the sensitivity setting and is quite low at 200uV - 100kHz.
The thing initial reeked or cigarette smoke and the build up of (conductive) cigarette tar in some unusual places has been causing problems. I initially could not get anything up on the CRT. The first thing to sort out was the low voltage regulated supplies. There are three LV regulated rails; -150V, 100V and 350V. The -150V rail is adjustable and is the reference to which the other rails are regulated. As a consequence if the -150V rail is out of order, so are all of the others. The -150 rail was well out of whack, with about 80% mains ripple and an average output of ~-175V. The electrolytic filter capacitors were dried out. I made a temporary fix by soldering a new electrolytic capacitor across the tag terminals of the chassis mounted filter capacitor for the -150V regulator and the -150V rail then came up good. The other rails came up within specification, voltage wise, but still with excessive ripple, though still passable for preliminary testing. The 3 regulated rails are derived from their own transformer secondary windings and filter capacitors.
The next step was to get something up on the CRT, which at that point was still blank. I started by checking the static deflection plate voltages to rule out the beams being deflected off screen by either a pair of defective vertical amplifiers or a defective horizontal amplifier. They all came up fine and they sit at approx 230V nominal/mean. I then adjusted the vertical and horizontal position controls for 0V between each respective pair of deflection plates (thus ensuring that the beams are centered on screen). However the CRT was still blank. I then checked out the -2900V regulated rail for the CRT cathodes and that came up OK, but the fault was eventually tracked down the the grid supply. The 2M intensity potentiometer measured ~300 ohms and as a consequence grid bias to the CRT remained in a state of beam cut-off. Upon opening the pot I found it to be full of stinky yellow cigarette tar, but otherwise look quite OK. Giving it a good clean out with electrical cleaning solvent restored it to 2M and I then got some nice and bright traces on the CRT, though the focus was badly out and the front panel controls had no effect. Turned out to be the same issue - the two 5M focus pots initially measured ~400k each but came good after a clean out.
The scope now appears fully functional, with the exception of a timebase triggering fault, which can be seen on the odd display generated when triggered on a 1kHz sine, as shown in one of the attached photos. The fault appears to be induced by excessive mains ripple on either or both the 100V and the 350V regulated supply rails, which are still operating with their original dodgy electrolytic filter capacitors. Before progressing any further with the fault finding / calibration / restoration I will have to replace all of the electrolytic capacitors in the power supply.
The scope has some weird packaged plug-in transistors. There is a total of 3 transistors in the scope and one selenium rectifier, forming a fully solid-state regulated PSU of 6.3V to supply the heaters of the input stage tubes of the vertical amplifiers only. This measure was taken due to the high sensitivity of the vertical amplifiers, to prevent any significant mains hum being induced into the desired signal via heater to cathode coupling.
A pretty straightforward resurrection, this one. The scope is also in great shape - the front panel is almost flawless.
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