Tektronix WFM601i Power Supply Repair - With pictures

[TEKhacker]

I have just fixed a Tektronix WFM601i Standard Definition Digital Waveform Monitor, fault was no power.
Initial inspection revealed a sticky substance that had removed some of the coating over the tracks, under a 5v output capacitor, these capacitors were changed but still no power.
Power input resister was very burnt, according to the diagram it should be 3 ohms, on test it was nearly 3k ohms, resister replaced still no power.
I worked through the Service Manual section on power supply all as it should be.
With the device on an isolating transformer I carefully checked the operation of the UC3842 controller chip.
It was working fine, the 16v supply would fire up the chip, run for a short time then shut down as the supply dropped to 10v, this is how it should be.
With a scope probe on the UC3842 pin 8 (+5V OP) as the positive trigger and another probe on pin 3, I could see the shutdown logic switching correctly, with the power on button latching the output on & off. Pin 4 the timing pin had a good sawtooth but there were no drive pulse on the output pin 6. My attention focused on pin 2 the Error Voltage, this should be 2.5v, it was over 3v, this will force the output drive off.
The error voltage comes from the transistor side of the feedback optocoupler, the collector is at +5v Primary.
Looking carefully I could see green slime creating a path between collector & emitter of about 2k2 ohms, this is enough to raise the error voltage above the operating point.
With the optocoupler removed, cleaned and re-installed the power supply sprung into life.
The +11v & -11v were a bit noisy, causing the menu overlay display to jitter, new capacitors fixed them.
I don't know what the green slime was, I suspect it came from a leaking capacitor, the optocoupler is at the bottom of the board so it may have just drained there?