Smoking 2465B (Rifa/30-ohm) + Recap = PS will not start


Yesterday was using the 2465B (SN+50K) and a loud POP and a puff of smoke came out the back. After inspection sure enough I spotted the remains of one of the RIFA caps. The 30-ohm resistor in series with it was also toast. I have been putting off the full recap of this scope for a while now, with new Mouser parts waiting in a bag. Since I am on 120VAC (more like 127-129V in my area) I figured I had ample time, although the Rifa caps looked quite crazed last time I saw them when I did the NVRAM/A5 work. So maybe now would be a good time to recap, not just fix the damage.

Full recap, only one cap at a time to avoid mixups or mistakes. The only ones I did not have was the two 2200pf 250VAC, one 0.01uF 250V and one 0.056uF 250V film caps, back then forgot to order them. The two 2200pf look a bit crazed, definitively will replace them asap. The 0.068 Rifas where replaced with 0.1uF X2 300V Kemet Safety caps that where suggested as a better substitute in one of the cap lists.

Also replaced the 4.7uF 10V Tantalum with a fresh 4.7/50V same composition type unit. The 3x 1uF 50V BP caps where also replaced, and the new ones are much smaller, tiny, in comparison to the originals so hope they still are ok to use (Mouser PN 647-UMP1H010MDD1TP in case anyone wants to check). Plus two new beefier 15-ohm 3W AC input resistors, and 2W for the 270K on the power supply. Even replaced the two Sprage main caps with 330uF 250V Radial Nichicons by extending their negative wire to reach the board. The originals still look pristine but since I already had the new ones on hand went ahead and swapped them as well.

BTW, all of the tall blue caps (180uF and 250uF) where more or less starting to leak from underneath, not much residue on the board, but definitively the smell when desoldering some of them was the biggest tell, and all of them had a slight oily residue between the leads. C1111 was the worst offender. All other caps overall looked quite good, have not tested any of them yet.

Before putting the PS back together double-triple checked that all caps where at the right locations, correct value and polarity orientation.

Upon scope power on the fan runs, all the front panel indicators flash only once in unison and then turn off, except for the green Trig/Sweep led staying on. The 3 GPIB Leds stay on continuously. Nothing else happens. Nothing on screen, no trace, nada.

I checked that all cables in/out of the power supply where plugged in correctly: the one going to the A5 is foolproof. The other two connectors on the power supply (the one with the multi-colored cables under the fuse and the other that only has white cables next to it) are properly seated and aligned with the pins on the board. The only other plug gors to J2474 has 3 small cables going towards the GBIB board and it is orientated correctly (printed letters/numbers up). Checked continuity of all the "stab" pin connectors as I understand they sometime can create a loose/intermittent contact if inserted the wrong way. All pins checked out fine.

Here are the the rail voltages at J119:

+15 -> +10.4V
-15 -> -11.7v
+5D -> +4.3V
+5A -> +3.3V
-5 -> -3.5V
-8 -> -5.3V
+10 -> +7.3
+42.4 -> +28.8V
+87 -> +59.6V

Needless to say all rails are way low. I even measured the voltage across the two new main caps on the J231A/B connector and I do read about 150V on each, total a bit over 300V, so I'm thinking that juice should in fact be getting into the PS.

Verified that the voltage selector switch was securely connected to the two bottom tabs, and the wires from fuse holder and line filter go to the remaining two other taps further up the board. Just checked continuity of the voltage selector switch at the tips of the connectors that go into the board and its almost zero ohms as expected (115V position). I would think that if this switch or the wires where not connected properly, the PS would assume 240V input, and with only 120 going in it might create a low voltage situation like the one I seem to be having. But no dice.

At this point I have the PS out again and on the bench, no way to safely poke into it while inside the scope. First thing again checked all the new caps and everything is confirmed correct. At this point was also looking at the PS troubleshooting chart in the SM, and the W1060 test jumper procedure. But it requires a test load for which at this point I don't have the correct specified resistors at hand. BTW just to know, what is the spec of the diode and voltage of the 3.3uF cap used for the load? Manual only provides Tek PNs.

I would think that with the exploding Rifa cap and burned resistor being well ahead of the actual power supply nothing else would have been damaged. Not even the main fuse blew, and I think it was just time for the cap to let go, and not because of an incomming huge spike or surge although could not be 100% sure. But I had my 2247A also turned on and some other equipment at the same time and on the same AC circuit, and everything else its fine. Nothing else around reacted to any abnormalities in the AC line either.

Any words of wisdom as always much appreciated.

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