Hi Steven,
The TDS460 and TDS420 both have a known no-boot failure mode, easily identifiable by the MC68020 µP /RESET pin held constantly low.
Culprit is an open-circuit 27.4 kOhm SMT resistor, at the top of a voltage divider connected to the +5 Volt bus, on the PFI pin (9) of MAX695 µP supervisor.
PFI remains at essentially zero Volts, /RESET never goes high.
The repeat offender is visible next to MAX695, under clear potting. I *think* it's on the CPU card, or adjacent card.
RF+ Tech
Thank you, I checked over the PS, it looked OK, I did not change any caps in the PS, nor have all the set caps come in yet, so I pulled off the one from the CPU Board, since this set has 1995 date codes, I think it is likely beyond most of the "cap" problems. So I checked the 10uF set cap, it measured 9.9uF. basically on spec for 10uF., no leakage, so I soldered it back in.
I then soldered a new 1/4 Watt 25.4K across the suspect 27.4k set part near the Maxim chip, replaced the PS, and started it up.
It blinked and blinked for a while, the CRT came to life, it displayed a few more leds on, leds off, and was taking a bit longer than expected. I turned it off, then tried it a second time. Again the CRT came to life, it seemed to be going faster now, with what are usually for the starting sequence and then indicated "test passed". and went to the graticule and all four channels showed background noise when pressed too become active. However, only channels 2,3,4 showed a random noise when I put my pinky in close contact with the center of the BNC. So I think the Attenuator for channel 1 may have a problem...
Others have found blown parts on the ceramic substrate of channel 1 and repaired that, now I just have to locate that info again.
Next time I fire it up I'll do more extensive testing with a probe and the CAL signal.
Looking good at this point, at least 3 out of four working.
-Steven