A colleague found an old Tektronix 2235 in his shed, and thought nicely I might like to have it. Sadly, it blew its fuse after working for a few minutes when he plugged it in – and I decided to have a go at fixing it.
MOSFET P9070 had died a violent death, with notable effects on the MOLEX connector it’s in, as well as the plastic surround strapping it to the heat sink. It had killed R909 – the 39 ohm resistor between it and Q908, not on the schematic but in an addendum – Q908 and CR907 as well.
The MOSFET can be an IRF720 it seems, which is a nice and cheap component. Q908 I ended up using a 2N2907, which was slightly more difficult to come by. It’s just a PNP switching transistor, so might be easier to find something else that’ll stand the voltage.
The diode I first tried replacing with a 1N4007 – this didn’t turn out well. Another colleague found me a BYV 26E, 1000v/1a 75ns fast recovery diode, which works fine. However: After the first killing of the diode, it appears the MOSFET and transistor were taken out again, and I failed to notice. Caveat: Check these components every time a line power trial fails, as they may just be fried…
I ended up replacing U930 with a TL494 (about $1 for that) as well, and found a good way of testing it: Apply ~10v between TP940 and pin 12 of U930, and look for a sawtooth signal at pin 5.
Some of my testing I did with line voltage, but as mentioned before, I had problems with components dying on me. I ended up using a lab DC supply, set to between 25v and 50V, connected between TP950 and the upper leg of R926, so just after the rectifier. When connected this way, with a BAD MOSFET in place, output voltage could be seen at measuring points 42 and 43 – just after Q946/7 that is.
With a good MOSFET in place, the switch-mode circuit wouldn’t start (and P9070 not open) as the trigger voltage across VR925 does not rise high enough (at 50V DC supply) – shorting VR925 briefly opens Q930, and the supply starts running. I’m not entirely sure why this is necessary; I hope it will work just fine once I put in the proper 400v MOSFET and give it 240v AC …
Other observations: I disconnected the high-voltage wire to the voltage tripler, and taped up the end securely, while working on the prereg. This removed the nasty 2Kv from the board, but there’s still 100v around there, so don’t touch it … 🙂
I changed a few capacitors, since I was changing components anyway, but it seems that the ones I pulled out of it – from 1985 – were just fine, and didn’t need replacement. Quality stuff.