Yes, a Variac is very usefull (remember to get one that is galvanic isolated)
Electrolytics can look fine, but still be defective. And in these old instruments, you might want to renew them all while you are workin on it
If you drive the output transistor fully on (for a "long" time), it will blow the fuse. But this will not destroy the transistor
So for the transistor to explode (it will short internally)
Either
1. the transistor gets very hot
- not driven fully on / off (remember that the transistor is used as a switch)
- overload
or
2. it is subject to high voltage (flashover/breakdown inside)
- defective snubber
- wrong input voltage
This SMPS is a flyback type and the transformer kicks back every cycle
a snubber is attached to "eat" this spike
CR16, R16, C20, C21
The connections are shown in the schematic on page 6 (but do check on yourself, as the picture is hard to read)
J2
1 +5,15V
2 +5,15V
3 +5,15V
4 +5,15V
5 GND
6 GND
7 GND
8 GND
9 +3,5V
10
11 -5,2V Analog
12
13 +12V Analog
14
15 -12V Analog
16 GND
17 +12V Dispaly
18 -5,2V Analog
19 +15,5V
20 GND
No need to build a new supply. These old SMPS can be repaired, but it can take a while. At least - you learn a lot
Components to investigate
Snubber resistor R16
Capacitors C17 & C46
Thermistor on B-E Q3 (51 Ohm NTC)
But your best approach is to measure the thing with a scope, while it is running
First measure all output voltages with some DMM's, to see if one supply is shorted