I can’t help myself, I find I need to further improve this signal generator with the replacement of a failed pass transistor socket so I found some new replacements at RS components. The items I ended up using is a Keystone item KEYSTONE 4602 TRANSISTOR SOCKET, 3 POSITION, THROUGH HOLE from Element 14, order code 1231819.
The Keystone part is not exactly the same as the original and I found it tricky to solder on the new one so that it is the same “height” above the PCB as the others but I got it close enough. The problems though is the legs of the mounting socket which are also the connectors to the circuit protrude through the PCB and is at a 90 degree angle to the power supply board behind (which the Pass transistor feeds). As it turns out one of the legs of the new pass transistor socket was sticking through just enough to come into contact with the heat sink of one of the transistors on the power supply board, the end result when I applied power was to blow one of the 3/4 amp 120V fuses and so I lost the +20V rail. Oh well, I fixed something and then I broke something – a familiar story.
Back onto Element14 and had a search around for some fuses to match, ended up finding one and it was received yesterday – LITTELFUSE 273.750 FUSE, PCB, 750mA, 125V, VERY FAST ACTING, order code 1200264. When trying to replace the old one I learned that in fact the original fuse is easily removable because it is pushed into 2 little sockets soldered to the PCB, one for each leg of the fuse – pretty neat! The issue though is the new fuse didn’t have the required leg spacing so I just soldered it to the back of the board where the fuse sockets are soldered so no harm done to socket if I ever want to replace with an original again.
Lastly, I had to replace the Rifa mains power capacitor – it sitting there in its sorry blown state – having let go the smoke years ago was doing my head in. Bought a Safety Capacitor, 2200 pF, Y2, B32021 Series, 300 V, order code 1609515 again from Element 14. Installation of that went fine, no new problems caused during the process.
Offending heat sink on the left – blown fuse top rightt 
See – it just touches 
The fix 
Replacement for Rifa mains power capacitor
I’m fixing mine too. All the emitter positions are weak, and one has a missing leaf. Looking through Mouser, it looks like Keystone pn 4606 is a better match. dimensionally it measures the same and looks to be similarly constructed. https://www.mouser.com/datasheet/2/215/4606-742215.pdf
I’m going to order some and see if they work.
I received my Digikey order and indeed the Keystone 4606 TO-3 socket is a direct replacement for the ones used in the 8640b.
I’ll be making my repair this weekend.
Good work mate, another one comes back to life (perhaps)…