Hello everyone. I thought I'd register on this forum to thank the uploader of the schematics for the SM620, so thanks.
I brought 3 SM620s and an SM630 from an eBay based surplus/clearance seller last week so I went looking for some schematics before they arrived as I suspected they would be faulty, which they were. I didn't really need the schematics in the end as all 3 SM620s had the same fault, they were dead and drawing about 20mA from the DC supply. To cut a short story even shorter, there are 6 x 470u 10V caps, 3 100u 10V caps and a 220u 25V in the multi rail supply circuit and all were leaking, not from the top as is normally the case, but from the bottom of the caps around the rubber bung. All caps were good quality Elna long life parts. The transformer is driven by an LM3578 which was also found to be faulty as a result. All three units are working fine now.
The SM630 had an intermittent channel 1 BNC input which turned out to just be a broken wire.
It also had worn lugs on two of the PLASTIC (??) BNC sockets so I changed all 3 for metal ones. I also replaced all the caps in the power supply although none were leaking but...
The backlight on the SM630 is not that bright, not sure if this is a fault or if that is just the way they are, perhaps someone who's got an SM630 can let me know.