On 21/08/2014 23:38, Matt Burke wrote: operation? After a bit more searching of the archives I found a thread started by Julian Wolfe who noted some burning on R11, R21 and R55, which is consistent with my two power supplies (no mention of R27 though). A few people commented that there was probably an issue somewhere but unfortunately the thread does not run long enough to see what the eventual outcome was. Has anyone here ever worked on a VT100 power supply? I just want to know if I should investigate this further or whether it's designed to run this hot. Personally I can't see anything wrong, especially as it seems to work just fine. Thanks, Matt

On 21/08/2014 23:38, Matt Burke wrote: operation? After a bit more searching of the archives I found a thread started by Julian Wolfe who noted some burning on R11, R21 and R55, which is consistent with my two power supplies (no mention of R27 though). A few people commented that there was probably an issue somewhere but unfortunately the thread does not run long enough to see what the eventual outcome was. Has anyone here ever worked on a VT100 power supply? I just want to know if I should investigate this further or whether it's designed to run this hot. Personally I can't see anything wrong, especially as it seems to work just fine. Thanks, Matt

If the diode does not appear to have been replaced, replace the diode. Don't even question it, just do it. It will take 4 or 5 things with it if it goes. That's what happened to mine. On Thu, Aug 28, 2014 at 3:08 PM, Matt Burke <matt at 9track.net> wrote: > On 21/08/2014 23:38, Matt Burke wrote: > > I'm currently restoring a couple of VT100 terminals. After checking the > > power supplies over and replacing the odd capacitor on the output side > > I've had both of them running on a dummy load. They seem to work OK but > > I've noticed that R27, which is a large 13W power resistor, gets quite > > hot after only a few minutes of being powered on. This resistor is part > > of a snubber circuit on the primary side of the main transformer. After > > about 5 minutes I measure a temperature of around 95C (200F) on both > > power supplies. > > > > Another component that appears to have been quite hot over time (burnt > > PCB) is R55, but perhaps this is just an underrated part? > > > > Does anyone have a VT100 and can confirm whether this is 'normal' > operation? > > > > Many Thanks, > > > > Matt > > > After a bit more searching of the archives I found a thread started by > Julian Wolfe who noted some burning on R11, R21 and R55, which is > consistent with my two power supplies (no mention of R27 though). A few > people commented that there was probably an issue somewhere but > unfortunately the thread does not run long enough to see what the > eventual outcome was. > Has anyone here ever worked on a VT100 power supply? I just want to know > if I should investigate this further or whether it's designed to run > this hot. Personally I can't see anything wrong, especially as it seems > to work just fine. > > Thanks, > > Matt >

Are we talking abotu the same supply -- the SMPSU from a VT100 (and not the VT101 supply which is very different)? I find the VT100 PSU to be quite pleasnat to work on. In common with many other DEC supplies of that period, the chopper cotnrol circuitry (oscillaotr, etc) is on the output side of the isolation barrier and runs off its own little linear PSU. So yo can remove the chopper transistor, power up, and clip the 'scope wherever you like around the chopper control IC. -tony

I will check... I've not got the printset in fornt of me (and having jsut come round from a gneral anaesthatic, I do not feel like hunting for it tongiht...) but if R27 is part of the snubber netowrk, then check the associated capacitors adn diodes _very_ carefully. -tony

I started down the path of designing a modern replacement for these-- which pretty much amounts to a carrier board on which one can mount modern, off-the-shelf switchers for +5 and +12 and then a couple small home grown converters fed from +12V generate the -12V and -23V rails. But-- this is an unfinished work. I really only need one so the economics of it are poor...