I have two failed VT100 (H7831) supplies in front of me. I gave up
trying to fix them because I think they're some kind of unholy circle
jerk-- where you can't have +12 unless you have +5 and you can't have +5
unless you have +12. I decided I will design a new replacement before
I spend any more time trying to fix these.
Anyway, on one of them, the bottom side of the PCB is totally burned at
Q8, D31, D33, Q1, and R37. It was so burned that traces were disappeared
and I had to reconstruct much of the board with copper tape and epoxy in
that area. This is the +12V rail. All semiconductors in that circuit
replaced, shorted caps replaced, and still no life (ie, no +12V out).
I think T4 is shot on this one.
On the second one, PCB is "dark brown" under D8, D11, and R55. In the
same area, a previous investigator has replaced C33 with two paralleled
470uF, 50V axial caps mounted high above the board and zip tied to a
nylon adhesive backed hold down stuck to the side of T2. This supply
has correct output voltages but will not deliver even 1A at 5V... and
is supposed to be capable of 11A.
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...
On Thursday (08/28/2014 at 03:29PM -0500), Julian Wolfe wrote:
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'
> > 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.