17 Aug
2016
17 Aug
'16
3:58 a.m.
2016-08-17 11:14 GMT+02:00 Peter Coghlan <cctalk at beyondthepale.ie>:
I could have filmed the behaviour but never did. But the size of the
picture was jumping which goes well with the bad input voltage. The
linearity was fine.
Yes. The waveform is present on the +12 output capacitor. There are dual
regulators. The main is only regulating the +5V and then there is a
secondary to regulate the +12V.
It is a VT100 PSU (H7831). I tested it with dummy loads and it worked
fine.
But when used in the terminal with the Basic
Video board and monitor
board
it gives a jumpy picture. Both horizontally and
vertically. First I
thought
that it was related to the monitor board but soon
recognised that the
+12 V
had a most peculiar waveform on it:
I've seen a lot of VT100s with poor vertical linearity and of those, many
jumped vertically as well. I never got to investigate whether this was
poor adjustment and/or noisy controls or a more real fault.
http://i.imgur.com/d0z0NQS.jpg
http://i.imgur.com/gQqmSN5.jpg
http://i.imgur.com/P0dt5y1.jpg
This waveform is only present on the +12V, not the +5V, not the -12V or
-23V.
So I connected just the Basic video board and a variable dummy load
instead
of the monitor board. I used a Variac on the
input. It turned out that
there were no problems now with the +12V. Until I pulled out and
reinserted
the keyboard. Then it was there. If I lowered the
input voltage it was
impossible to provoke this problem and also if I increased the +12V load.
Further testing also gave that putting a few amps extra load on the +5V
also made it resistant to this type of failure mode.
The amplitude and frequency of this waveform is shifting by +12V loading
and AC input voltage.
The VT100 SMPSU is a primary switcher regulating the +5V. Then the +12V
is
handled by a secondary switcher which is
synchronised with the primary
switcher. The other voltages have linear regulators.
It looks to me that something in the regulation circuitry is not
behaving,
thus oscillating. But what component has failed
(or is out of spec)? I
checked transistors. I checked the waveform from the 555 chip and ramp
voltage input to the 555. But I cannot figure out what the problem is.
I checked the 560uF output capacitor but my LCR meter said it was in good
shape. Around 700 uF and very low ESR.
If the waveform is present across the +12V output capacitor and +5V is ok
and
and the regulation only monitors the +5V, it sounds like the power supply
is
doing it's best and maybe the problem is in the +12V load?
Alternatively, could there be an abnormally varying load on the +5V which
the
power supply regulation managing to smooth out but in doing so causing the
unmonitored +12V to vary?
The +12V is not unmonitored since there is a switching regulator there
using a NE555 chip and a LM393 regulating the +12V as a secondary side
SMPSU.
The main regulator is a SG3524 chip. The waveforms from the SG3524 and the
output on the +5V looks perfect. It is just the +12V that is bad which
directs me to the NE555 and LM393 regulator circuitry.
Monitoring the current waveform in the +5V and the
+12V might throw some
light
on this, maybe by putting the scope across a small series resistance in
each
line.
That is an interesting idea. I will bring in my current probe.
I guess the 560uF capacitor referred to above is on the 12V line? The
corresponding capacitor on the +5V might also be worth checking in case
the regulation of the power supply is managing to compensate for it
being bad.
I will take a look at the +5V output cap as well just to be sure.
If I put in the VT125 option board the terminal behaves just fine, so the
loading seems to be of importance.
Regards,
Peter Coghlan.
/Mattis