On 16.02.2016 19:27, Cursitor Doom wrote:
Hi again,
I know I said I was going to mothball this scope for the time being,
but I keep thinking of new things to try and just can't leave the
damn thing alone. It struck me I ought to next check out the main
transformer because if that's toast, the whole psu might as well be
binned. I think I *may* be on to something.
Here's the circuit again:
https://www.flickr.com/photos/128859...in/dateposted-
Hi
It looks like the circuit is tricky and you aren't getting anywhere yet.
The caps you mentioned won't do anything with the board outside the
scope. Even if they were completely shorted, as long as the board is on
your table and with no loads connected, they won't be noticeable. So you
can skip them for now.
I think that you should first try to pinpoint, in which general
direction to look for the fault, that is:
- in the power circuitry
- in the feedback, control and regulation circuitry
- in the output circuitry (rectifiers and capacitors)
- in the auxiliary supply (diodes V1816 to V1819 and caps)
If the fault appears in the power circuitry, it is likely
- due to dielectric breakdown (capacitors and transformer)
- due to diode breakdown
- due to operation outside of resonance (also control related)
To this end, you'll need a way to slowly power up this thing at
controlled, limited power conditions and try to see what works
(to a limited extent, given the conditions) and what doesn't.
I'll try to include some step by step instructions, but please be aware
that this is mostly instructions for the desperate, so please take care
to exercise more caution than you might otherwise.
You'll need a 12V load, an isolated variac, and a signal generator
capable of outputting a square wave with variable duty cycle. The 12V
load should be easy to observe visually, therefore preferably a lamp. It
should be substantial (not a christmas tree light), but still sort of
within the capabilities of this supply under heavily reduced voltage
conditions (so not a car headlight either). A car taillight lamp is, I
think, Okay, but a brake light lamp is maybe already too much.
You will also (actually first and foremost) need to pay attention to
good safety practices, and also don't increase any voltage too fast and
don't "take shortcuts". Patience is a virtue and "I'll just quickly do
...." has no place when working with a defective and live power supply!
---------------------------
First, disconnect L1803 and R1807 and R1804. This will make the
controller inoperative and allow you to drive V1806 externally.
Connect the signal generator to V1806 B-E (in parallel with R1812) A
signal generator with 50 Ohm output, set to +/- 5 V peak square wave
should easily drive the transistor into full turn-off and into full
saturation.
Connect the board (mains input) to the isolated variac (or to an
isolation transformer that is itself being fed from a variac), but keep
the voltage at zero first.
Set the generator to the nominal resonance frequency that this board is
expected to run at (value was mentioned somewhere in the previous
thread), square wave, duty cycle somewhere around 30 - 35 %, 5V peak.
Connect the lamp to the 12.7 V output and some voltmeters to other
outputs for reference.
Turn on the signal generator, make sure the waveform is right.
VERY SLOWLY start raising the input voltage with the variac. Don't go
higher than about 70 V at any time.
Watch out for unexpected problems (heat, significant increases in
primary current consumption, other signs of overload!
Try to power it up so far that you can see the lamp faintly beginning to
light, but not much further than that. Watch for overload conditions. If
you notice shorted rectifier diodes in the output (they would heat up)
or capacitors (also), replace them and try again. Don't let the
dissipation resistor (R1814) overheat, reduce voltage if needed. If you
can't get the voltage up enough for the lamp to glow (because something
else overheats first), try a small 3 volt lamp from a pocket light
instead. It should allow the test to continue at much lower voltages.
Now, try to adjust it into exact resonance (it won't automatically be
there with your "first try" coarse signal generator setting). Slowly and
carefully adjust the signal generator frequency (still with 35 % duty
cycle constant) a little up and a little down, try to get a feeling for
the direction and sensitivity, and then set it for maximum lamp
brightness (maximum brightness = resonance frequency peak). Keep the
frequency there. If you (at any point) notice the lamp becoming too
bright, wind the variac down. Don't ever let it burn out!
If you cannot get good resonance, even at low voltage, check C1806 and
C1807. They may be open-circuit. Capacitors do not always fail shorted,
sometimes they fail open, and sometimes they even fail with a weak (too
high resistance) connection inside (this won't be noticed on a LCR
meter). Especially foil capacitors at high impulse loads sometimes will
do that. If in doubt, replace always both together.
Make sure that you really have resonance - check the waveform at the
transformer. It should be a sinewave. Possibly with some distortion,
that's okay, but still mostly a recognizable sinewave. This test is
better performed with the small 3 volt lamp at a low variac voltage. If
you cannot get a reasonable waveform, it means that some component in
the power circuit is most likely shorted out completely. It may (or may
not) be a diode. Also don't forget V1816 and its mates, C1819 and C1821.
If you found a problem in this area, just disconnect all those 4 diodes
for now, you can replace them later.
Always remember to switch it off (wind the variac full down) when
changing lamps, in this mode the board is not supposed to run with no
load at all, at any time and for any time duration. Not even for a
fraction of a second.
Now put the 12 V lamp back in and dial the duty cycle on the signal
generator very low (preferably around 3 %, at least smaller than 5 %).
Make sure that you have the correct polarity (so that the switching
transistor duty cycle is really 3 % and not 97 %)!
Now slowly raise the variac again. Keep an eye on the lamp, another on
heat dissipation. Make sure the variac is isolated (or connected through
an isolation transformer) because you will want to try reaching the
nominal mains voltage now.
Keep an ammeter in the power input. Watch out for sudden increases in
current draw. Keep an eye on the lamp and on the voltmeter(s) on the
output(s).
Watch out for power "inversion" behavior. This is important! Watch out
for behavior, where the lamp suddenly becomes darker instead of brighter
as you increase the input voltage, although the current draw has
increased. If this happens, then it's likely that you have a problem in
the power circuit due to dielectric breakdown (or due to a diode
breaking down). Set the voltage where this "just happens" and identify
the part that is in process of breaking down (it may start hissing,
smoking, getting hot and showing similar signs of distress).
If you found one, replace and repeat, looking for others. Whenever you
have replaced something, go back to low voltage and 33% duty cycle and
readjust for best resonance (because it may have changed when you
replaced a part).
If you cannot get the input voltage back to full mains, even with
minimal duty cycle (3 % or slightly less) on the signal generator,
because something overheats too fast, and you cannot get the lamp to
light (even a little), then something is wrong in the power section. It
is most likely some isolation in some part breaking down or a part in
the power section being shorted. Disconnect all rectifier diodes from
all transformer outputs except the ones where the lamp is (V1831 +
V1834) double-check or replace those and see if the problem goes away.
If you can now get it back to full mains without anything smoking, count
yourself lucky. This would mean that the power circuitry is basically
okay, and that therefore the problem must be elsewhere (in the control
circuitry most likely).
At full mains, but still at 3 % duty cycle, the lamp will probably light
very dimly (maybe hardly visible). That's ok.
Now try very slowly and carefully (and considering isolation and good
safety practices of course, after all there are high voltages now) to
increase the duty cycle in order to get the lamp to light up with
"normal" brightness. Remember that there is no regulation and no
feedback, so don't switch anything in "hard" steps and don't burn the
lamp out. With the load suddenly disappearing (burned out lamp) the
board would fail catastrophically and immediately, faster than you can
turn the power off!
Carefully set the duty cycle to normal lamp brightness and let it run at
nominal input voltage (variac at 100 %) and normal lamp voltage (around
12 V) for a while and watch out for signs of electrical and thermal
overload. Don't touch parts now, they are at 310 V!
If you can let it to run in this way for a while, then the power
circuitry is definitely good and the original problem is not in the
power part. You will have to troubleshoot the control and regulation
circuits later.
Drop the variac voltage somewhere low, measure the resonance frequency
in this (semi-working) state and write the value down. You may need it
later.
Carefully wind everything down, first variac to zero, then signal
generator off, and reassemble the board back.
-------------------------
Post here, how far you came and if you could really find a problem in
the power circuitry or if it was good and the control part is the next
in line.
@other regulars he If you notice something amiss with the steps
above, or something that is likely to be wrong, please indicate that.
Regards
Dimitrij