25 Jun
2023
25 Jun
'23
1:20 p.m.
On Sun, Jun 25, 2023 at 1:36 PM Rob Jarratt via cctalk
<cctalk(a)classiccmp.org> wrote:
Hello,
It has taken me ages to get back to this, but I think I have hit an
insurmountable problem.
I reassembled the PSU after the PWM started working again and tested it with
a dummy load and using two 100W light bulbs in series on the input side to
limit the current. However, it did not work. I traced this to the 7812
regulator that supplies Vstart (PSU Sheet 1) not getting enough voltage to
run, about 3.7VDC. In the same conditions the working one gets about 10 or
11VDC. I also noticed that on the good PSU the 100W bulbs pulsed and were
fairly dim, but on the bad PSU they were brighter and glowed steadily.
This made me suspect that the startup transformer may have been damaged. So
I checked the output of the startup transformer. On the bad PSU this was
about 2.4VAC, while on the working one it was about 12VAC. I lifted all 4 of
the diodes connected to the startup transformer and they all tested OK.
Sadly, I think this means the transformer must have been damaged. I have the
facility to do a ring test on the transformer, but don't want to remove it
unless there is a need to. Is there any other failure mode that I am not
considering? I didn't check the inputs to the transformer, so something
could be short on the input side and pulling the input to the startup
transformer down. It is worth reminding anyone reading this, that the 7812
on PSU Sheet 1 did fail with a short to ground.
I wouldn't be too hasty to condemn the transformer.
This is a mains transformer, not part of a switch mode power supply
circuit. It takes in mains via the voltage selector switch, the centre
tapped secondary is full-wave rectified twice to produce the +12V and
-12V rails needed to get the rest of the supply going.
Now, you mention the lamp limiter bulbs (series light bulbs, whatever
you call them) are bright. That means they are dropping significant
voltage. So the input voltage to the startup transformer will be less
than mains. Which will lead to a low output from its secondary
winding.
Try disconnectng the mains feed to the main chopper circuit. Perhaps
remove the mains bridge rectifier, or both of the inrush limiter
thermistors. Anything to prevent power getting to the main chopper
circuit. Power up again with the lamp limiter in series with the mains
(of course). My guess is that then the bulbs will be dark and the
startup transformer will give the right voltages. This would point to
a problem with the chopper circuit.
-tony
I have included a couple of minor responses to Brent's last email to me
below.
Regards
Rob
-----Original Message-----
From: Brent Hilpert <bhilpert(a)shaw.ca>
Sent: 26 May 2023 17:24
To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Subject: Re: [cctalk] Rainbow H7842 PSU Fault
On 2023-May-25, at 1:43 PM, Rob Jarratt via cctalk wrote:
> This evening I went to check Vstart
for any oscillation. However, all of
a
sudden, the current draw is down to 85mA and PWM
has started working. I
am at a loss to explain it. I wondered if there might be a dry joint, but
I have
tried a few light taps and shakes and it
continues to work. Perhaps your
idea
of some debris causing a short might explain it,
otherwise I just don't
know.
Operation with only VStart+12 places the circuitry into an unspecified
operating region - a region outside of the design intentions. In part,
several
semiconductor junctions and portions of circuitry
are polarised opposite
to
their normal/designed-for state. It is not
surprising that you are seeing
odd/unpredictable behaviour under this operating environment, nor is it
surprising that it's different than the 'good' supply under the same
operating
environment.
So why was it in shutdown earlier the other day but not now? :
Who knows - it's operating in an unspecified region. Perhaps the room
temperature is 2 degrees higher. That's a serious point, not phase-of-the-
moon satire.
Well, the weather here has indeed got a bit warmer.
When you supplied the proper startup environment with both Vstart+12 and
Vstart-12 both the bad and good unit behaved as expected for the design.
Why is the VStart+12 current draw higher when it was in shutdown versus
when the PWM controller IC is pulsing? :
Because in shutdown the 'Chopper Driver' transistor (PSU Sheet 2) is held
hard ON (conducting) (see datasheet).
Holding this transistor ON subtracts it's off-state current (~ 17mA) but
adds
it's on-state base current (~ 37mA) and
it's on-state collector current (~
73mA), for a net up-to ~ 93mA increase (may be less dependant on duty
cycle of PWM), to the Vstart+12 current.
Thanks for explaining that, it makes more sense now. I need to learn to look
at other parts of the schematic beyond the immediate thing I am interested
in because I keep missing things like this.
There remain two unexplained things here:
- Where was that unusual current-sense voltage that sends it into
shutdown coming from?
(I provided one potential explanation earlier, but it remains
unknown
at this time).
- The 51-ohm current-sense resistor in the -12V supply vs the mode
of operation of
the -12 supply remains unexplained/non-sensical. At the max current
you mentioned (150mA),
the V drop across that R would be >7V (!), which makes no sense. If
I
had it in hand, I'd be
double-checking the drawing of that current-sense circuit around the
51-ohm R as a start.
Agreed that this does not make a lot of sense. I have checked the actual
value of the resistor (in circuit) and it matches the schematic. I have also
checked the schematic against the actual circuit and it really does seem
like the schematic is correct.
But this is not to say that either of these has anything to do with the
fault you
were/are dealing with, they're just things
that aren't understood at this
point.
Either or both could be pursued out of curiosity
or for the sake of
completeness.
> I am thinking I may put it back together and test with a light bulb in
series.