25 Jun
2023
25 Jun
'23
4:33 p.m.
Thanks Tony. I removed the mains bridge rectifier and found it measured short across two
of its terminals, so I am ordering a replacement. I tested the PSU, without the mains
bridge rectifier and without the diodes on the output of the startup transformer. I got
24VAC on the output of the , the lamp limiter bulbs did not light and the fan in the power
switch unit turned. So it looks like the startup transformer is actually OK.
Regards
Rob
.> -----Original Message-----
From: Tony Duell <ard.p850ug1(a)gmail.com>
Sent: 25 June 2023 14:21
To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Brent Hilpert <bhilpert(a)shaw.ca>ca>; Rob Jarratt
<robert.jarratt(a)ntlworld.com>
Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
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
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.
> >
>
> -----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