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.p850...@gmail.com> > Sent: 25 June 2023 14:21 > To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts > <cctalk@classiccmp.org> > Cc: Brent Hilpert <bhilp...@shaw.ca>; Rob Jarratt > <robert.jarr...@ntlworld.com> > Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault > > On Sun, Jun 25, 2023 at 1:36 PM Rob Jarratt via cctalk > <cctalk@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 <bhilp...@shaw.ca> > > > Sent: 26 May 2023 17:24 > > > To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic > > > Posts <cctalk@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. > > > > > > >
