VAXmate PSU

I did use a faster timebase, 10us, triggering on the negative edge of channel 2, and it looked identical to this: https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-detail-2… Admittedly the trace you see was taken on the slower timebase and then zoomed in, but there was no difference, the result was identical. Regards Rob From: Mattis Lind <mattislind at gmail.com> Sent: 09 April 2020 05:52 To: rob at jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>; Rob Jarratt <robert.jarratt at ntlworld.com> Subject: VAXmate PSU onsdag 8 april 2020 skrev Rob Jarratt <robert.jarratt at ntlworld.com <mailto:robert.jarratt at ntlworld.com> >: I will look at all the suggestions, particularly of a failure on the secondary side. Something must have burned up, because there was a distinct burning smell after the initial failure, although I have never been able to see any physical damage to anything, despite looking many times. Aha. Don?t think I seen you writing about that before, or did you? It might be very hard to find the source some times. Even just a small burn will give quite some smell. Check ALL semiconductors very carefully. But the thing that really puzzles me is that, after correcting the probes to include the D19 anode, there doesn?t seem to be anything that would cause D19 to trigger. Am I reading the trace wrong? It is very hard to tell from the traces what is going on since the resolution is too low. Use a faster timebase. 5 or 10 microseconds. Find out if you can trigger on something that happen only when it stops. Like channel 2 negative slope. /Mattis Thanks Rob From: Mattis Lind < <mailto:mattislind at gmail.com> mattislind at gmail.com> Sent: 08 April 2020 07:42 To: <mailto:rob at jarratt.me.uk> rob at jarratt.me.uk; Rob Jarratt < <mailto:robert.jarratt at ntlworld.com> robert.jarratt at ntlworld.com>; General Discussion: On-Topic and Off-Topic Posts < <mailto:cctalk at classiccmp.org> cctalk at classiccmp.org> Subject: Re: VAXmate PSU Den ons 8 apr. 2020 kl 00:34 skrev Rob Jarratt via cctalk <cctalk at classiccmp.org <mailto:cctalk at classiccmp.org> >: via <cctalk at classiccmp.org <mailto:cctalk at classiccmp.org> > supply, a something +V action. the but crowbar - Oh dear! After Brent's question about D19 anode, I realise that the probe was connected to the cathode! I have now done it again with the probe connected to the anode. I have taken two images of the same capture, one at low resolution to show the overall behaviour https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-1.png And one zoomed in to show what happens when the SCR shuts down. https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-detail-2. <https://rjarratt.files.wordpress.com/2020/04/primary-side-shutdown-detail-2.png> png The channels are the same as before, namely: Ch1. 555 timer. Ch2. D19 Anode (now corrected as it was previously the cathode!) Ch3. D19 Gate. Ch4. Q1 Source. I got an earlier trace which showed the D19 anode at 9V, which is under the Undervoltage Lockout threshold, but I have not been able to repeat it. I don't fully understand the debate about using the variac. I am not going to debate this either since I know what I have been doing for years and it works perfectly well for me. I have fixed the bigger PSUs in a VAX 11/750 (one broken switch transistor and multiple broken output rectifiers). PSU in NORD-10/S (most carbon composition resistors had gone out of spec). PSUs in many smaller machines as well. I prefer to work in circuits where I can fiddle around without the danger of getting killed all the time. Regardless of use of HV differential probe it can be dangerous. Running it on 50VAC with a protection transformer do expose a lot of problems already and you can poke around safely in the PSU. I have not yet seen a problem that wasn't seen at low voltage, but I expect there could be semiconductors that experience breakdown that occur at lower than specified voltage. However, my measurements appear to suggest that when I use the variac the SCR triggers because of what appears to be a genuine overcurrent detected by R13. I think this is because the duty cycle at low AC input voltages is 50% (rather than about 10% or less as per the trace I have just taken), and I measured 2V across R13, which does seem to be enough to trigger the SCR. When I use 220VAC, the voltage across R13 does rise to 6V, which should also trigger the SCR I think, except that the peak last a lot less and so perhaps the fact that the 6V last for a brief period is insufficient to trigger it? On the issue of duty cycle. If we look at this from the start up perspective rather than the steady state perspective. At startup there are no stored energy in the output filter capacitors. The voltage on the output is thus 0. As soon as the PSU is doing its first switching pulse energy is transfered as the main switch transistor is cutting off. The energy is transfered into the capacitor and into the load. The voltage is starting to increase. The duty cycle generated by the PWM circuitry is in pure relation to the voltage error, i.e. the difference between output voltage and reference voltage. In essence it is a P-regulator. When there are 0 Volt out the duty cycle will be at the maximum. Nothing strange about that. But what is maximum duty cycle? It depends on the circuitry used. The UC3842 can do up to almost 100% duty cycle. However it may be wise to limit duty cycle in a flyback design so that the transformer is not saturated. I am not sure if there is some kind of duty cycle limitation in this circuit though. So if it can handle 50% duty cycle at startup it should be able to handle it at any time. Besides it would be incredible weird to design a circuit to use a 10% duty cycle at its standard operating point and detecting over current at 50%. Then you have much less head room for load and input variations. I am more or less convinced that what you see on the primary side is a result of some kind of fault on the secondary side. A very common problem is short-circuit rectifier diodes on the secondary side (D12, D11, D21, D22, D23, D24). They can be difficult to measure correctly in circuit since the resistance of the secondaries of the transformer is so low. Depending on type you can either desolder them completely or just lift one end of them. My experience is that electrolytic capacitors seldom short circuit. They probably boil and explode instead. Tantalum capacitors often short circuit. Some of them goes into fire other just stay short circuit. So check for tantalum capacitors and try to measure them for short circuit. You have a crowbar on the secondary side. Are you sure that one hasn't triggered? If you still run on variac you can disable the crowbar circuit by removing the SCR and ramp up the voltage slowly to see if that makes any difference. Breaking the feed back loop: R23 seems to be in the feedback path. If you lift it and insert a voltage from a lab supply here you could simulate the output voltage and study the behaviour of the UC3842 for different feed back voltages. You will see that it will stay on max duty cycle up until close to the nominal voltage and over a very small span change to almost no pulse out at all. This is due to the gain of the circuit. /Mattis I have seen the suggestions to study the waveforms at a much higher resolution. What I am doing is setting the overall timebase in the 100ms range so that I can trigger on when the 555 starts to oscillate and capture the whole period of operation until the SCR triggers. I can then zoom in, as can be seen from the trace provided in this email. I hope that is good enough, or am I missing some problem with doing it this way? I would like to follow Mattis's suggestions (and other people have said it too) to break the feedback loop, but it does look difficult to know how best to do it. I also understand Brent's suggestion that the gate spike is just the result of the SCR triggering, rather then the cause of the trigger. I had wondered if that might be the case. Regards Rob