I've seen a lot of VT100s with poor vertical linearity and of those, many jumped vertically as well. I never got to investigate whether this was poor adjustment and/or noisy controls or a more real fault. If the waveform is present across the +12V output capacitor and +5V is ok and and the regulation only monitors the +5V, it sounds like the power supply is doing it's best and maybe the problem is in the +12V load? Alternatively, could there be an abnormally varying load on the +5V which the power supply regulation managing to smooth out but in doing so causing the unmonitored +12V to vary? Monitoring the current waveform in the +5V and the +12V might throw some light on this, maybe by putting the scope across a small series resistance in each line. I guess the 560uF capacitor referred to above is on the 12V line? The corresponding capacitor on the +5V might also be worth checking in case the regulation of the power supply is managing to compensate for it being bad. Regards, Peter Coghlan.

It's certainly an odd one... There are several things that are curious. The first is the frequency of the waveform. 329Hz or so according to your 'scope (assuming I am reading it right). That is far too high to be mains ripple. It is too low to be the oscillator in the SMPSU. So where is it coming from? My first thought is to look for anything else that runs at that frequency. The second oddity is that if it is 329Hz and you have a 700mV ripple then (assuming the output capacitor is what it says it is), you have about 200-odd mA of ripple current. That sounds a bit high to me. Maybe OK... Certainly the output capacitor is my first suspect. ESR meters do not tell the full story always. I would try sticking something in parallel with it to see if that has any effect. Anyone else looking at it, be warned the printset is not easy to follow. The one on bitsavers has a couple of pages swapped round, so the PSU schematic is not on consecutive pages. And DEC have drawn the drive transformers split between the 2 pages with the control circuitry drivng the primary widings on one page and the secondaries and chopper transistors on the other. -tony

On 2016-Aug-17, at 1:48 AM, Mattis Lind wrote: A line of thought for consideration, with the usual disclaimer: I've seen these (VT100) supplies before but I haven't had to repair one, the following is based only on a cursory purview of the schematic. Let's look at some of the design nature of the supply. All the outputs extract their energy from the main switching transformer (T1 here). The drive for T1 (the primary driver) however, is controlled only by the +5 regulation, that is, T1 is inside only the +5 regulation feedback loop. This is typical of many multi-output computer switching supplies. An increased load on the +12 output must extract more energy from T1, but the +12 output has no means to directly increase the primary drive into T1 as it doesn't have a control line into that regulation loop. Instead, through the interaction of the magnetic fields in the T1 windings the additional extracted energy for +12 lowers what the +5 output can get out of T1, so the +5 regulation loop kicks in and tells the primary drivers to increase the power into T1 till the +5 is happy again, the +12 now being happy too. In consequence, there may be a minimum load requirements on the output of the major regulation loop (+5 here) or a relationship (ratio) between the output load demands to allow the outputs to function properly. The preceding accounts for why an increased load on the +5 output can affect the +12, and cause the problem to go away. Let's hypothesize that something in the +12 switcher is 'weak', say the gain of the +12 switching transistor (Q8) has fallen below design requirements. When the +5 is lightly loaded, for some given demand on the +12 output, Q8 can't conduct enough energy into the +12 storage inductor (L1) to maintain the +12 in the time it has to do so, because the low +5 demand has the T1 drive down to a narrow switching pulse. (Note that the reason the +12 switcher is synchronised to the +5 switcher is so that Q8 'knows' when to extract energy from T1 to push into L1.) As the load is increased on the +5 output, the +5 regulation widens the T1 drive pulse to supply it and now Q8, though weak, has more time to pass sufficient energy into L1 for the +12 output. The 'ripple' you are seeing is not 'capacitive' ripple as would result from inadequate filter caps - it's the wrong waveshape (aside from the wrong frequency). I haven't thought out a full explanation but I suspect it's an oscillation set up by the load demand and an (inadequate) corrective response attempt by the +12 regulation. Note I'm not saying Q8 is bad, go replace it; I'm just trying to describe how these functional parts interact, to account for (some of) the behaviour you're seeing, there are other parts of the +12 switcher that could produce the scenario, like limited base drive into Q8, or pulse width or phase issues around the +12 555 monostable. Something may have slipped out of tolerance and upset the design limits for the output relationship mentioned above.