Beaten by VT100 PSU.

Mattis Lind mattislind at
Fri Aug 19 05:35:02 CDT 2016

> 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.

There is a minimum load requirement. 1.5A on the +5V and 1A on the +12V.
The basic product should consume 2.5A  and 1.8 A respectively. Adding the
AVO option which is supposed to consume 1.1A solves the problem.

> 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.

Agreed. As you say the shape of the ripple is not caused something that is
charged. It is more like a half sinus.

> 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.

Thank you Brent for this! Very well thought through and well described. It
is inline what I have been thinking as well since checking all the caps,
there are only six aluminium electrolytics, gave nothing. The two on the
input  were in perfect shape. The ones at the output were fine as well.

Something is out of tolerance. Yes. Maybe I can replace a few things to see
if it gets better. What can be a priority list for this? Would you say that
Q8 is likeliest, then Q1. Isn't it more likely that the active components
is out of spec than the passive? I just did a quick in circuit check of the
transistors and diodes. But there were none of them that were really
upsettingly bad. Could be marginal though. Hard to tell in circuit.

Unfortunately the GE D44Q1 is not a very common part. Hopefully it can be
replaced with something similar. The switching frequency is not so high.
And it is not high voltage nor high current.


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