How can I unit test an observable that is ObservedOn a scheduler?

Partly this comes down to the documentation for TestScheduler just not being very helpful, but...

I have a class which my DI framework provides as a singleton:

public class Foo : IObservable<Bar>
{
    private readonly IObservable<Bar> _feed;

    private readonly IBarSource _remote;

    public Foo(IBarSource remote, IScheduler scheduler)
    {
        _remote = remote;
        _feed = Observable.FromAsync(async token => GetBarsAsync(token))
                    .Merge()
                    .Repeat()
                    .RefCount()
                    .ObserveOn(scheduler);
    }

    public IDisposable Subscribe(IObserver<Bar> observer)
    {
        retu _feed.Subscribe(observer);
    }

    private async Task<List<Bar>> GetBarsAsync(CancellationToken token)
    {
        var raw = await _remote.GetRawBarsAsync();
        token.ThrowIfCancellationRequested();
        retu raw.Select(ProcessBars).ToList();
    }
}

Basically it tus a data stream that has to be polled into a nice observable; having it as a singleton class means all the different components of the application that need data from this stream get it from a single source rather than each of them spiing up their own background thread to get it.

I'm using ObserveOn to make sure updates go out on the UI thread; I take in a scheduler for that so that the DI framework can provide DispatcherScheduler.Current in the actual application but I can provide a TestScheduler in unit tests so I can test the class...

...except I can't seem to figure out how to use the TestScheduler in such a way that events actually get published. I have this:

[Test]
public void BasicTest()
{
    // arrange
    var remote = Mock.Of<IBarSource>();
    var sched = new TestScheduler();
    var instance = new Foo(remote, sched);

    // act
    bool published = false;
    var sub = instance.Subscribe(_ => published = true);
    sched.Start();
    sub.Dispose();

    // assert
    published.Should().BeTrue();
}

But it always fails. I assume I'm not using the TestScheduler correctly but all the examples I can find seem to assume you're using it to control periodic events rather than using it in ObserveOn. What am I doing wrong?