slow signal triggering fails with DSO Tek TDS-420

[gotnoise]

I just treated myself with a Tektronix TDS-420 from ebay (vintage digitizing scope, 150 MHz, 100 MSa/s, 4 channels). I quite like it, and it is surely one of the better pieces of equipment on my beginners bench. But I have a little problem when trying to trigger slow signals. For some of my projects, I am working with signals at 1-100 Hz. And I am not able to trigger them properly with the TDS-420. When I switch to anything above 20 ms/dev (next step for this scope is at 50 ms/dev), triggering fails, and the scope switches to rolling (untriggered) mode. I fed the scope with an external function generator, and the problem seems independent of amplitude (0.1 - 5 V) or waveform (sine, sawtooth, square wave) of the input. Also, I could not get it to work by feeding the sync output from the waveform generator into channel 2, and then triggering from channel 2. I assume, the scope is simply not capable of triggering at that slow time base. Is this normal, and am I asking for too much? I don't quite understand why a slow time base would limit the trigger function. Also, is there a trick to overcome the problem? Just pressing the stop button to look at the frozen signal is not good enough for my purpose.
Thanks a million for any input!

[Bored@Work]

The manual says two out of its four roll modes trigger, two are untriggered. Check the manual to see how to switch between roll modes.

[gotnoise]

Thanks for your feedback, bored@work!
My TDS420 has only two roll modes (auto=untriggered roll & normal=triggered roll). I did find your reference to 4 trigger modes in the manual for the TDS420A. There, it explicitly states that at 50 ms/div and slower, roll mode can be turned off. Unfortunately, in my TDS420 this option does not show up in the menu. I could not find a menu for the 420, only for the 420A. But I assume that the difference between the 420A manual and the 420 menu functions has to do with the older 420 version. Maybe a firmware upgrade would fix this problem. My current firmware version is FV:2.5e. I could not find any firmware upgrade yet, but I will keep looking for one.