I picked up an old Spectrum Magnum Ultra Power Pack. It has a varying pulse (of maximum DC about 20v) as you turn up the pot. Any tips on what may be wrong with this?
I know 30 years ago there were power packs that pulsed the motors with duty cycle DC pulses. This yields lots of noise on my engines when I use it. I’m about to dig inside it to see where I can add a capacitor to smooth it out. My prelim on doing this outboard is that without a load it goes from low duty cycle to high duty cycle in a very limited range of the pot.
Questions:
Was this a pulse type unit and it’s working as designed?
Is there a way to smooth this out in a reasonable way?
All this would be answered if I had a schematic, but finding one - no luck on that. I’ll figure it out eventually but thought I’d go on the forum and get some input first.
You need to test it under load. Testing it without load is like jacking up the driving wheels of a car to test the car’s acceleration. No coorelation to the real application.
I have one of the Spectrum top of the line packs, with volt and amp meters - it provides smooth and quiet control. It does not have the “feel” of a MRC Tech II, but it controls the trains just as well.
The purpose of Pulse Power on those old packs was to give an extra kick to the Pittman open frame motors used in those days. Pulse power can kill a can motor because of excessive heat buildup.
Most Pulse Power packs were in pulse mode only with a very low throttle setting to help a train run at creep or switching speeds, and the voltage smoothed out as the throttle setting increased.
There’s not going to be room inside the cabinet for a capacitor large enough to be of any benefit.
What you’re going to need is a non-polarized Electrolytic capacitor of several hundred Microfarad with a working voltage of at least 60 Volts, wired across the output terminals. This can be done anywhere along the wiring between the power pack and track.
All Electronics in Van Nuys, California, for example, has 150 Mfd 100 Volt non-polarized caps for $2.50 each, plus shipping. You could even wire 2 or more in parallel to increase the total Mfd rating; i.e., 2 in parallel would give you 300 Mfd; 3 would be 450 Mfd, etc.
A drawback to using these capacitors would be that they act like a momentum switch – when you turn off the throttle, the train will continue to run until the capacitor(s) drain off their charge.
I’d be more inclined to relegate this old pack to the antique pile and get a new MRC controller.
Thanks for the detailed replies especially about the All Electronic capacitor. I have ordered from them in the past but had not located a good value on a non polarized one. I’m now wondering if maybe there is some smaller cap inside that has gone bad and is causing the output to be totally a pulse width situation. It goes from basically zero pulse width of “full voltage” to 100% pulse width at full throttle. Maybe I can figure out why this is happening. I’m thinking that the pulses are supposed to be smoothed out and the level applied to say the “gate” of a voltage regulator that would pass a DC level to the track. This unit has a momentum and brake button so I suppect now that something is not right inside so I’ll look at trouble shooting it.
I have 3 Tech II Model 2400s with switchable on/off pulse. I usually forget and leave it on even though I maybe do 5% of my running in a mode where it actually helps. I have not burned up any motors, can or otherwise, in 30 plus years. If you ran a heavily loaded engine very slow for a long time I can see where that might do some damage, but I don’t do that. The relative pulse amplitude decreases at higher throttle settings.
There were some straight pulse width modulation throttles with little or no smoothing made back when. And you are correct, they used 60 or 120Hz pulses. The 2 questions are 1) whether the Spectrum model you have was designed with no DC component, or has become that way through an internal failure, and 2) how to fix.
The momentum and brake could just as easily be implemented on a straight pulse width modulation scheme - many of the simpler DCC decoders do this today. The better DCC decoders have a higher frequency pulse scheme to get away from the noise, and provide more protection for newer motors. I just don’t know how the Spectrum was designed. Perhaps a knowledgeable person with a known working Spectrum Magnum can measure his output and report. MRC deliberately smoothed their pulses and added DC injection - but their power packs were not as cheap as others, nor could they make an open frame locomotive crawl as well as some of the other packs with steeper pulses.
Obviously, the capacitor will smooth things somewhat at the expense of some of
Save yourself some headaches and get rid of it. I have one of those old Spectrum power packs that I no longer use, for the simple reason that it caused two locomotives pulling in consist to overheat, seriously damaging the motor in one of them.