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Model Forum / General / Railroads / November 2004Repairing old MRC Ampack
Yeah I know I know--why bother? But I'm curious. The darn thing shoots up to 20 DC with load as soon as I start cranking the knob. Opened it up--no solid state. The 50 ohm rheostat checks out OK on my ohmmeter--a gradual 0-50. There are two silicon rectifiers in the circuit. The DC track voltage comes out of the transformer center tap. Also the AC accessory line uses one of the rectifiers. Why? I plan to convert this unit to electronic anyway but it's puzzling me why the voltage goes so high and fast when the rheostat checks out good. Any advice appreciated if you have a few idle minutes. Robby >Yeah I know I know--why bother? But I'm curious. >The darn thing shoots up to 20 DC with load as soon >as I start cranking the knob. What load? >Opened it up--no solid state. The 50 ohm rheostat >checks out OK on my ohmmeter--a gradual 0-50. 50 ohms will not be enough to stop most modern motors so adding an electronic control will be worthwhile. >There are two silicon rectifiers in the circuit. >The DC track voltage comes out of the transformer >center tap. Also the AC accessory line uses one of >the rectifiers. Why? Probably because its an unconrolled DC output. >I plan to convert this unit to electronic anyway but >it's puzzling me why the voltage goes so high and fast >when the rheostat checks out good. Use your multimeter to check the Ac voltage on the transformer secondary. >Any advice appreciated if you have a few idle minutes. > >Robby Make friends in the hobby. Visit <http://www.grovenor.dsl.pipex.com/> Garratt photos for the big steam lovers. With no load the voltage will not drop across a resistor. > Yeah I know I know--why bother? But I'm curious. > The darn thing shoots up to 20 DC with load as soon [quoted text clipped - 14 lines] > > Robby Robby again--I used an old Roco switcher on a test track with multimeter leads on the rails. DC track voltage shot up to 20 volts soon as I cracked open the rheo. Of course the little Roco shot off like a mouse when the room light goes on. The AC voltage across the secondary is about 24 Volts no load. The accessory terminals are labeled "AC accessories" yet one of the rectifiers is in series with one screw. ???? The rectifiers both read 9 ohms in one direction but I didn't check the other way. Now I'm thinking somebody re-wired the unit because when I bought it, a racecar terminal track was wired to the screw for DC track. Could be the unit was boosted up to make the racecars run fast. The typical for that type of powerpack is to have 3 wires coming out of the transformer and 2 of those wires have a diode on them, both pointing towards or away from the transformer. This is full wave rectification where each half of the transformer is used for 1/2 cycle to provide the power. Either lead goes to the reostat and then to one side of the DC power while the other goes out as the other side of the DC power, going through the reversing switch on the way to the output terminals. Due to the desire of the toy train types to go fast and the cheap construction of the transformer, the voltage is usually on the order of 16-20V no load. If your loco takes only about 100ma or so to run, even the 50 ohms won't drop the voltage enough to make any real difference in the speed of the loco - 50 ohms will only drop 5V at that current so you still probably have 14V to the loco! -- Why isn't there an Ozone Hole at the NORTH Pole? [...] > -- > Why isn't there an Ozone Hole at the NORTH Pole? There is. It hasn't been publicised much. The Innuit up there are suffereing a much higher rate of cataract and skin cancer than they used to, on account of the increased UV radiation coming through the hole. And the effects of climate chnage are more drastic up there, too. The arctic and ant-arctic have more fragile ecosystems, a small change in temperatures suffices to cause major changes in breeding cycles, vegetation growth, and so on. Result: the larger animals and birds are disappearing. And so forth. Doesn't make for thrilling pictures on the TV news, though. Empty tundra is kinda boring.... Yes, there is a thinning of the ozone hole at the North Pole. This isn't the same as the total elimination of ozone at the South Pole tho. Please note that most of the ozone depletion chemicals have been used in the Northern Hemisphere which would make the North Polar Region have a big hole while the Southern Polar Region have a much smaller hole or just a depletion of the ozone but that isn't the way that it is. -- Why isn't there an Ozone Hole at the NORTH Pole? Dear folks, The Ampack wasn't a cheap train set pack. I put my money on a badly fungled center-tap transformer arrangement. This used to be popular, presumably when the cost of the extra 2 diodes made it cheaper than a bridge rectifier, even with the fancier transformer. I do not have an Ampack, but it should be wired something like this: http://www.geocities.com/kezelak/ampack.JPG If you want to use it as a power supply, you can get 12 v full wave across wire C and the junction after the two diodes. There might be a slightly different order of the components, and there probably is (should be) a fuse before the transformer, maybe a power switch too. If there is a pulse-power switch, it will most likely be just ahead of one diode, cutting it out of the circuit to change the output to half wave DC. Obligatory MR warning: DO NOT EVER ATTEMPT TO CONNECT OR MODIFY CIRCUITS OF OVER 50 VOLTS POTENTIAL OR THE BAD NASTY ELECTRICITY WILL GET YOU! STAND TO THE SIDE WHILE OPERATING A LIGHT SWITCH. WEAR A FULL FACE SHIELD, A POWERED AIR PURIFIER, GOGGLES, AND A RAD SUIT WHEN POURING A GLASS OF WATER. BE AFRAID. BE VERY AFRAID. Cordially yours, Gerard Pawlowski President, The Sparta Railroad |
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