MRC Railpower 1370 problem

[Anonymous]

Could those with a Model Rectifier Corporation Railpower 1370 power pack and access to an oscilloscope answer this question ? Have been trying to get the answer from MRC themselves for weeks, but they are not cooperating.

I have 2 Railpower 1370's and have always assumed the full wave rectified sinusoid output at all speed and voltage levels was normal. On talking to owners of Railpower 1300s it turns out that the 1300 has half wave rectified sinusoid output at low speed and voltage, just like my MRC Tech II 1400, 1440, and 2800. (Incidentally, the 1400 and 1440 half wave outputs are out of phase, so if a loco straddles blocks powered separately by these 2 units the loco will speed up momentarily while powered by both packs and receiving full wave). This halfwave is much better than fullwave for really smooth and slow locomotive creeping.

Well, on first asking MRC about this i was told that the 1370 is supposed to be half wave, just like all the rest of the MRC line including the 1300. So why were both my 1370s showing full wave? My scope does not lie.

Made a special tool (filed a notch into the end of a slot screwdriver) to undo those darn tamper-proof screws and dig into the 1370. What i found was very surprising...

First, there is a missing resistor, an empty spot on the PCB labelled 'R1'. This resistor is what develops the AC polarity offset that is supposed to result in halfwave output at low voltage.

Second, there is a PCB trace pattern layout error on the revision 1 pcb that makes the presence of R1 useless anyway if installed in the pcb holes.

It is very obvious that someone in the manufacturing plant recognized the error, but rather than fix the PCB layout error problem or jumper resistor R1 to one of the transformer leads ahead of the bridge rectifier, they instead chose to eliminate R1 and ship a product with a known design flaw.

R1 can be a value between 5.6K and 10K (MRC will not tell me). If your MRC Railpower 1370 is full wave output (either test on an oscilloscope or compare slow speed locomotive response against a power pack known to be true halfwave) you can repair it by adding a resistor, 6.8K works fine, at the empty R1 pcb location.

Be sure to UNPLUG the AC power cord first before taking the unit apart !!!

Do not insert the R1 lead into the PCB hole that connects to the diode bridge recifier, connect instead to one of the transformer AC output wires (the low voltage secondary side, NOT the 120 V side primary). Which secondary wire is unknown, one or the other will work but will result in opposite phasing of the halfwave pulse. Since my 1400 and 1440 are already opposite, best to select the phase that matches any other power packs you own if you run a loco across blocks powered by individual power packs.

The main question - is this problem only in the first revision of the MRC Railpower 1370? Or is it an ongoing problem that has never been fixed? Again, if you own a MRC 1370 please test if possible and respond with your findings.

Thanks,
Glen