Saturday, October 27, 2012

Parallax 7345 SMPS Repair Tips

I started this as a reply in the Parallax 7345 RV Power Converter (SMPS) post. It quickly grew to the point I decided to tidy it up and make a new post out of it. Most of the specific info is for the Parallax unit though it may apply to other SMPS's.

Most of what I know about SMPS repair comes from reading: If you intend to work on a SMPS I strongly suggest reading and digesting the contents of it first.

Possible Problem Areas

Quick Disclaimer or "Safety Third":
Be very careful! There can be potentially lethal voltages all over the board even when disconnected. At least one heatsink is live! Proceed at your own risk!

Again, be careful, It's hard to diagnose a SMPS without powering it up. Usually a isolation transformer is used. I've toyed with the idea of using an inverter or generator for this. Basically you don't want the neutral tied to ground, you want it floating, then it's only dangerous between the line and neutral rather than line and ground (that you're standing on). All household / shore  power is referenced to ground at some point.  Depending on the wiring of your generator it may or may not be. I'm not sure how to advise you on this, no matter what you do there is a risk. Try to minimize it.

Check the obvious and make sure the problem lies inside the unit.
That said, I'd follow the power through the unit as much as possible. Start where the 120v line connects, I've spent hours searching boards for a problem only to find it wasn't powered to start with. make sure you have 120v going into the board from your generator / line connection before disassembling the unit.

After you can disconnect power, let it sit for a couple hours then disassemble it. Still be careful what you touch, use the one-handed method until you get the board out. turn it over and check the voltage across the two big caps, it should have bled off, but could be over 150VDC (I don't recall the voltage spec on them, assume it's fairly close to it). If they are zero (or at least below 20V) check the continuity from each power-in line to the bridge rectifier (just after the MOV, Square thing with a heatsink) it should read almost zero ohms (if it's several thousand your problem lies here).

Always look for obvious damage,burnt components, etc. 
Sometimes a burnt component is caused by a faulty but perfectly good looking component, so don't assume you found it, you may replace it only to have it go again. (A failed triac on a Maytag Neptune washing machine will destroy a resistor, replace the resistor and it will destroy it again, replace both and you can wash clothes)

That's as far as I got on this one the problem was rather obvious, so I'm out of specific information on it. As I noted it's basically a big smps, The document at the beginning of this post should help you with the basic operation. I marked some things on the above image, you may need to check how to actually test these, but hopefully it will give you a start. As always Google is you friend, just looking for more generic SMPS info than specific Parallax info should help.

These are worth checking, roughly ordered from most likely to least (imho):
  1. Bad Capacitors, look for bulged , burst or leaking caps, they cause all kinds of headaches
  2. Output resistors. You can see these have been really hot, but are working fine on this one. However if one failed I'll bet the rest would go within seconds or the output would drop dramatically. They should be a low ohm value(0-1R). 
  3. Switcher Mosfet(s) - handles all the power, it should switch on and off really fast (100khz to several mhz, depending on design) this is connected to one of those big heatsinks, probably the live one.
  4. Logic / Startup power. I might be off the mark with this, but I think without this resistor the unit will not start. It requires some power to get thinks going, I think it comes from here. Not sure of value, but shouldn't be open and probably fairly high(100K+?).
  5. Bridge rectifier - if this goes you will get low(or no) voltage on the caps, unit may not start.
  6. The logic section, the most complicated part of the system. Controls the whole thing, depending what's wrong it may be un-repairable (bad microcontroller, even with a new micro you won't have the firmware) or could be something replaceable (op-amp, pwm controller, etc.) I put this last because I know of one that survived 240V on the input, so I assume it's pretty rugged.

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