XANTEX XPH75-2D Repair FIXED, still baffled - Page 1
Forum section: electronics / repair

author: Seekonk
author_rank: Super Contributor
timestamp: December 08, 2018, 08:12:06 pm
content: Not many postings for repair of Xantrex power supplies. Thought this might be helpfulto someone. There are some partially drawn schematics that show major power flow thatmay not be totally accurate, but enough detail to provide circuit understanding. Theseare the steps I use in approaching a repair. Understanding and luck are the two factorsin a sucessful repair. A little understanding makes you a lot luckier.This nice 2006 Xantrex supply has a switch on the front panel it can be switched fromdual 75V 2A to single 150V 2A. It can also be "paralleled" for 4A. The two suppliesdon't really connect together, one of the supplies is just beefed up for 4A. This supplyhappened to be made in England.The supply would only tick about every seconds with a short LED flash. The switcher wastrying to start up, but the inverter was not sustaining it. Scope trace shows the inverteroutput top and the shutdown signal from the over voltage opto isolator bottom. For betteror worse, supplies have become a lot more complicated. This one has power factor control.The main supply cap will not even charge up unless the control electronics work. That capthen powerers the inverter transformer which powers the two nearly identical supplies.That inverter transformer also powers the control electronics. Everything has to work inorder to remain working. One nice aspect of this is that any failure immediately shutsdown the supply. It does make it hard to service when the initial power bump up circuitonly lasts a fraction of a second.Interesting to note that the AC switch and fuse are after the AC line filters. No protectionin the event of an AC filter component failing.

author: Seekonk
author_rank: Super Contributor
timestamp: December 08, 2018, 08:14:37 pm
content: I find it helpful to do a simple sketch of the board and major power circuits. Thisferments in my mind where everything is and where I want to look. I always use a dim bulbtester in the AC power to protect against severe damage when testing. However, these donot work well with PFC supplies. At lower voltage they tend to just draw more current.Here are some diagrams that might help someone with the same supply, not guaranteed to beaccurate. Initial ohm checking of major power components came up with nothing.I then poweredthe PFC chip with an external 16V supply. That showed some life and indicated up to andincluding the inverter was working. The problem had to be in one of the isolated outputsupplies.From each of these supplies came an opto isolator monitoring the 100V power supply capacitor.The output of these were paralleled for a single voltage feedback. I assumed that thesejust regulated the inverter output. The output supplies had an inductor, diode, capacitorand FET. What you would expect in a buck converter. What I had drawn out didn't look right,but I hadn't been too careful with the details.

author: Seekonk
author_rank: Super Contributor
timestamp: December 08, 2018, 08:18:31 pm
content: I was getting nowhere fast and became impatient. Time for a little destructive testing.Nothing on the board was really that sacred. I decided to momentarily short out the optoLED. That couldn't cause any problem! Immediately after shorting with the scope probe thefront panel came to life and it kept workin after the probe had been removed. Must havezapped some intermitant. Then I started to see smoke. Before I could hit the power switchthe capacitor exploded. At least I'm getting somewhere.An ohm check now revieled that one zener had shorted and the other opened on the optomonitoring circuit. Besides that two 200 ohm 1/4W resistors had opened up. Not a sign ofheating or cracking. I found that unusual. can usually get 1/4W resistors to glow redminutes with hardly any change in resistance. Here two of them failed and yet the optoisolator LED was not damaged. Something was really crappy about these resistors! Now thatthe bad channel had been isolated I dug a little deeper and found another 100 ohm resistorof the same type had failed open with no visible damage.The simple buck converter was not a buck converter at all. It looks like they are usingthe DC saturation of an inductor to control the output and the FET was operating in thelinear region. Sure would like to see that circuit in detail. So it appears that anyonewith a free Harbor Freight meter could have fixed this by the drudgery of measuring eachresistor. Xantrex usually takes a lot of care in selecting parts.

author: JFJ
author_rank: Regular Contributor
timestamp: December 09, 2018, 12:35:15 am
content: Quote from: Seekonk on December 08, 2018, 08:18:31 pmThe Xantrex XPH75-2D is also marketed as the TTi EX752M. An EEVblog member obtained a service manual or circuit diagram (it's not clear which, from their post) for that power supply, free of charge from TTi:https://www.eevblog.com/forum/testgear/does-anyone-have-a-tti-ex752m-bench-power-supply/Quote from: Seekonk on December 08, 2018, 08:18:31 pmThey may have been fusible film resistors, as used in other TTi power supplies. In the attached diagram, for example, the "+" sign after resistors R32, R100, R103 and R110, indicates that they are fusible:(Taken from a TTi EX354D service manual).Quote from: Seekonk on December 08, 2018, 08:18:31 pmThe voltage drop across the MOSFET linear regulator is used to forward bias the Base-Emitter junction of the KSE350 transistor (in your last circuit drawing). That transistor provides a feedback reset current to the magamp (MA2), which keeps the MOSFET voltage drop to under a volt. Thereby, minimising the linear regulators power dissipation.

author: Seekonk
author_rank: Super Contributor
timestamp: December 10, 2018, 01:38:30 pm
content: Thanks for that schematic, it is very similar and lets me understand the circuit. All those resistors were certainly fuseable. The two on the opto isolator though seem to defeat the purpose of limiting damage to the supply.