I made a speculative purchase from ebay on a “faulty” Multiplus 48/5000, I was quietly hoping it was a configuration issue and a bit of software tweaking would sort it, alas not. My background is more electrics than electronics but i consider myself competent when it comes to a bit of pcb work so lets crack on.
the fault: the unit was working in AC mode, pass-through was all good, reassuring clunks from the relays and power through was all good. However, the low battery warning alarm and led was always on and charger not working. First thing i did was update firmware and reset all setting in VEConfig, ditching all assistants while i was there. All of this had no effect.
so I set to dismantling the unit and commenced fault finding, these units are a pleasure to work on as there are very few micro smds its all quite big in electronic terms. Firstly, control PCB (the one with the LEDs on) all looks nice and clean, good, move on. Next came the connection pcb (under the main connection points, not so clean, pretty grubby in-fact with indications of black marker pen which leads me to think this has been looked at before. Then the main power boards with the heat sinks and FETs again all looks good. But bad for me as there are no “smoking guns”. More detailed fault finding required …
There are no circuit diagrams readily available for these things which commercially I can understand so it’s now it’s case of piecing together how it works.
Picture 1 shows the top side of the connections pcb; the connection AC1 on the top right supplies the board with mains power with then goes to the big black block just below it which is a 240v to 34v transformer the output of which goes to the bridge rectifier G1 which sits just below the transformer and converts 34V AC to 48v DC which then goes the battery connections and power this pcb should no batteries be connected.
In between the big transformer and the big cap in the middle there is another transformer which supplies different voltages for the unit, 24v 16v an -3v, on of the places the 24v goes is to J7 (white junction block bellow the big black transformer, this when live this 24v activates the mains relay when the unit is switched on.
On my unit this was all working while on mains but not on battery only. The plan now is have a closer look at the battery powered side of this board: have a quick first pass with a multimeter checking what I can but again as im not an experienced pcb fault finder and no smoking gun it’s a bit of a shot in the dark really. After many hours I eventually find a short between pins 5 an 7 on IC1 which is a pwm driver for T1, pins 5 and 7 are ground and V+ and T1 is a Mosfet which provides the frequency for the multivoltage transformer (TR2). But wait a minute if that was the case the unit would not work on mains either. So I put it all back together and guess what, the unit is now completely dead now lights nothing, mains, battery, nothing. Now clearly ive made things worse but I think not I think what ive done is accelerated an existing fault which hopefully will now be easier to identify. My plan now is to replace IC1 and T1, possibly T2 and T3 too although im not sure what T3 does at this point. I’ve put in an 8 pin socket I had laying around, just till I get this sorted as I don’t want to damage the board while testing.
Anyway, let me know your thoughts and if you’ve got any pointers I’d be grateful.
New PCB is £430 by the way!
