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reparing truecharge 20i

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sup8pdct

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I am having trouble fixing a xantrax truecharge 20i battery charger.
I have recreated part of the schematic that I am looking at the best I could:

**broken link removed**.

It stopped working while it was connected to a small 240V 720 watt generator. Had 2 identical chargers running at the time. One quit when the ac voltage went to 265 volts. The genny didn’t have the power to blow anything up, not even a 4A fuse, but several components were shorted so fully loading the genny.
I have replaced all transistors, both MosFets, all 3 zenners, both optocouplers and the uc3845. Have checked all resistors (in circuit). The other diodes appear to be ok (in circuit) and the capacitors also seem to be ok (again in circuit). The only test instruments I have are a multi meter, An esr/low ohms meter and a simple transistor checker.
The circuit doesn’t self start. I am reading 7.7 volts at pin 7 of the ic when it should be over 8 volts.(needs 7.9 v min)
I have tried to reduce the value of R5 with no effect. C28 reads ok with my ESR meter
I have got the circuit going by injecting 12V to pin 7 till the processor (not shown) that drives the opto couplers, fires up, then circuit runs. The voltage at pin 7 is then still low at about 9.5 volts when it should be somewhere near 14V. Also noted that the 2n2222 that drives T1 gets quite warm when it shouldn’t.
Have swapped C19 from another location and even swapped T1 from the 2nd charger. Have changed c25, all with no effect.
Have measured the resistance of each pin of the ic to ground on both chargers and they match quite well. Have swapped the ic in both chargers as well with no effect.
Have measured the voltage on each pin of the IC while it was running. Have noted that while measuring pin 4, noise comes from the charger, where as doing the same on the 2nd charger, nothing happens. Same happens when I measure the voltage on the upper opto coupler output
Can anyone shed any light on what to change or try next?
Also, does anyone have a full schematic
I really don’t want to swap componants over too much. There are many heavy tracks and have damaged some while desoldering in the charger I am trying to fix.

James
 
Looks like Q2 may be dead shorted, causing Q1 to heat up excessively. I would check both transistors (Q1 and Q2) and replace both of them, only if either Q1 or Q2 are shorted out.

Those transistors are driver transistors for Q6, and if the drivers don't drive Q6 properly, the power supply output will be out-of-whack.

-Ben
 
I have checked Q1 and Q2 with my transistor checker and both are ok.
Have replaced C28 and swapped c27 from another location. Still no go.
Have swapped T2 and T3 from good charger figuring one had a shorted turn. still no go.

If one of the snubbers wasn't working, what would happen??

James
 
The noise you are hearing is likely frequency pulling from the transformer, this is because the transformer is not running at or near it's resonant frequency as designed. My money would be on one of the low ohms source resistors being high causing lack of drive, or perhaps something loading the supply excessively in the secondary causing the supply to trip or hunt constantly. If this happens the supply on pin 7 will drop as it's only really given a kick to get it going, then the smps supplies the voltage. Can you disconnect the secondaries and provide it with a dummy load?
 
I have rechecked all resistors (in circuit) on the primary side and compaired them to the 2nd working charger. all are the same .
I can get the charger working by injecting 12V between pin 5 and 7 till the processor on the secondary does it's boot up sequence then I can remove the injected power.
Have compaired voltages between both chargers on the uc3845. Pin 7 is low at 9.5V. (should be near 14) and pin 6 is higher by .4, so something is being driven harder.
I am begining to wounder if the IRF840's I got are within spec. Am going to check R20-r23 out of circuit tomorrow then start replacing the small capactors.
Would the snubber network cause something like this?
James
 
The reason you only have 9.5V on the startup supply is because the smps is not running, the initial pulse on that pin should be sufficient to start the supply oscillating (>8V), however, if the supply doesn't continue to oscillate then your supply at the junction of the diode and C29 in your drawing will not feedback the required supply volts to keep it going it will just sag back. The supply at pin 6 being a little over would suggest that the supply is trying to start and is in foldback, probably due to an abnormal load or insufficient drive of the primary tx winding. The snubber will not cause this, more likely it would cause the fuse to blow. Replace C19, 26 & 29 with brand new components, don't swap them with others. Also make absolutely certain it is the right version and manufacturer of uc3845 that you are using, there were about 4 different versions of that fet driver before I left the trade, and quite often strange behaviour was seen when the wrong version or manufacturer was used. Grundig TV's come to mind as being particularly picky about which one would work properly in their circuits.

When you apply 12V and the circuit runs, does it continue to run after you remove the 12V? Is the charger full operational when you supply the 12V to pin 7?
If the answer is yes to both questions, I would suggest that you are using the wrong uc3845.

rgds
 
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Will try Those capactor changes when I can get to the shops. I have however swaped in those capactors from other scraped items and charger still won't start.
I too thought about the uc3845 so I socketed them in both chargers and swaped them around before I started asking for help. All chips I have work in the working charger..
Re the 12V injected, Once the processor has done its boot up and battery voltage measure, it can supply the full 20A till the processor sets the uc3845 back to startup state so it can measure battery voltage again. When this happens, the charger fully shuts down because the startup voltage isn't enough. The main supply for the 3845 from the winding on T3 is still low when pumping out 20A. I have noticed that if the injected 12V is left on for too long, I can hear noise from the transformers, so something isn't right.
I have injected power into the secondary side and measured the current draw on both chargers, both are the same. that only leaves the diodes and the snubbers on the secondary side left to check.
If either D2 or D3 protection diodes are leaky or partly breaking down at high voltage, would they cause what I have? I have ordered replacements and some new irf840's and another uc3845 just in case.
Also am on the virge of getting some more 2n222a and 2n2907a transistors again...

James
 
The charger is finally fixed.
Thank you to every one who replied.
I am not sure what was wrong as I replaced every small low voltage capacitor except for C25 as it is a monolithic and I couldn’t get one locally, I also swapped it with another one from a different location. Replaced both D2 and D3. This naturally didn’t fix it so I started to remove the irf840’s and both 2n2222a transistors trying to track down what was pulling down the start up voltage. It wasn’t until I lifted the uc3845 out that the voltage raised right up to 14.5V. I had proved the chip I got does work so something had to be preventing it from working. I looked around and found a small bit of solder from my sucker near 2 pins of the 4n25 I also lifted C25 (it broke apart) and replaced it with a ceramic one. Put in new 2n2222a’s, irf840’s and a new uc3845. It fired up at last. Checked out the other uc3845 I have and it works as well. Not sure if it was that capacitor or the transistors or the solder dag.

James
 
Congrats on getting the charger working again! It's always good to hear a success story and a little feedback from folks :)

rgds
 
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