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Interesting Power Supply Issue

Journey11

May 23, 2018
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May 23, 2018
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Hi All,
Been a while ...
I have an interesting issue with a washing machine motor control board. The washer works as in going through the cycle but motor won't spin. In rush current thermistor decided to unsolder itself from the pcb. Owner resoldered and it ran for 12 loads and then burned itself out. Some other repair idiot just jumped it and it wouldn't work. After I got hold of it ... I found a cap that was sploded.
So replaced Cap and Thermistor and tested ... no go ... then decided to draw schematic and during that process found broken solder joint at R75 in included partial schematic. Broken conn was on side of R75 that supplies 160V to Q10 and Q9. Once that was fixed everything seemed to be fine. I decided to take one more voltage measurement at Q9 pin 1 and Q10 popped it's lid. (????)
I ordered 4 KSP44's ( a slightly beefier unit than original KSP42 ) ... replaced blown Q10 and also replaced Q9 as well just in case. Hooked up AC and nothing ... no smoke and no voltage on zeners. Found that both zeners were blown ... D11 shorted and the other (D10) crippled. Replaced both and retested ... this time all 3 transistors blew in a blaze of sparks. ( WTF I thot ? )
So when looking at the damage ...I found D8 had been blown apart.

So here's my question ... if D8 was open ( tested/measured was open ) ... would that have caused all 3 transistors to pop off ? My thinking is that the answer would be YES ...
Any thought out there ?
I have found a reasonable replacement for D8 and had extra Q's for 8,9, and 10 ... Q8 is now the old Q9 I decided to swap out. I just have to replace the zeners again and ready to test again but wanted to get some input here on it.2022-07-12_01h09_29.jpg

All the voltages shown on schematic were taken after I found and fixed broken solder joint at R75 and pins 1 and 3 Of Q10 and Q9. Again all I did was try to see what the voltage was on pin 1 of Q9 and all hell broke loose. I must have powered that brd up at least ten times to take what voltage measurements are shown and no issues.
 
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Journey11

May 23, 2018
88
Joined
May 23, 2018
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88
Hi All,
Sorry about the messed up posting ... I added some edits and for some reason it went haywire. I wanted to add that this control board is from a Kenmore 417.44042400 front loader and the brd part number is 134393900 ...I have drawn up a full schematic of it as well.
 

Journey11

May 23, 2018
88
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May 23, 2018
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88
Hi All,
I have discovered an error in my schematic while replacing blown components and testing for more I may have missed.
Please disregard the above drawing and refer to this corrected version ... Thanks.
12v-5v ps section.jpg
 

Journey11

May 23, 2018
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May 23, 2018
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Hi All,
I think I have resolved the issue ... After looking at the old Q10, I noticed that they had printed the part number on the backside of the transistor ... which at first appeared to be the flat side where they normally print the part #. This part had barely noticeable curved edges on the side of printing, hence I put the new part in backwards. I have also revised the schematic once again to accommodate Q10's new configuration where the two collectors are tied together where the voltage comes in ... and now makes a whole lot more sense to me. Also looking at the silk screen foot print on the pcb ... it clearly shows that the new transistor I installed was backwards.
Not sure how/why I missed that. I built a copy of this circuit section on a breadbrd and it worked perfectly as per revised schematic. Although at an 88VDC input, the 12v zener was not at 12v but at 17 to 17.8V which I'm finding odd. But I think it's fine because the one IC it feeds (IR2136S) can have a Vcc range from 12 to 20V.
Only thing left now is to test it now and hopefully no smoke show and sparks ! Wish me luck ...Revised Schematic.jpg
 

Journey11

May 23, 2018
88
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May 23, 2018
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Well ... I did some testing of the most recent CORRECTED circuit just above and was able to use separate power supply input, rather than the 160V coming from the main rectifier section. I used ~40V from my bench supply. The circuit seemed to work properly but noticed luckily that Q10 is getting very hot. I found a trace break from topside of R77 going to R76, D8, and Q10 pin 1 ... I thought after fixing that Q10 would operate normally but it is still over heating. Still scratching my head on this one.
Any ideas will be helpful ! Below is another image with the voltages around the circuit using the 40V bench supply.Test voltages.jpg
 

Sunnysky

Jul 15, 2016
529
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Jul 15, 2016
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529
Not sure on purpose of Q9 but Vbe is reverse biased to -8V and with DMM probe between Q9-b and GND will certainly trigger Q9-be to short circuit failure and start a cascade of reactions.

This is a puzzle since Vebo = 6.0V so why use it at 8V reverse bias? and Q8 is also reverse biased except on power up.
R73 must source 400V with 11:1 attenuator to create 40V to charge up the 5V output dropped from the 12V zener.

But it appears to be some AC=> DC power on sequence regulator.
 
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Sunnysky

Jul 15, 2016
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Jul 15, 2016
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It would be easier to analyze what output is needed and replace this mess with something better.
It looks pretty low power. It seems the source was measured at 160V and R73 scales that down to 1% at R32.



With the revised schematic comes revised hunches. Now it looks like a zero-crossing detector with the 12V Zener to create ~ 12V pulses to 0V at the zero crossing on the Emitter of the top Q10-1.

There are certainly easier ways to do this.

There is no power being converted with all the high resistors. This looks more like a pulse generator from line-rectified ripple.
 
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