PWM Woes....

[GooeyGus]

seems like the zener diode on the 'intermittant' board is just very slow to respond.. I got home and it didn't work, I pulled the board out and charged up the diode myself and put it back and it worked

Looking forward to getting my new diodes in the mail... hopefully it will take care of this problem.

Although I do have a backup plan... I picked up another 4240PS. I guess you can just never have too many

[mixedgas]

two cards , one for each side. Ok.
Thats not bad.

Steve

[drlava]

Typically when you see the 'resistance' increasing on your multimeter as you probe components, it is because you are charging a capacitor that is parallel somewhere in the circuit. There is a rather large tantalum right in that area. Tantalum caps are very sensitive to reverse voltage and over-voltage because of their extremely thin dielectric. What could have happened is the dielectric has a micro 'pierce' that increases the caps leakage. You might try desoldering the cap and testing it, and if its bad replacing that cap with a higher voltage rated cap and see if the 'intermittent' problem goes away.

[GooeyGus]

Yup that cap is tied to the Zener. Maybe that is the issue. I'll order some tantalums. It's weird though that if I charge it with my dmm it works, but if it sees the stronger voltage of the psu it doesn't. Shouldn't a Zener gain resisitance as voltage is applied as well or am I wrong?

[mixedgas]

Yup that cap is tied to the Zener. Maybe that is the issue. I'll order some tantalums. It's weird though that if I charge it with my dmm it works, but if it sees the stronger voltage of the psu it doesn't. Shouldn't a Zener gain resisitance as voltage is applied as well or am I wrong?

No, not really. they do very voltage with respect to current and temperature, but they dont really shoot up in resistance on a ohm meter.

Its the cap breaking down, which dumps pulses into the zener, which causes your chain of failure. Right about the time your PSU was made, there was a scandal about bad tants flooding the market, Then again in the 90s, and again around 2002. i avoid tantalums like the plauge, but sometimes you need them, and often Uncle Samual and Uncle Ivan insist on them.

Steve

[GooeyGus]

Yahtzee...

I swapped the cap on one of the boards that was totally dead, it clicked right on

Thank you both very much...

So, in this application, I'm guessing It's necessary to use the tantalum cap right there? On some of the newer boards I have, all the tants have been replaced with electrolytics EXCEPT that one, so I'm guessing it needs to stay tantalum.

I tore my house apart trying to find a 25V 33u tantalum cap... I realized that the spare boards I had, had a few of the same caps in different areas so I just borrowed one. I guess I'll have to order some now.

Thanks again... my hair was getting a bit thin because of this thing

[drlava]

Consider yourself very lucky! Your photos and testing feedback were good enough to allow remote informed conjecture/diagnosis. That sort of thing is pretty rare!


If the new boards still use tantalum there, then I'd assume it's for a good reason. Tants are typically more expensive than their electrolytic counterparts. You could probably get away with a ceramic/electolytic parallel pair, or just keep a few spare tantalums on hand.

[GooeyGus]

I just cant believe that all three caps failed within a couple months of each other...

Maybe it really was effected by that 'bad capacitor' batch, or maybe they were just getting old. The funny part is on that board where the cap wasn't totally dead but it still wasn't 'good', every time I turned the key off that side of the power supply would fire for a split second. I think the higher voltage was enough so that it wouldn't work, due to your explanation of a 'micro-hole' inside the cap. BUT if I used my low voltage on my DMM to charge up the cap first and THEN plug it in, it would work... crazy electronics...

[heroic]

Tantalums will gradually fail if left unused or overvoltaged. My guess is that those ones were just old.

The little circuit you have there with the zener and the UJT seems to be a relaxation oscillator of some kind. My guess is that it's responsible for starting the whole shebang going. And by shebang I mean switchmode.

[GooeyGus]

Good news... I got my replacement 25V 33uf tantalum capacitors today. I changed every tantalum capacitor on the boards with the corresponding values (I had a bunch of other tantalums already, but none of the bigger 25v 33uf). Every card fires right up now!! Even the totally dead ones clicked right on. I cant thank you guys enough