PWM Woes....

[GooeyGus]

Wow... more troubles today. I dont know if these caps are bad or what but i'm having issues again. Everything worked flawlessly last night. Ran it for a few hours. I went to start it today and the cards would only give weak little pulses... clicking at maybe 2Hz while trying to start. Usually it starts instantly, but if it has to click a few times, it's MUCH faster. On the order of 50Hz or so until it starts, so you really only hear it click once or twice. It's almost like these new caps cant dump in pulses to that little starter circuit fast enough... I swapped the caps back to the 'old' ones on two of the boards and slapped them in and they worked and its running now... but it's really weird that everything worked fine last night and today it wouldn't even fire up.

One of the boards was acting like it did when it had the 'almost dead' capacitor in it. I would start it and it wouldn't do anything, but then when I turned the key off and the voltage dropped it would click on for a split second. Very weird. I'll bring these caps to work with me tomorrow to test the capacitance.

[GooeyGus]

Old Cap on the right, new cap on the left. Am I missing something? "336" means 33uf, right? OR are the old caps in the same notation? Meaning '33' would actually be 33pF. That cap seems a bit too big to be only 33pF!!! It must be uF...

Opinions?

[mixedgas]

Hum 103 is .01 uf. 104 is a .1 uf, 105 would be 1 uf. so 106 must be 10 uf,
so 336 SHOULD be 33 uf. I'm sure heroic will check my math in a few hours.


Steve

[heroic]

Hum 103 is .01 uf. 104 is a .1 uf, 105 would be 1 uf. so 106 must be 10 uf,
so 336 SHOULD be 33 uf. I'm sure heroic will check my math in a few hours.

336 is indeed 33 microfarads.

I know you're not stupid and all... but did you respect their polarity? Tantalum caps are polarized.

-J.

[GooeyGus]

336 is indeed 33 microfarads.

I know you're not stupid and all... but did you respect their polarity? Tantalum caps are polarized.

-J.

Yup I made sure the side with the line on it, on the yellow cap, went to the positive (+ is actually marked on the board on every capacitor... makes it kinda nice!). Also, one lead was longer than the other, and I've always known that to be the anode lead, so I had triple verification on polarity.

So now I'm just stumped... these caps came from an ebay seller from Taiwan so who knows.. maybe they are just not good caps. The 25 has a 'K' after it... don't think that means anything though, probably the brand.

So on the red capacitor (the old one) since the 33 doesn't have another digit after it, am I just to assume that it's 33uf? I dont think they used the three-digit notation that long ago.. but if they did it would be a 33pf cap, which just doesn't sound right to me.

The PSU has absolutely ZERO issues once the PWM circuits get going... it's just the actual starting of the card that is miffed up. It has to be something to do with these new caps since it worked once I put the old ones back in.

[mixedgas]

While I am loath to suggest a change, a 33 uf non polarized electrolytic or poly cap might work just fine there, just pick the right voltage, hopefully twice what it sees in the circuit.

Steve

[andyf97]

I am far from an expert in this subject but one thing reminded me of the past.

Those boards are old and so are the connections, is it possible that all those caps are failing due to poor contacts from the board to the socket?

I have this feeling you fixed the symptoms but might not have cured the problem.

if you have further failures in the futer, before swapping the boards again, have a good look at those contacts.

Best Regards

Andy

[GooeyGus]

It could be a poor connection at the socket, although I thought of that before and gave the cards a light 'wiggle' with no effects. I'll give the leads that go into the socket a light tinning for better contact.

[GooeyGus]

I got my (third) new power supply today anyway, and it seems to work perfectly so I'll have plenty of time to mess with this other one

Its a couple years newer than the other two, so I'm curious to check out the PWM cards to see if they changed those caps to electrolytics or anything other than tants.

I've only ran it for today, and the other supplies seemed to work for the first day and then have problems, so I guess we'll see tomorrow!

[GooeyGus]

So either I just have horrible luck, or there is something else wrong here. The third power supply worked flawlessly yesterday. I went to start it this morning just to see if everything was still as it should be and only one side of the psu lit, just like the other two. I really don't think there is anything at all in the head itself that could cause this... It's really just a tube and a very simple control card for the He and Cd heaters...



So frustrating....