Capacitor Venting

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Could it be an electrical discharge? Try viewing in the dark (with precautions against contact!)...

Mike.

Feel the capacitors and find which one is getting hot. An IR heat gun may be better to keep the shock hazzard down.

I like your thinking so I tried it. But no sign of any arcing at all. Anyway it really does sound like more of a hissing than a rasping.

I'm not really happy about prodding around this thing while it's running for that reason. I've tried feeling for hot caps after switching it off again, but at 150V I'm guessing it's not enough to heat the dud cap up enough to be able to feel the difference. I have tried using a temp sensor & DVM to check for heating but that proved inconclusive as well.

I have heard high frequency arcs that sound just like air escaping, at least to my ears. Eric

Interesting. What component was failing? There's a 1.5kV transformer on this board which is encased in opaque grey plastic so if it's something arcing inside that it won't show up in the dark. I thought perhaps a old AM radio next to the board might indicate something useful, but I'm guessing it would be swamped by all the spurious noise these things make even when they're running right. Spectrum analyser, perhaps? Bit drastic but I do have one!

Well, that could just be the switching action of the power supply. Some generate pure ultrasonics, but some have lots of other frequencies in them and the part we can hear sounds like hissing. I've heard lots of SMPS make hissing sounds under some particular condition (no load, full load, intermittent current pulses to the load, etc.) Undervoltage might have the PS jumping back and forth between start-up mode and normal switching mode very fast. MANY SMPS' will make a hissing sound as the main input cap runs down, which could be exactly what you are seeing with your variac test.

If you do just put full AC to it, wear ear protectors and don't poke your face near it, just in case it IS a cap getting ready to blow.

Jon

Sometimes an smps run at low voltage (or with a defect) will not osc cleanl y and make noise like hissing air. I've seen some supplies sound like that when one or more of the secondaries are feeding a short. Most modern desi gns shut down completely but a lot of the older ones throttled back to a sa fe current but otherwise would continue to run.

Try the Grand-Dad's Ear Horn approach. Get a length of 1"-diameter plastic pipe, hold one end against your ear, and move the other end around the board. This will let you "listen" to the individual components, while still maintaining a safe distance from any high voltage.

Seems to me that you could be hearing either:

- Actual venting. I'd be a bit surprised, though, if this were happening and you couldn't spot the culprit. Maybe fire it up in a really cold location, and look for a cloud of vapor?

- Switching noise from a failing cap. I've heard SMPS capacitors begin to hiss like this when they began failing due to the infamous "'lytic capacitor electolyte plague" a few years ago. They'd go high-ESR well before they'd start to leak and vent.

Checking all of the caps on that board with an ESR meter (with the power off and fully discharged, of course) might locate the culprits.

- High-voltage leakage. I had a Tek 7904 scope which was suffering from corona discharge and hiss (and an occasional "snap") due to some contamination/deterioration on the high-voltage lead from the power supply to the CRT. Cleaning didn't help. Coating the lead (from the PSU, through the cage) with a bit of high-voltage silicone putty made the problem go away.

I've never been in a position to work on a switcher before owing to the lack of an isolation transformer until very recently, so I was unaware of that. Well, if push comes to shove and there's no alternative, I'll have to jack up the juice to 230V and we'll find out then one way or another.

You're running a switcher at way out of spec input voltage. The magnetics making awful noisy or hissing sounds is not unexpected under those conditions as it fails to startup correctly.

Well I've now wound it up to the correct supply volts and it's still hissing. Nothing's blown that I can see; there's just one power resistor that's running too hot to touch (because the psu is unloaded?)everything else, including the transformer is cool. Also, by use of better gauge pipe, I've been able to detect the sound is coming from the grey 1.5kV transformer in the middle of the board:

Could this be just normal behaviour that we don't normally hear since it's normally buried deep within the scope casing??

BTW, those burnt areas are previous problems that someone else fixed years ago and not relevant to the current issue.

It can be so hard to tell sometimes. Try scoping the output from the transformer (taking suitable precautions of course!) and look for any irregularities that might indicate internal arcing.

I would look for anything shorted on the secondary side of the supply. Pulls the switcher frequency down into the audible range. (for those of us who can still hear anyway...)

Usually the root cause is a regulation problem (bad caps mostly) causing the monitored voltage to "appear" low, the switcher duty cycle increases, the actual DC output rails out, usually shorting a zener diode which gives up it's life to save components downstream.

Example: an 18 volt zener across a 14 volt line shorts because now that line has shot up to 35 volts.

Common problem in older consumer vcr's, but 'scopes etc often do the same thing.

My .02 worth...

Mark Z.

Thanks for that. Worth more than 2c I'd say. In that case I'll check the duty cycle unloaded by scoping the B/E junction of the chopper transistor. That should be a good indicator of anything having gone low-res on the secondary side from what you say. I'm still curious as to why that power resistor is getting so hot. Even if the tranny is fine something's still amiss somewhere.

Power resistors often get hot. Can you measure the voltage drop across it and knowing the resistance calculate the wattage it is using.

No visible markings on it at all (typical). In the photo I posted, this is the one near the top right hand corner. You got the two big bulk storage caps, then to the right of those the bridge rectifier, then to the right of that and down a bit is the resistor in question - brown coloured one lying between two black diodes. It reads 20 ohms, but could be in parallel with god knows what, so that's not much help I know. I'll lift one leg and remeasure if you think it'll do any good.

No load could also cause weird noises- that board looks really old. If that transformer is burned or arcing it's already trashed- more testing won't make the problem worse, but giving it with no load or with no load and at a weird input voltage won't help troubleshoot anything easier.

Yes, the world of vintage has its pros and cons. The plus side is everything is big enough to work with - handy if you are half-blind and clumsy like me. But the con is things get very fragile so you have to think thrice before disturbing anything (and there's quite a bit on this board that's been disturbed before due to previous issues.)