Strange electrolytic cap behaviour...or not?

[jezzbo]

Hi all,

I just fixed my friends Rivera amp. (Overheated, blown electrolytic caps in the power supply)

There is still a crackling and cloudy noise when switching on the standby switch.
After a few (10 to 30) seconds the noise goes away.
And the amp behaves normally.

Anyway i was thinking it had something to do with those replaced filtercaps.
So i was wondering if I would tough the filtercaps on the top (radial caps) the noise would be more. Just to be save, I connected one lead of my DMM to ground and touched one of those filtercaps with the other lead of my DMM.
I measured 217VDC! on top of the 100uF 250V cap!
And 239VDC on the 330uF 250V cap.

Is this normal behaviour for elko's? (conducting + 200VDC on the TOP of the radial cap, not on the legs)

I was happy that I didn't touch the top of those caps with my hand!

What do you guys think?

Thanks,

Jesse

[dynaman]

Yes, I've gotten zapped by touching the top of a charged radial cap.

[Ace Carson]

Replace them. AFAIK thats abnormal.

[Firestorm]

Is this normal behaviour for elko's? (conducting + 200VDC on the TOP of the radial cap, not on the legs)

Yes, that's perfectly normal. The caps are in series so half the voltage (in this case) of the top cap is on the "sleeve" of the bottom cap (and on the negative leg, too). Since the power supply "blew up," I'd think some of the associated resistors cooked. That can make them noisy.

[jezzbo]

thanks for your reply guys.

So it's pretty normal.

@ Firestorm: If those resistors would generate the noise, wouldn't they do that all the time instead of only during the first 30 seconds or so?

[Firestorm]

Depends on what kind of noise (and what kind of resistor). "Normal" resistor noise increases with current and/or temperature. But if the resistor lead connections were damaged, they might be noiser UNTIL the connection reaches a certain temperature. Just a guess. Course, if you put new tubes in, they could be noisy in that way, too.

[selloutrr]

I would trace threw the whole power section make sure nothing else was damaged in the explosion. It's not a small thng to gave amp release that amount of stored voltage at once. It may have boogered up a tube aswell.

The voltage on the cap can is normal just becareful and discharge before you go poking around. Most shocks accure when the plastic shrink is scuffed and you touch metal.

Here is the thing... It's probably going to be faster to replace a handful of parts then search for a bad one.

I'd replace the diodes in the rectifier and the resistors near the caps after I tested the tubes!

What worries me about this Rivera are a reletively new amplifier and shouldn't be having cap issues or over heating so what actually caused the problem?

[jezzbo]

I would trace threw the whole power section make sure nothing else was damaged in the explosion. It's not a small thng to gave amp release that amount of stored voltage at once. It may have boogered up a tube aswell.

The voltage on the cap can is normal just becareful and discharge before you go poking around. Most shocks accure when the plastic shrink is scuffed and you touch metal.

Here is the thing... It's probably going to be faster to replace a handful of parts then search for a bad one.

I'd replace the diodes in the rectifier and the resistors near the caps after I tested the tubes!

What worries me about this Rivera are a reletively new amplifier and shouldn't be having cap issues or over heating so what actually caused the problem?

thanks for your replies guys.

The amp overheated because he (my friend) used it on a concert with the amp completly covered. There was absolutely no airflow at the back of the amp.
So probably 1 or more tubes overheated and when it died it took several components in the amp with it.

I allready replaced all filter caps, rectifier diodes and tubes.
It's a few of those new filtercaps that meassure + 200V on top of them.

There are a few solderjoints that look not perfect so I'm gonna redo these first.

How do I know if the resitors in the powersupply are bad?
They meassure normal.

Thanks again!

Jesse

[Gibsonman63]

I would replace the resistors soldered to the power tube sockets with 2W metal film resistors. They probably cooked during the overheating.

[selloutrr]

resistors are tricky because it's possible to test good when cold or not in use but as they warm up of have high voltage ran threw them they can shift and or stop working. as long as you are in the amp i'd just replace any that are in the path or direct contact with the power caps or first gain stage esp. the large 2W resistors. the noise should calm down. from what you discribed it sounds like the amp is sabalizing and getting it's aches and pains out if you will. if the tubes are caps and diodes are new next would be resistors.

[sootboots]

I have dealt with similar noises and each time I traced it to arcing in the power tube sockets. make sure there is no carbon or residue in the sockets and retension them appropriately.

Good luck!

jp

[jezzbo]

I have dealt with similar noises and each time I traced it to arcing in the power tube sockets. make sure there is no carbon or residue in the sockets and retension them appropriately.

Good luck!

jp

how do I see there is carbon residue in the sockets?
Can I just blow compressed air through it? (from an spraycan not a compressor)

I detected two cold solderjoints that I've resoldered.
It sounds that the noise isn't that loud anymore.
It could be whishful thinking though.

I'll test more tomorrow. And let the amp stand on for some time.

thank you!

[selloutrr]

burned carbon is visiable as a blackish trace connecting pin holes. use a caig cleaner deoxit i found a great set of wire brushes on ebay perfect for cleaning sockets a few years back. and a dental tool is stll the best for retension or socket posts. compressed air doesnt break burned traces down it takes a solvent. i like the compressed air or an oil less compressor to blow out the amps dust and what not.

[jezzbo]

Ok guys,

The amp is fixed. No noise
It turned out to be two cold solderjoints.
Everything works just fine now!

Thank you all for your help!

Jesse