Help needed: Is this cap fried?

[deadbone]

A couple of months ago I inherited an old Sony receiver (STR-AV920) and CD player (CDP-C705) from one of my father's coworkers. They're not the greatest gear but for the price of free I'm pretty satisfied with them.

Soon thereafter I noticed there was a channel imbalance with the receiver; the left channel was significantly louder and clearer in the high frequencies than the right. I swapped the speakers around--nope, not them--and noticed the imbalance with headphones too, so after a bit of online research I assumed it was probably the result of aging capacitors or something and decided to even it out with the balance control.

Lately the problem has been more noticeable, requiring the balance to be set around 2 or 3 o'clock and this with a clear loss of clarity. I opened up the CD player tonight to adjust to tension of the chucking arm and decided to take a look inside the receiver as well.

So one of the two largest caps on the board is slightly bowed out on top, and there is a pool of white paste around the base of both of those caps (see pictures). I don't know much about electronics but I assume these are the capacitors for the output stage, one for each channel? At first glance I thought the white substance was thermal compound (do caps produce heat?) or something but at second thought and smell I wonder if the bulging capacitor is leaking.

If that's the case, should replacing those caps be relatively straightforward, and is that likely to fix the channel imbalance? I found capacitors of the same spec (?) on digi-key for less than $10 for a pair (part no. P7492-ND). Would these be a suitable replacement or am I going to need some kind of special audio-grade caps? Obviously this is not a great receiver. If I can fix it cheaply I'm all for it, but putting too much into it (say more than $25ish) doesn't make sense.

Please forgive any ignorance in this post and feel free to correct it. Thanks!

 

[thermocouple]

Boy that bowing on the top is a bad sign. The white stuff at the bottom might just be old glue. I would replace the caps, clean the switch and pots with contact cleaner and check again.

They are probably the power supply caps, so I wouldn't go to expensive audio-grade, but a good name brand, like Panasonic which you have chosen, with ones that match in capacitance and voltage like you have.

 

[Sir.Byrd]

That is glue. The caps are probably fine. I doubt that those are your problem.

 

[elnaldo]

I would replace them anyway, it's an easy fix, and you'll be sure if the problem is there or not.... Get 105º caps if you can. Check diameter, you don't have too much space there. Write down position and polarity before removing them. It's very easy to forget how they were 1 minute ago....

 

[TWTRTECH]

Those are the power supply filter caps, if they were bad you would likely have problems with both channels such as hum. And the white stuff at the base of the caps is just mounting glue, no biggie. The bulge on top of the cap is just the plastic cover warping with age, simply cosmetic. Your problem more likely are dirty swithes or coupling caps

 

[deadbone]

Ok, thanks for the replies everyone. Didn't know those were power supply caps.

Occasionally when the receiver powers up, the right channel is fuzzy and extra quiet for 10 to 20 seconds, then snaps back to it's usual imbalance.

Neither the volume or balance pots are staticky, though I can give them a proper cleaning if that could be the issue. I ran them back and forth a bunch of times after I noticed the imbalance to see if that changed anything--it didn't.

I have a multimeter, would that help me identify the source of the problem? What should I look for with it?

 

[Sir.Byrd]

Try checking DC offset and Bias, if you can get ahold of the service manual.

 

[markn2wae]

Those are the power supply filter caps, if they were bad you would likely have problems with both channels such as hum. And the white stuff at the base of the caps is just mounting glue, no biggie. The bulge on top of the cap is just the plastic cover warping with age, simply cosmetic. Your problem more likely are dirty swithes or coupling caps

+1 :thmbsp:

Mark T.

 

[KingBubba]

The bulge on the top is not plastic, it is aluminum. The bulge is bad juju and a early sign of deterioration. The grooves in the top of the caps are there to allow expansion in the case of internal strife due to heat, etc. Every bad motherboard I have ever had had caps that looked like that. Caps should be flat on the top.

Replace them before they do go, in a catastrophic way, and take out more components down the line. The advice regarding the Panasonic caps is good. They are reasonable and good. 105deg caps are also a good choice but not 100% imperative. Check with the Digikey search engine to find the closest replacements. With caps that size, the price will be a bit steep but worth the investment. Use a Dremel and acetone or lacquer thinner to remove all traces of the glue. It is oftentimes corrosive. You need not use glue when you replace them. Remember to measure the old caps correctly and do you best to match the size. Height is more crucial because if the new cap is too tall you may not be able to restore the cover; shorter is okay. As far as diameter, smaller can be corrected with tape but too large of a diameter will be a real PITA to deal with.

Take pictures underneath and make notes on the chassis regarding the cap's position and polarity. You do not want to wire those backwards unless you like the sound of cannon fire. Have fun with your project.

 

[deadbone]

Seems to indeed have been a dirty switch. Not sure if it was the ladybug that fell out of the speaker selector, but something I did between taking the front plate off and now eliminated the issue.

Thank you all for your advice. This further reinforces that you should never write something off as broken without at least trying to fix it, and now I know a little bit more about what's inside a receiver.

 

[Sir.Byrd]

Make sure you get the right caps as far as lead type goes. Those are probably snap in with 10mm pitch. Be careful not to order the 4 or 5 lead snap in caps instead, they will not work.
Why don't you desolder the caps to check out what kind of leads they have?
They could also be solder lug, but that is unlikely.

 

[TWTRTECH]

Well an easy way to tell if the cap has a bad bulge, just press down on the top of the cap with your finger nail if it goes flat then you are good to go, if there is no movement then yes the aluminum has bowed up from a presure release and may be leaking. But I have seen many a cap where the plastic cover just warps, be it from heat or age. Though the ideal way would be to put a scope on the caps and see how bad the ripple current is.

 

[2chanman]

The output relay ( if there is one) may have dirty contacts which could intermittently cause what you describe. The same could be said of the speaker selector or mode or tape monitor switches.

Sorry to hear about the poor lady bug.

 

[deadbone]

Turned on the receiver today and the channel imbalance is back. Love intermittent problems. I set up a tone test in Audacity, 1000Hz panned 100% left then 100% right, and noticed on the front-panel spectrum analyzer when playing the left channel tone, the 1KHz band is full on with sidebands to the left and right. When the right channel tone plays, the 1KHz band shows lower volume and there are no sidebands.

This makes me question whether the problem is in the output circuitry at all, or in the input section/preamp. I suppose it depends where the spectrum analyzer is placed in the chain. Anyone with some insight on that?

 

[deadbone]

Also, in regards to the power supply filter caps--when I had the unit open yesterday, I did try pressing on the top of the bulged cap, which gave as TWTRTECH suggested. It felt like plastic though I suppose it could be thin painted aluminum as well.