Leaky capacitors need replacement?

All is well except the bias won't zero down and there seems to be a low hum over speakers at idle which I don't think was there before.

Those caps may not be the only ones that were bad. But your tubes can cause both hum and bias problems- have they been tested?

There's nothing weird or crazy about these amps- they can be easily repaired by anyone competent with tube circuits.

Replaced my RM10 caps with the Nichicons George recommended in 1st paragraph, thanks George!.  All is well except the bias won't zero down and there seems to be a low hum over speakers at idle which I don't think was there before.  When the caps went bad the bias couldn't be zero'd down either and the hum was louder.  Otherwise it's holding bias steady at 28mV and sounds fine.  Could 12AX7 input tubes be the culprit?  Uncoupled the CDP player (which I run direct into the amp) from RCA inputs and same low level hum at idle.  Sure wish Roger RIP was around....

I'm in the same boat with my RM10 MkII....same caps have gone south and leaked....anybody know of a good, available replacement that fits easily?  TIA!

Talk about going around the world and the thread becoming a cluster**** of info just thrown out there.
This could have been fixed over a week ago, if you just replace those leaky cheap caps to start with.

Cheers George

I replaced all the tubes and my amp is still sputtering so next I’m off to a tech to replace those caps and hope that’s the issue.  

@ dletch2
The + side of the capacitor connected to the 1kohm resistor go to the screen grid(pin 9) of the output tube via a red/orange color wire, can you see it?

For those who are curious, please take a look at the photo from 6moons.com:
https://6moons.com/audioreviews/musicreference/hero_open.jpg

Those two 33uf 350v capacitors are smoothing/filtering purpose, you can clearly see it connected to a 1kohm resistor go to the output tube screen grid.

Those are polarized capacitors and should be replaced with the same type and value. Film won't fit! Non-polars won't fit either.

Replace the capacitors. Be careful though there may be other components affected by the leaky caps.  An O Scope will help to see if the signal looks clean.

ALWAYS!!!!!!!!!!!!!!!!!

Definitely.

I cannot quite see from the picture if they are polarized or not. But it looks like they are polarized by the little black band on the side indicating the negative terminal, but I cannot be sure. Please check.

But not to worry either way, if they are bi polar (non polar), you can always put two 68uF (standard capacitance value) back to back in series and it would do perfectly, which would give you 34uF and with the +/- 20% tolerances it would not matter.

Do NOT buy another direct bipolar capacitor because their lifetime is VERY limited.

Here are your options (I increased your voltage to 450V for even longer life).

If your capacitor is polar:
https://www.digikey.com/en/products/filter/aluminum-electrolytic-capacitors/58?s=N4IgjCBcoOwEwFYqgMZ...
Note that these are VERY long life capacitors (20,000 hrs at 105C)

If it is bipolar, you need two of these back to back (- to - or + to +).  The good thing with back to back capacitors is that their voltage will double to 900 volts in this case.  They will obviously occupy more space however:
https://www.digikey.com/en/products/filter/aluminum-electrolytic-capacitors/58?s=N4IgjCBcoOwEwFYqgMZ...
These are also very long life too (20,000/12,000 hrs at 105C).

They will last a looooong time.

OK, here’s a bunch of pictures:
Imgur: The magic of the Internet

You can still use the radial Nichicon’s

OK. Now we can see they were cheap IC axial, so get these Kemet’s just as good as the Nichicons.
https://www.mouser.com/ProductDetail/KEMET/PEG124UH2330QL1?qs=%2Fha2pyFaduiIzF561dUvaRbfng7X1%252BD%...

Or these 400volters
https://www.mouser.com/ProductDetail/KEMET/PEG124VJ2330QL1?qs=%2Fha2pyFaduhcwO2qxWSDEzBRXsrUKlP2qluB...

https://americas.rsdelivers.com/product/kemet/peg124vj2330ql1/kemet-33%CE%BCf-electrolytic-capacitor...


Cheers George

I'm not as in depth about amp building and repairs as others around here, but now that I see this,  I don't see why you couldn't do what you wanted to in the first place and use a decent metalized polypropylene there, say 33uf/400v.  I don't see any reason why a bipolar wouldn't work there.  Since Ralph chimed in here @atmasphere    I would get his expert advice

OK, here's a bunch of pictures:
Imgur: The magic of the Internet

I have amp #1249, if that means anything regarding how old it is.

The caps needing replacement are around 15-20mm wide and 40-45mm or so long and are axial and connected as show in the pictures (picture #2, at the bottom right, the two grey ones). 

It looks like one side of each capacitor runs to the ground from the AC plug and then goes through the cap and a resistor to what I imagine is the transformer.  The two capacitors are connected together at ground and the two resistors are connected together at the transformer but they don't connect at the tabbed thingy in the middle, which is just a separator.

Can I shove some small radials in there?  Probably.  I have some vertical height to work with but will the leads be long enough to reach where they need to go?  I'm not sure.  That's my concern with radials.

I could also just use a couple of axials of approximately the same size.  I found two on Mouser:

336TTA350M Cornell Dubilier - CDE | Mouser
PEG124UH2330QL1 KEMET | Mouser

Here's detailed specs for the KEMET since no ripple current is listed:   PEG124UH2330QL1 (kemet.com)

It looks like either will fit fine.  Should I order some of these and take them and the amp to a local tech for replacement or are the radials mentioned preferred?

+1 georgehifi

I need to open the amp back up and take some pictures and measurements and I'll post them later.  It's just a matter of what will fit.

I will see if @clio09 has any of the original capacitors he can send me

Geez! why go that way, they look like old cheap stuff that may need to be formed, because they are so old, and the same thing may happen again.
Just get the better Nichicons I linked to for only a couple of bucks each.

Cheers George

@shawn3997

Just a FWIW, if those caps are used in an application where they are seeing a higher voltage closer to their rated voltage (in the photo it looks like its in a DC Blocker to me, but who knows), I can tell you from experience that those Illinois caps can do that well short of the time you expect of them. Switching to a Nichicon will assure longer life. As @clio09  recommends, replace them with the same value and voltage.

@imhifan Ah, I see. That is more comforting...

I will see if @clio09 has any of the original capacitors he can send me and if not I’ll buy the Nichicons as suggested and find someone local (if possible) to replace them.

If this is truly a DC blocking capacitor for a power amplifier, then those capacitors are grossly under-rated for ripple current. That would lead to premature failure.

Is it right that those capacitors have a life rating of 10,000 hours?

Rated 10,000 hours at 105°C, life of a capacitor doubles for every 10 degree Celsius decrease in temperature, that means the capacitor operating life at 55 °C is 320,000 hours!
If a capacitor rated 10,000 hours at 85°C, its operating life at 55 °C is 80,000 hours.

Is it right that those capacitors have a life rating of 10,000 hours? Will I need to change my capacitors again that soon?

If it fits check Mouser part # 647-UCS2V330MHD, the difference in size is 16mm diameter versus 12.5mm, but the ripple current is 900mA versus 380mA for the UCY series.

My gosh, I’m rebuilding 2 amps right now that are 35 and 38 years old, every cap in both amps were in better shape than those. I didn't catch in the thread which amp that you have. I don't think that you'll find most of those caps in what I recommended in the voltages that you'll need. But I would definitely change any caps with the best that I could find and make sure that you get high enough voltage ratings.  Good Luck,  Tim

The guy I bought it from said it was 3 years old.

Looking at how tarnished the solder is, the grey color and cosmetics of the plastic wrap on those caps, and how long they are (much shorter now), and that they’ve already leaked, I’d say more than 3 years old.
Or they were NOS caps of the shelf that were used and not formed, as electrolytics also have a shelf life and need to be re-formed very slowly when new but old.
https://www.youtube.com/watch?v=vA8YksKJSsQ
Boring as bat **** voice on this guy

Cheers George

I set the bias.

Which MkII do you have, the fixed bias version (you set the bias) or the cathode bias version (amp sets the bias)? I ask because all fixed bias MkIIs are well over 3 years old (mine is at least 15) to my knowledge. A cathode bias version may be within that time window.

The guy I bought it from said it was 3 years old.

hello shawn3998.....I'm curious how old is the amp?  My RM10 Mk II is 10 yrs olde now....perhaps I should check under the hood at some point.

Mine is alive and well loved here so it's not going anywhere.  :)

It's just a little sputtery so I'll get that fixed sometime soon, hopefully locally so I don't have to mail it.

@clio09 has been super helpful and great to work with!  Thank you.

iF you are in USA i would suggest shipping this superb amplifier to tech at Brooks Berdan in Monrovia, CA. They are very familiar with Music reference and the tech will look beyond this issue and get amp properly cleaned up from leak.

just my two cents, they take care of my RM-9.

if all this seems a hassle, i need an RM-10... dead or alive...

From what I can decipher from Roger's schematic (which has a series of iterative changes written in pencil on it), BOM, and notes it is an electrolytic power supply filter capacitor, 33 uF/350V. As I said previously, the OP should replace it with the same value.

If this is a DC blocker, I question whether the lowest impedance capacitor is the best choice. That means more high frequency noise is passed through. Thoughts?

No- it doesn't work that way. Actually though if this is a DC Blocker as it appears the voltage only need be 16 volts not 350.

@clio09 is associated with service of these amps. Tony, can you confirm that this is the DC Blocker circuit?

If you have some extras I’d love if you could send a few to me.  I may have found one or two local techs who might be able to do the work.

It’s a Music Reference RM-10 and he should use the exact value for the replacement. I can't remember the brand that Roger used, we may still have some lying around, but the Nichicon’s pointed out by George are a good choice and should fit inside. They were one of Roger’s favorite capacitors. As for getting someone to put them in any competent local tech can do it.  

I keep getting 403 errors.

Try this it’s the 2nd one
https://www.mouser.com/Search/Refine?Keyword=33uF+350v+105c

Cheers George

atmasphere9,377 posts04-25-2021 2:15pm

Are you saying to replace the electrolytics with new electrolytics or use something else? These are right off the AC mains between the IEC plug and the rest of the amp.

These parts are being used in the DC Blocker inside the amp.

They should be replaced with the same value and voltage. @timlub has mentioned some nice quality parts to replace yours, which have a tendency to do this long before their time.

I am not sure what amp this is inside, but the ripple current rating seems a bit low for a power amplifier.


If this is a DC blocker, I question whether the lowest impedance capacitor is the best choice. That means more high frequency noise is passed through.  Thoughts?

Also, I need an axial version of that Nichicon and mouser has gone mental it seems.  I keep getting 403 errors.  Deny me will you... 

Thanks to everyone for their help, by the way.

Yeah, you're probably right....  I'll check around locally.  I'm not sure anyone actually does gadget repair here anymore that isn't just iPhone battery replacement.  If not I'll have to ship it off and listen to my old stereo again for a while.  Blech.

Anyone want to put out a shout for a great tech just in case I have to ship?

Ok, thanks, will do! I have a bunch of soldering questions but I will do a lot of research online before I try it out on the amp. If anyone wants to help out:

Sorry to be blunt Shawn, for you to ask these questions, this job is not for you to do, get someone that is good with soldering, and component id to do it.

Cheers George

or parts connexion 

Try this https://www.quadhands.com/products/quadhands-workbench

I got mine on Amazon, and it has made soldering much easier.

Ok, thanks, will do!  I have a bunch of soldering questions but I will do a lot of research online before I try it out on the amp.  If anyone wants to help out:

1. What can I use for extra hands?  I would say that is 1/2 of all of my soldering problems -- not being able to hold everything together so it doesn't move when I apply the solder and all the used-to-be soldered stuff is now free to move around. 

2. What can I use as heatsinks to keep the heat to the area where I am soldering and not running out to other solder joints through the wires?

3. Is regular boring old solder from a plastic tube good enough or should I go all fancy silver solder?

I guess that's really all the issues.  :D

I should add I would be most happy to put something in that might last longer.

Just get the Nichicons I linked to for $1.13 each, quality cap and will last longer than you and the next owner will probably keep the amp for anyway.
https://www.mouser.com/ProductDetail/Panasonic/ECA-2VHG330B?qs=sGAEpiMZZMukHu%252BjC5l7YR8X6v1kBION9...
Cheers George