Cap failure in Seduction

[Nick_Marshall]

Hi,
One of the PS capacitors started to smoke/melt in my Seduction (with C4S). It's the 220 uF 200V cap here:
http://wardsweb.org/audio/seduction/build/build21.html 

I turned off immediately. Can I simply replace this capacitor and power up again or is there likely to be damage elsewhere?
Thanks for any help!
Nick

 

[vetmed]

At the least you should do a simple resistance check on any other electrolytics in the HV supply. Very high resistance is good, low(in 10K -100K) would make me suspect the cap was damaged even if it shows no obvious signs of such damage. Also, caps will vary in value until they settle on a hopefully high value, but you probably already knew that!




Robert Lees

 

[Nick_Marshall]

Thanks - will check (don't count on me knowing too much...)
Nick

 

[fullheadofnothing]

Was this the first time turning it on to do voltage checks? Yes, you can replace the cap, but it's important to figure out why it blew up. Was it in backwards? Was anything around it put in backwards (diodes, other caps)? Also check other parts for damage in case they were overloaded.

 

[Nick_Marshall]

This wasn't the first turn in etc - the unit has been working fine for months and just started smoking one day when I turned on. I will thoroughly check all the other components for signs of stress, bad solder joints etc.
I should have a bit of time this weekend. I don't have any caps so will have to order these.

Is it possible that caps can just fail like that?
Thanks again.

 

[Doc B]

The caps are overrated by a pretty wide margin, about 100V above B+. So it's not likely that they would fail from over voltage. I would be inclined to make sure the tubes are good and that nothing has move3d under the chassis that might crate a short.

 

[Nick_Marshall]

ok - thanks Doc - will check tubes and for shorts.

 

[Grainger49]

This wasn't the first turn in etc - the unit has been working fine for months and just started smoking one day when I turned on. I will thoroughly check all the other components for signs of stress, bad solder joints etc.
I should have a bit of time this weekend. I don't have any caps so will have to order these.

Is it possible that caps can just fail like that?
Thanks again.

IF my memory serves me, there has been one or more instances of power supply capacitors installed with the wrong polarity that didn't fail immediately.  It is a tribute to what a cap can take.

Re-verify the capacitor orientation.  It is a possibility.

 

[Nick_Marshall]

Thanks for the further tips. I checked and I installed the PS caps oriented as in the photo of the Seduction in the advert on the Bottlehead site and Wardsweb. Negative terminal at the top (north) in the photo. I have checked for further damage to components and shorts and there is nothing obvious - will look further.

Can I check the tubes without powering up and installing? Should I replace the tubes anyway - just in case?
I have ordered the new caps but they will take a while to arrive.
Thanks again for the help - it's much appreciated!
Nick

 

[Nick_Marshall]

I have replaced the capacitor. Measured the resistances as specified in the manual and all are within ~10% of the expected values.
I have bought new valves (E88CC) and will install and measure voltages tomorrow (provided the cap holds up!).
Nick

 

[Grainger49]

Within 10% of expected resistances is good.  I just looked at the Seduction product page, all stripes, negative terminals, are North, toward the top of the page EXCEPT the one for the heaters.  The stripe on it is South.  It shows the same on Wardsweb's site.

New capacitor failures are quite rare if they are installed right.  Well, you have to have the rectifier diodes in the right direction too (banded ends toward Terminal 25 for both diodes).  You have verified both so I would have to say this was that one rare cap failure.