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Need to do some work...


Marvel

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Was listening to the new (old) Sansui SP-2000 speakers the other day, powered by my Moondog 2A3 amps. Sweet! Heard a pop... Shut it down and finally got to have a look today. Looks like a cap on one of the amps went. Pics attached...

 

Bruce

cap_sm.jpg

underside_sm.jpg

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I'm just thinking I'll replace them on both amps. I'm the fourth owner of these. Awesome amps, Magnequest iron on the outputs... Not quite sure of their age, but I've had them around 10 years I think.

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John, you are possibly correct. There's not much room (or no room) between the cap and the bottom cover. Something went, though and there is the smell of something that fried/popped/burned.

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5 hours ago, NOSValves said:

those Moondogs only have a few hundreds volts in them no where near the 600V those caps are rated at. No reason those Solens should fail for 50 years or more... it looks to me like a mouse ate the damn thing. 

If I recall correctly (these days a bit of a challenge!) but I seem to recall a dead mouse (or should I say fried) preamp  thread a few years ago!

 

Josh 

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Nah, no mice inside of these. The bottom plates fit very tight.

 

They've been off for almost two years, so that nay have been the problem. I just hooked them back up and flipped the power on.

 

Bruce

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On 6/5/2017 at 2:35 PM, NOSValves said:

those Moondogs only have a few hundreds volts in them no where near the 600V those caps are rated at. No reason those Solens should fail for 50 years or more...

 

 

I agree with this. I don't see a poly cap failing so soon especially with a 350-0-350 transformer it will never see anything over 500v, actually factor in source resistance and a tube rectifier and it probably never sees anything close to that. One thing I am not sure of is the ripple current rating of them, maybe something you should look into. They don't specify in the datasheet but say this;

 

"They have lower dielectric absorption factor, lower

equivalent series resistance, lower inductance, higher
resonant frequency, more linear impedance,
lower dissipation factor, higher curr
ent capacity, faster rise time,
unrivalled handling of fast high current
pulse, high temperature stability, excellent long term electrical and mechanical
reliability and a better damped mechanical self
resonance under dynamic signal conditions."
 

 

***Make sure your rectifier isn't shorted from plate to cathode.***

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7 hours ago, alzinski said:

One thing I am not sure of is the ripple current rating of them, maybe something you should look into.

 

That's why kinda crossed my mind. The cap sees raw rectified DC pulses being the first cap right after the rectifier tube...I dunno...

 

2 hours ago, Marvel said:

It plays, just at lower volume, and distorts/fuzzy.

 

Is that still with the damaged Solen capacitor?

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3 hours ago, Marvel said:

It plays, just at lower volume, and distorts/fuzzy.

 

While I'm at it, I'll redo the crossovers in the Sansui SP-2000 speakers.

 

 

 

Still using it? You are brave.

 

You should check B+ voltage.  You may have a L input filter instead of a C input now if that cap failed open giving you much lower supply voltage.  Hard to believe that cap would fail open.................Maybe it originally failed short and gracefully destroyed itself open?

 

Anyway it's safe to say go ahead and check voltages before replacing anything.

 

 

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Yes, Mike... I haven't had it  back on after the initial check, but I still had audio out, low  volume and distorted.

 

I've not ordered new caps yet. I've also not used it, and won't until I do some checking and repairs.

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On 6/5/2017 at 3:43 PM, joshnich said:

If I recall correctly (these days a bit of a challenge!) but I seem to recall a dead mouse (or should I say fried) preamp  thread a few years ago!

 

Josh 

 Your memory has not failed you! Yea I had a mouse die an electric chair like death in my Juicy Music BBX the funny thing is the preamplifier just kept working... I found it inside in a rock hard petrified state when doing some changes to my system. Without the mouse in place the BBX would no longer work from a fried trace on the circuit board. The mouse was completing the connection! The tread title was "juicy Music builds a better mouse trap" or something along those lines. 

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As others have said, the Solen 10Uf used as the input to filter capacitor is well within the limits of the B+ coming off the rectifier.  What you are seeing is simply a mild deformity of the outer insulation.  However, there is in fact another issue related to BOTH the 10uf cap and the rectifier Ron W. chose for the Moondog circuit (and as you older forum fellows might remember, I'm also a Moondog owner/user, and have been so for nearly 20 years.  Here's the thing (and I know the amp circuit so well I can see it in my minds eye):  I have run into a number of more modern designs, both single-ended (as with this amp) and PP, where the 'designer' did not take into consideration the maximum allowable capacitance for a given valve rectifier when using a cap-input-to-filter design.  So, if you are still using the stock rectifier, Bruce, the max value given for it is 4uf.  There seem to be varying degrees of attention to this detail, as well as how important it ultimately is; but it seems tube data would not just attribute random values of reflected capacitance for the many rectifiers using the same general filter design.  I'm NOT saying that this is your issue with the POP you heard, but just mention it as something I think to be very worth considering for modification since you have the amps on the operating table anyway.

 

I'm in agreement with John and Craig on this.  in my experience, popping is often associated with PSU connections, such as broken/intermittent connections on either the primary or secondary sides of the power transformer, connections to filter capacitors, etc.  Don't only suspect high voltage secondaries -- check also filament winding connections for the rectifier, input/driver stage, and output tube.  Confirm the grounded high voltage center tap connection too; dropping resistors, plate load resistors, etc.  The power switch is also of course in the recessed area of the wooden chassis, so you might also inspect there, as well as fuse connections.  Even something as simple as dirty or loose tube socket pins can cause such a problem, and pins can be easily tightened up.

 

And while you're at this, put one of those MQ grid chokes I mentioned years ago on the grid to ground in place of the grid leak resistor on the 2A3.  I must acknowledge Dr. J. Lessard for sharing this with me when I built his marvelous parallel-feed Horus amps.  I still kick myself for ever selling those things.  With the K-horns and Lowthers we were using at the time, that amp sounded easily like a crystal-clear 25 watts.

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