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Mark Levinson has odd choice for cap upgrades in one of their service bulletins. Why?

Pursuant to one of the Mark Levinson service bulletins, for Model 331, 332, 333 etc, they have outlined one of many things that should be performed on any amp that comes in for service.

One of the line items has me baffled. They recommend replacing four caps on each voltage gain / input board. These eight caps are PP type of .01uF @ 160v. They recommend replacing them with Ceramic X7R .01uF @ 200v. These are axial configurations.

This is not an expensive upgrade but I thought to myself that polypropylene caps had low failure rates and good longevity compared to other types especially if they were used in a proper operating envelope.

I just finished watching and reading some information on the perils of using ceramic caps in certain applications. For one, they tend to drift heavily with temperature changes. In a monster like the Model 333 there will definitely be a large temperature swing. The ceramics also tend to exhibit piezo effects with vibration. While vibration is only inducing small voltages, I can imagine the sum of many caps being subject to vibration not being a good recipe for an audio signal.

ML has stated that the ceramic replacements should be installed with spacers to keep them lifted from the circuit board. I am guessing that this could address temperature concerns, vibration or parasitic capacitance issues. They do not provide any reason.

I would really like to learn a little more behind their reasoning as it seems this particular "upgrade" is counter-intuitive. Can anyone shed some light on this?
generatorlabs
erik_squires
15,986 posts
 
By the way, important to note that thermal stability ant temperature/longevity are not the same thing.

You can have a very stable cap that has a low thermal rating.

Also, who cares. These are RF filtering bypass caps. You could use 1 or 4 and the audible effects would be 0.  A cap with 20% variance in value is just fine here.
generatorlabs OP
25 posts
 
@imhififan :
Due to voltage rating of those caps? Are they working on same voltage or the caps working at lower voltage on current gain board?
I took it apart today to compare the caps on the current gain and voltage gain boards....they are the same. If I had to take a stab at why Mark Levinson chose to ignore the CG caps:
a) difficult to do it properly without desoldering all the output devices?
b) they are shielded from rising heat by the large red film caps?
c) input gain stages are more susceptible to RF?

Here is a pic:
https://drive.google.com/file/d/0B0yvjhlC62G7UUdweEVOeGtva0U/view?usp=sharing

Like I said I am no engineer and I know enough about this stuff to get it working if it is broken. I learn something new everyday and working on this type of equipment is very calming to me. I disappear in my lab trying to solve these little mysteries.

Thank you all for your responses so far!
danpe
1 posts
 
Hi Generatorlabs,

In one of your messages you stated that you replaced '6 small electrolytics'. I suppose this is about  6 x Sprague 68 µF 25 VDC. Can you tell me the replacement type that you used? The service manual mentions part number 240-200013 but nowhere I can find more specifications. Your help would be appreciated.

Thanks,

Daniël
generatorlabs OP
25 posts
 
@danpe1 

It has been so long since I have had the covers off this monster I cannot really remember the caps you are referring to. Where are the caps located? Since the original is a Sprague you could find a suitable replacement from the The Vishay/Sprague lineup. However these particular caps could be replaced with low ESR caps from Illinois Capacitors (I like IC caps and I know for certain there are a few spattered in my ML), Panasonic, Nichion etc. Admittedly the hardest part is matching terminal dimensions, not so much the value. Also you can go slightly higher with the voltage (ie 35v) with no bad side effects.  


pragmasi
212 posts
 
@georgehifi
The XR7’s are great for HF filtering, I use them on dac I/V stage voltage rail decoupling.That’s why ML are recommending them.
That’s the answer, XR7 would be the first choice for this application... makes me wonder why they used PP in the first place.

C0G are great for setting time constants (e.g. RIAA equalisation) bot not necessary or most effective here. It’s actually the limitations of the dielectric that make X7R the best choice as high frequencies (such as RF) will get shunted to ground... these caps are on the DC rails where you don't want any AC.
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