parallel silver mica caps opinions please

[musicamex]

I just found the noise problem with a fender prosonic amp i had on the bench. It was a 150 pf silver mica cap leaking dc. I dont have a 150 pf but a 47 pf removed all noise. I never heard the amp like it should be but this is probably an important value re tone. It has plenty of vol now. What are your opinions re running 2 or 3 47pf caps in parallel to get closer to the right value. At least until i can get the right cap from up north. I live in a parts desert and have the best inventory for miles round, but often there is something i seem to be missing.

I should mention the cap is between the plate of v2a and the treble pot. Marked c11 on the board and schematic.

Thanks in advance. This site is such a resource.

[Justin Thomas]

If the voltage rating is high enough, go for it. I wouldn't leave it as a permanent repair, though. If you're just curious about tbe sound, though...

Loud is good. And, these amps sound better loud. I know it has a Master Volume for the gain channel, but it still sounds better loud.

Justin

[g1]

Agree with Justin, use a single cap for permanent repair. However, in the meanwhile, try the different values you mentioned (and larger like 200 or 250) to see which you like best. There's nothing that says you must stick to the stock value.

P.S. we know the guy who started the whole "silver mica" thing. rofl (titu)

[musicamex]

The sm caps i have are all 500v. I might see if adding 1 or 2 in parallel make much difference. Since it isnt my amp, and too many years around loud drummers and rr in general make me an unlikey tone judge candidate. To be honest, it sounds balls on bitchin with 47 pf. I started realizing just how many different possibilities are possible with this amp. I looked up an owners manual and it slices and dices more different ways than a vegamatic. (That dates me amigos. A pre blender tv ad joke). Ill be glad to finish this amp. It has a reverb issue too.

The next two amps today also left me cussing at engineers again. One was a pignose 40. To remove the chassis, the wooden blocks nailed to the cab that held the tube guard in place had to be removed since the trannies hit the blocks. 7 bulging leaking 100mf 400v filters WOW, they must have gotten a deal!!!

The next was a roland ac60 which i own one of too and really like for limited use within its design limitations. The one for repair had a broken pcb mounted input jack. I never have opened mine up before but often replace plastic pcb mounted inputs with a metal Switchcraft jumpered to the board. I told the client that forgetting that even Roland, whoz gear I've had good luck with, employ young engineers, who's frustrated repairman fathers beat them daily as children after trying to fix stuff like this amp. And they are trying to get even with ever more complex sardine can layouts that are more dissambly puzzles than thought out assemblies. The one thing likely to break first on this amp is a plastic inpot jack, levered and wiggled into broken solder (like this one), and finally into actual mechanical brittle plastic jack failure syndrome.

So wouldnt a responsible engineer 1 mount that jack on a seperate little board? NO Or 2....make it and its solder joints easy to get to? NO Or 3......make the board easy to remove to replace the jack? NO NO NO. THIS BOARD IS ONLY EVEN MOVED ENOUGH TO DO JACK YOGA WITH THE BOARD STILL AN INCH FROM WHERE IT STARTED AFTER DISCONNECTING AND REPOSITIONING EVERYTHING BUT ONE SMALL BOARD. INCLUDING THE HEATSINK AND TRANNY! I finally got the jack pieces out only to find the Switchcraft jack wont fit and of the 20 odd plastic chassis mount jacks i have on hand none come close.. except if i rob one from an unused position on another board in the amp. Ill go through the bone pile but am not overly optimistic.

I know i sound like Rodney Dangerfield and meant this venting to be humorous, but it is true.... I am going crazy with modern stuff, and i know this stuff is macro compared with tablets and cellphones. Do you think maybe most electronics arent designed meant to be repaired? Unless you can easily swap a board? With smt components smaller than rice and thousands on a chip they would be hard to replace, even if you could trace a problem to one of them. Maybe its time to say no more stuff built after 1980. 😥 except female groupies. 😈

[Chuck H]

AFAIK cell phones and tablets are NOT meant to be repaired. At least not in the conventional sense. Also, due to some hands off construction methods and other cost efficiency measures there are many consumer electronics use to be "repairable" that no longer are. Including some guitar amps. That is, careful technique is required to do the work and particular models are considered DNR (do not repair) my the manufacturer for warranty purposes because it's cheaper to replace them than to pay a repair man to even look at them. That's just the macro story though and some such amps (and other things) are made just well enough to stick around beyond the warranty period. They can end up on repair benches much to the dismay of both customer and service tech because the repair can often be half the cost or more of a new item.

re: that capacitor... It doesn't need to be silver mica. Any film type cap will do. I think this must be the treble cap you're talking about? So it will have HV on one end and must be rated for that. I probably wouldn't use a ceramic cap but many have for the tone stack treble cap without problems.

[Jazz P Bass]

Hah!
My son's IPod needs a battery.
Guess which is the very first item installed in the device?
If you answered 'the battery', then you win the fluffy bunny.
And to top it off, there is not a screw to be found holding it together.
The unit is literally 'glued' together.

[Chuck H]

Hah!
My son's IPod needs a battery.
Guess which is the very first item installed in the device?
If you answered 'the battery', then you win the fluffy bunny.
And to top it off, there is not a screw to be found holding it together.
The unit is literally 'glued' together.

That sort of thing makes me so mad I could punch a baby.

[g1]

It gets even more ridiculous. I have seen things that are glued together with fake plastic screw heads molded into the case so it looks like it comes apart. Yep, looks real "quality"!
The funny thing was, it was something I would normally not look at (aftermarket ATV tach), but I decided to have a peek because at quick glance I thought "at least you can take it apart". Had a good laugh at that one!

[Enzo]

made just well enough to stick around beyond the warranty period.

Oh, come on, Chuck. Most products have a year warranty, some like peavey or Fender a five year warranty. You build amps, tell me, how you would design it to fail right after one year or after five years?

Anyone who does repair work knows that cheap amps, that is entry level ones, the kind that are on the do not repair lists belong to novice players and beginners, and most repairs are for broken jacks and controls. Beginners step on their cords and break the input jacks. Most guys who have $1500 amps are past that stage.

I know it is frustrating to look at a surface mount board, but we can't expect amp makers to revert to decades old technologies - like the simple to work on 1980s Peaveys - just so out of date techs like us can still feel comfortable. Imagine if the cell phone people did that, we go back to mobile phones the size of VHS cassettes. Remember those?

You can buy what I will call macro parts for cells, smart phones, and tablets. But they amount to case, screen, board, and like that

[Chuck H]

I was just trying to flesh out how some otherwise DNR products make it into repair shops. Indeed MOST items make it past the warranty period. If they didn't manufacturers would shorten it

[drewl]

I never have any problems repairing Roland Cube input jacks.

Also some items that are glued or pressed together can be popped open by applying a little heat to the seems.
Like SMPS power supplies for computers or monitors, I always end up fixing those for everyone around the office.

As far as caps, in parallel the capacitance adds, in series the value is like resistors in parallel.

[musicamex]

The ac 60 is WAY more sardine can like than a roland cube. There are quite a few of those in service down here and like most roland and edirol gear are fairly reliable.. the guy with the ac60 is a wd40 freak. I think he must even put it on his pancakes. He and others here think it is ok around electronics especially on the coast. If it were only being a dust magnet /preserver that would be bad enough but not for pots, boards, and especially not pickups. I have a box pup parts waiting to be rewound with dissolved sticky potting or varnish and shorted coils.

I think of the ac60 as an intermediate small amp. They are box store 400ish now but i paid close to 500 for mine when it came out. It is a good coffee house size amp. Easy to pack in on one shoulder and axe on the other. I try to use right angle jacks when they work because stuff on small stages gets stepped on. I actually had a drunk come up, stand on my roland vg88 and tell me he liked my slide playing. I tried to be polite. It survived. Ive been in his shoes and i did too. The vg88 made it through beer and rain storms ant lots of sweaty gigs for about 8 years and i bought it used.

[J M Fahey]

Nobody who offers a warranty designs product to fail (that's a positive action) after a certain time because it can and does backfire and becomes a reliable business killer.

It works the other way: products are designed not to fail within warranty time, any extra life it has afterwards (be it 1.5X or 100X the warranty period) is a buyer bonus.

Being a statistically distributed event, failures have a large spread and all you can predict (if you have the time and resources) is an average, with quite a few units living much longer, and same amount much less, so you try to push that average as far in the future as possible, so the short lived percentile does not bite you in the *ss .

Or to be more precise, not many.

Failures, as many other important parameters in our life, such as age, IQ, weight, possibility of heart trouble/cancer/diabetes/car crashes/being mugged/whatever (which can also be considered failures), usually follow a Gaussian curve distribution:



showing that many/most events (failures in this case) will cluster around a center value (which they call "0" ) but many smaller/larger ones exist and are normal.

In this case it would be shorter/longer life time.

This is an over simplification but to apply it to product duration and warranty servicing:
if parts quality is chosen so they, in average, fail after 1 year , Gaussian curve tells us that in fact 50% will live longer, 50% will have to be repaired or replaced for free (to the customer, of course).

A truly Business destroying situation.

To put some amp related numbers into it and to avoid having to draw a curve, let's use this borrowed one, although applying different names to what it shows, and very slightly shifting it to the left:



suppose I sold 20 amps, offered 10 months warranty and chose parts quality so average failure is 10 months also:
a) it can be seen that by 10 months, 10 units (50% total) would have failed ... and I would soon be broke .

b) Curve is so useful that it can predict that product would be very reliable up to , say, 4 months , by 8 months I already had to replace 2 (I'll survive but that will probably eat up my profit) and beyond that it's a Holocaust.

c) so in fact if I want no Warranty trouble eating my profit, I want to design my product so "4 months" a.k.a "quite reliable/no trouble" becomes "10 months" a.k.a. "factory warranty" by shifting the entire curve to the right.

d) how much?: 10/4=2.5 X
So I must aim to 10 months * 2,5= 25 months average life , way longer than offered warranty, and also meaning that a sizeable quantity will live way way longer than expected.

e) and why does not everybody just aim at, say, 10 or 20 years average failure, so practically NO amp needs servicing and you forget about that headache?
Basically because thay means that parts cost and build quality will become very expensive, a real deal killer, so reasonable design aims at allowing, say, 1 or 2 % failures within warranty period.

Which is about what experienced Techs, such as Enzo, usually find out.

When somebody complains in Forums (even here) about "low quality Behringer/Crate/Peavey/(insert favorite hate brand here) fails all the time" his usual very accurate answer is something like: "do you have any idea of how many of them are sold every Month?" and/or "yes, I repair a few ... given the huge amounts sold not significantly more or less than , say, Fender/Marshall/Ampeg/Laney or any other major brand".

f) so many Companies design so price is reasonable , they don't over design by much because it's expensive, and allow for, say, 1% replacements, no questions asked, to cover the few actual warranty claims they'll meet.

g) there are some areas where incredibly high reliability is needed and no servicing or replacement is possible during actual use, think a bomber electronic countermeasures unit dodging enemy missiles or a transmitter on a Moon or Saturn mission craft, and there they will pay 100 bucks for a Pop rivet , if they use one.

Sad to remember this, but I seem to remember that Challenger was destroyed by a stupid piece of polyurethane foam insulaton which failed (think beer cooler stuff).

[Chuck H]

Excellent post. I did understand that there must be a formula, and now I know what it is! But holy moly, I was just being snide for a second!

Permit me, for the record, to amend my above statement and simply say that there are many products that are considered DNR by the manufacturer for warranty purposes. That being the case, the designer isn't going to make any serviceability considerations. Once the warranty period is up some of these products, designed without serviceability in mind, wind up in repair shops where a repair man must take the time to determine the cost of service for this item that was never intended to be repaired, ever, and then give this info to the customer who will make a decision. A cumbersome situation with few opportunities for gratification for either party.


I'm not saying it's a bad business plan (though I will). I'm just saying that things were simpler for repair shops once. If this product strategy continues to radiate into all facets it means the certain death of repair shops, obviously. We've already seen too much of it for my liking. Soon there will only be makers, sellers, buyers and the garbage man. Not only will most of the revenue from manufacture remain in a separate economy, there will be no peripheral income for service or maintenance. The profits for this business plan all land on the executives and the revenue from manufacture stays abroad. So... Where's the market demographic and how can they afford to buy the product? This is typical short sighted planning on the part of big business that destroys economies to line the pockets of the rich. The real flaw is that it doesn't build long term strength in the company because they deplete their market like a plant depletes the soil in a pot. No where to go and when it's over, it's over.

[Jazz P Bass]

Personally, I feel that most products are 'designed' very well for there application.
(Why would an engineer 'design' something incorrectly?)

Enter the flies in the ointment.
Bean counters.
Production managers.
Solder station operators that are being harrassed to 'move more pieces through'.
Solder station operators that have not a clue what they are doing.