Marshall 5210 - Microphonic Capacitor

[moegreen77]

I've had the rumbling issue that has been addressed in other threads, and replacing C13 seems to have solved it. C13 got my attention because it was very microphonic. The thing is, it's STILL very microphonic with the new cap installed. And just putting my finger near it causes an increase in hum, let alone tapping it.
Btw, my amp is the older version - the hand drawn schematic, op-amps are all 1458's and it does have C14.
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/marshallamps/50w_5210.pdf

So, I'm touching up any solder joints that seem remotely relevant, and maybe I'll try replacing the transistors at each end of C13, and maybe go ahead and replace C12 fwiw.
Anyway, any thoughts about the microphonics?

[Roly]

Much rarer in solid-state than with valves.  Preamp valves "jingle" even when they are good.

Low value Hi-K disc-ceramics used in tonestacks are often a bit microphonic, but it has never been a problem (but don't use them for microphone coupling).  Microphonic electrolytics are rarer but do turn up.  Even had a 6CA7 output tube so microphonic the combo started howling as soon as it warmed up.   

Then there are the mechanical things, dodgy connections in valve sockets, pots, failed solder joints, PCB cracks.

Seems to me that what your C13 really needs is grounded screening, a little ducktape and tinfoil hat.  :tu:

[gbono]

As mentioned high K capacitor dielectrics are microphonic. Capacitor temperature coefficients to avoid are Y5V, X5R, etc. These high K dielectrics will also drop their capacitance value when bias is applied. These types of dielectrics can cause a capacitor to loose 50 to 70% of its original unbiased capacitance value depending on the DC voltage applied and when you add the change with temperature you will be surprised at the total "retained" value of capacitance in the actual circuit.. Try a film part in your application if it will fit.

[J M Fahey]

Surround it with Silastic or equivalent.
Imagine yourself decorating a cake and covering a cherry with some whipped cream.
It won't really cure it, but will damp resonances.

[Roly]

These types of dielectrics can cause a capacitor to loose 50 to 70% of its original unbiased capacitance value depending on the DC voltage applied

Most Vari-Caps aren't a good as this.   

[Ja-son1024]

I have a marshall 5210 amplifier. The reverb does not work on it and I've traced all of the wires to the reverb box and all looks firmly connected. Aside from that issue I also noticed that one of the ceramic disk capacitors located at c33 (labelled "10n") was not fully connected to the circuit board. Could this be the issue resulting in no reverb? Also, what are the specs for this capacitor so I can order a new one to replace it?

[Roly]

Welcome!

I also noticed that one of the ceramic disk capacitors located at c33 (labelled "10n") was not fully connected to the circuit board. Could this be the issue resulting in no reverb?

Certainly - well spotted.  :tu:

On the "'orrible print" circuit C33 (a.k.a. C41 on the nice print) is the input signal coupling for the reverb driver amp, so if this became disconnected then it would most certainly stop the reverb working by preventing the signal from getting there (but you should still get a clang! if you slosh the reverb tank by rocking the case with the Reverb control turned up; it's still listening to the tank, it just isn't being driven with anything).

what are the specs for this capacitor so I can order a new one to replace it?

Not critical.  Just about any 10nF "ten nano-farad" (= 0.01uF "point oh-one micro-farad") cap, say poly or greencap, will suit, just avoid ceramic (for the reasons given above).  Even the value isn't particularly critical, +/-20% will work fine, and most are rated for 100 volts and this one has only 15 volts on it max.  Just about any electronics supplier worthy of the name should be able to fix you up for only a few cents.

HTH

[Ja-son1024]

ank you Roly! so I can put a 50 or 100v 10uf cap in that spot? Also, I'd like to mention that I was tinkering with the amp today and was trying to reuse the existing capacitor and just resolder it. The job was clean and went well but when I plugged amp in and started playing the volume was low low low. Even after cranking both master and regular volumes up all the way the signal is weak. Is it possible I shorted something out in the process of attempting to repair that capacitor located at c33?

[DrGonz78]

ank you Roly! so I can put a 50 or 100v 10uf cap in that spot?

It's nano-farad rating is equal to 0.01uf and a 10uf is way way higher. 50-100v is fine but not 10uf. 0.01uf is what your looking to replace. Perhaps 10uf would work but it is not the specified farad rating. best regards.

[Roly]

so I can put a 50 or 100v 10uf cap in that spot?

The original was probably rated at 100 volts, it's about as low as they come.

As the Doc pointed out it's 10 nano-farad not 10 micro-farad.  It could get a bit confusing because a 10uF/100V non-polar cap would be physically HUGE, but it would most likely still work, you'd just get a hell of a lot of bass into the reverb line which would sound pretty strange.

You shouldn't have any trouble getting a 10nF cap, and anything +/-20% in value and more than 15-20 volts will do, just avoid a ceramic type (which would work okay, just not the best type for the job).


I'm afraid I have no idea why the amp is now weak.  You were working in the "sidechain", not the main signal path, so you are going to have to go back and take a careful look at what you've done, perhaps a wire broke off while you were handling the board, or you forgot to put back an internal connector you removed.

It's kinda odd that a cap was broken off in the first place and suggests something like a previous owner has been messing about in there, and perhaps you have disturbed some other broken component, hard to say at a distance.  Just go back carefully over what you've done - "your eyes are your best service instrument".

You can try plugging your guitar into the Main Amp In socket and confirming that the power amp is working okay (or not) and that the problem is in the preamp (or not).

Try plugging a known good lead into Pre Out and Main In and see if that clears the problem as these sockets can sometimes give trouble.

[Ja-son1024]

Hello again Roly! Thank you again for your speedy response. I figured out why my volume was weak and repaired the issue. It ended up being the treble pot on the gain channel had a weak solder joint and was moving freely on the board. Nothing that a little touch up solder couldn't repair. Anyways, I put the cap in that you advised and still no reverb. I checked of the wires to see if the connections were flimsy and all looks intact and firmly. I'm still going to mess with this amp until I hopefully discover what's prohibiting the reverb channel not to work as soon as I have free time.

[Roly]

Ah, real good so far :dbtu: , but a bit of a sideplay, incidental damage, tho it could be another clue to rough handling, single big fall, who knows with older gear?

Conceptually
I'd start by confirming output from some point, such as where the dry and wet reverb return mix, and working back along that signal chain towards the reverb return, and the first major milestone is to hear the reverb line crash when you rock the case.  If you have to you then work along the drive pathway.

In my currently limited state I'll leave it to others to detail tests, and putting a cap in series with an audio source to use it as an injector probe for signal tracing.


---{future form element}---

[b ]What have you got in the way of test gear?[/b]
-Multimeter/DMM?
-Oscilloscope?
-Signal source
-Audio Signal Generator?
-MP3 Player
-CD Player
-Laptop/Sound card?
-Cassette player?
-Signal Tracer
-Powered computer speaker?
-Dummy Load?
-other

-