Rumbling Marshall 5210 combo, HELP !!!

[Chris C]

I will find a replacement for the IC1 and replace the chip on the board. You may be correct here, and THANKS for the alternative circuit diag. The one I have is for the 5210 not the 5212 i believe.
IC1....It's an 8 pin chip, TLO72 CP looks like. Might have to do some digging to get my hands on one

[Chris C]

FIXED

For noise the same in both channels and affected only by the master volume, IC1 is the prime suspect.


There are more than one version of this amp, and some part numbers are different. Here is an alternative, make sure you are using the right drawing.

http://www.webphix.com/schematic%20h...s/50w_5210.pdf

THANKS. Replaced IC1, Amp sounds better than it did before (even better that it doesn't sound like a wind tunnel. I bought the IC1 semiconductor here from Jameco (they're great)
Link:
https://www.jameco.com/webapp/wcs/st...10001_33195_-1

[Chris C]

Now I spoke too soon Still makes the noise......it just takes a bit longer to start now. I'm guessing a transistor may be overheating.

[drewl]

I got one on the bench now with at least 4 or 5 bad pots.
It seems the pot shafts are stressed a bit as the board doesn't line up too well with the holes/mounts.

It's probably a bad/noisy amp stage, but you might want to check the pots anyway.

Also in regards to C14 and C18, on the schematic there's a note that from serial # U29600, they are removed and a .22uf cap added between the boost channel volume pot wiper and the collector of TR6 where C14 was mounted.
Looks like a mod to reduce channel bleed.

At least on these amps you can run both channels together, without the footswitch plugged in, unlike the tube versions.

[52 Bill]

I got one on the bench now with at least 4 or 5 bad pots.
It seems the pot shafts are stressed a bit as the board doesn't line up too well with the holes/mounts.

I don't know if it's possible on that amp, but the solution for that is to reheat the solder connections on each pot after everything has been mounted, to relieve the tension.

[cgoepel]

Problem Fixed!!! - I am a retired electrical engineer with lots of troubleshooting experience, an oscilloscope, and lots of time. The problem with my 5210 was a noisey C12, a 2.2 microfarad 35 volt tantalum capacitor. This effort took alot of time of locate the source of the noisey cap. C12 couples the preamp and clean channel tone circuit to the master volume circuit. I tried new op amps, removed the JFETs, one at a time, and made a slow and concerted effort to isolate the noise source. There is no textbook explanation of why that capacitor is generating noise. It's not broken or burnt and it reads fine with an ohmmeter. Only by substituting a known good capacitor in place of C12 did I find out where the noise was coming from. Here's the next dilemma - This amp uses 8 of these caps (2.2 uF@35volts, orange epoxy coating). I don't like to shotgun a repair and replace all of these for no reason, but I'm wondering when the next one will go up and start generating noise just like it did in C12 position. Hmmmm. Thanks to everyone that posted here. I was able to get the schematic at drtube.com and look for parts at Jameco. Just to be sure, I reinstalled C12 and the noise came back. I replaced it with a 10uF electrolytic that I had laying around which seemed to work just fine. Good luck on trying to troubleshoot this noise issue. I got lucky.

[Steve Conner]

Well, tantalum bead caps used to have a bad reputation for reliability. Maybe you have a bad batch.

Another possibility: In some solid-state circuits, the DC voltage across the coupling capacitor is nominally zero, but because of production spread in offsets, it can take on a small value either positive or negative. Non-polar electrolytics should really be used, but many designers don't bother. The negative voltage could potentially cause reliability issues.

I had a circuit where I put a tantalum in backwards, and it survived -1.5V for several years, but eventually failed short.

[moegreen77]

I've had the rumbling issue too, 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. And just putting my finger near it causes an increase in hum, let alone tapping it. Btw, my amp is, I guess, the older version - the hand drawn schematic, op-amps are all 1458's and it does have C14.
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?

[gbono]

Also posted at ssguitar.com

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.

[Jazz P Bass]

Touching parts of a high gain circuit (& the resulting hum) does not really help, troubleshooting wise.
That's what they do.

[moegreen77]

Touching parts of a high gain circuit (& the resulting hum) does not really help, troubleshooting wise.
That's what they do.

On the contrary, that's exact how I solved the rumbling/crackling problem in my Marshall 5210. Inspired by forums like this, I was just snooping around the circuit board, looking for "trouble". Now the microphonics with the new capacitor doesn't seem to be causing any practical problems - the amp now seems fully functional - it's back together and being used. I was just curious about the nature, potential causes and possible remedies of a microphonic capacitor in the tone circuit of a solid state guitar amp.

gbono, thanks for your reply, but I'm afraid I don't know enough about electronics to take much meaning from it.

[Jazz P Bass]

My point was not that 'touching' does not assist.

I was mearly stating that certain parts Will make noise.
Like getting in the electric field of a cap.

As long as that is understood, tracking down a touched part that makes weird stuff happen is what we do.

I had a Fender 88K PI resistor that the body was crap.
It arced to my probe as I was testing a heater pin.
Zap.

[J M Fahey]

If that capacitor is in a low signal level (meaning that signal will be amplified a lot) high impedance (meaning that inducing hum voltage there is very easy) point, touching it or even putting a finger close to it means we *will* hear hum .... and thatīs normal.
Thatīs the way itīs supposed to be.
So it tells us nothing new, and definitely does not mean that part is "bad".
Thatīs what JPB meant.

Now if the part cracks in 2, falls fropm the PCB, etc. , well, that means something.

Rather than a finger, you should have touched it with something insulated , such as a Bic pen or something.

And then, yes, you could check whether itīs way too microphonic, connection is poor/cracked, even that some leg has internally cracked (it happens).

[Gibbo_335]

Hi Guys,
new to the forum, G'day to all!

This is the first time I have seen anything on the Marshall 5212's.
I have a quickish question, my 5212 all of a sudden developed an issues, when I plug a guitar in the volume is very low and cannot be adjusted with any controls. If you plug into the effects return there is volume which, as expected cannot be changed but, I think, it shows the amp is working but I have an issue before the effects loop. I am a newbie to amp repairs but there is no "Amp Doctor" in Alice Springs.
Also the schematics links at the top no longer work
Cheers mates