Marshall capacitors

[ukcol]

I have a small Marshall amplifier that I use for backing tracks. It has had a problem recently. When switched on from cold it would be very low gain (quiet) but without any distortion. Turning up the gain and volume controls would increase the volume slightly until a point was reached where the signal "punched" through the fault and the controls would have to be turned down again to get a normal level. The amplifier would then work fine for the rest of the session. The fault would only happen about (say) one in 10 times when first switched on after at least a days rest.

I have had problems with some polystyrene capacitors made by Suflex (and so have many others) where a similar problem has occurred due to bad connections between the capacitor and its lead-out wires.

I have changed some capacitors in the amplifier (see picture) which are of a different type and higher in value than the polystyrene types mentioned (0.022uF, 0.1uF and 0.22uF) in the hope that one of them may be the culprit.

Does anyone have any experience of failures of this type of capacitor as in the picture below? There are markings on them but too small me to read; fortunately I have a diagram and the component numbers are printed on the PCB.

The amplifier is working fine at the moment but due to the nature of the fault I will not know whether or not it is cured for some time.

[Omegaman]

I've never had problems with these caps in the Marshall Valvestates, but a common cause of the problem you describe is the effects loop jack switching contacts being dirty and the effects loop mix pot also dirty/intermittent. In fact all of the pots can be 'iffy' on these! Headphone jack (if fitted) also can cause problems.
Hope this helps

[ukcol]

Thanks Howard, I will take a look at the jacks.

I had already replaced the pots, when I first had it, as they were all pretty poor. I had also replaced the electrolytics at the same time for good measure.

[Diabolical Artificer]

Are there any zeners or diodes in current sources etc that and and iffy valve bases are worth a look.

Andy.

[ukcol]

Hi Andy,

Circuit attached to post No. 1

[joebog1]

I would suggest Omegaman has the answer!!! A close look at the jacks will show them for what they are. Cheap tin plated steel contacts. I clean the with a piece of 2000 grade emery paper ( from car shops, its called colour paper) that I fold over so I can clean top and bottom contact at once. I then use contact lubricant to stop the now exposed steel rusting.
That hint by the way took me a few years to fathom, its not something I "normally" checked. Now its the first thing I check.

Joe

[ukcol]

Hi Joe,

Yes, there is a very good chance that Howard is right. The amplifier has faulted again already so it was definitely not those capacitors.

[joebog1]

I have NEVER changed one of those caps !!! They are in another thread from an old timer.
and absolutely reliable. There are a couple of things that really "bug" me about your circuit, that add to Howards, and my comments.
A TDA 2030A is not a "huge" amp, but running THAT MUCH current through a set of contacts designed over 100 years ago to work on 600 ohm telephone lines, IS NOT suitable for speaker current applications. Speakons are the modern idea, and I use them.
When guitar amps, as such, were "invented" whatever was easily available was used!!
It makes sense from repair/service point of view, as much as it does for cost. Phone jacks just seemed to appear for almost every signal connection. At that time we didnt have multi function pedals and stomp boxes so to connect your guitar to the amp, a phone jack and phone plugs worked fine!!
BUT!!!! If you look at early guitar amps they were like 5 watts and built as combo's, the earliest Fender models I have worked on were in steel cases !!!!. SO the "trick" in reading this, is that there wasnt an external speaker, effects loop, or headphone switching of the power amp.
I dont know if you can buy one, but another trick I use for cleaning guitar amp jacks, especially those that are used regularly and need low noise characteristsics, is a brass pull through .22 caliber gun cleaner brush. This doesnt clean the contacts, but it does clean the ground or earth connection. That lead that gets dunked in spilled beer when setting up/ pulling down the equipment, not to mention you sweating all over the guitar and in some guitars the output jack, PLUS your greasy fingers on the jack as you plug in/plug out and perhaps change instruments.
Tis a roadies job !!!!! As well as mixer and engineer, I found the best roadie was the one who paid for the equipment, ( I started off building LOTS of gear before I even left school)
CLEAN gear equals CLEAN sound, and the above out of control ramble is what I would call "normal maintainance" for a working muso!!! After all, you change strings regularly, dont you ??. WHY not at the same time clean the cables and jacks?
OHH, AND neck? A nailbrush works superb

Joe

[Dillen_de]

I am not familiar with Marshall amplifiers but I've seen many other cases where oxidation on relay contacts, typically at the speaker outputs, caused a low signal until the volume was increased enough to "burn through" the oxidation.

Martin

[ukcol]

Hello Martin and thank you for your help.

It looks like the collective wisdom of all you chaps is pointing towards a faulty jack and specifically the headphone jack. The input jack won't be responsible because turning up the volume has no implications for the signal at the input. It won't be the line output jack because the signal through the amplifier does not pass through the contacts of this jack. This just leave the headphone jack the contacts of which feed a normal low impedance loudspeaker.

I shall temporarily bypass each contact in the headphone jack and leave it for a while to check that it has solved the problem. If as expected it does cure the fault I shall replace the jack rather than just clean it and risk a repeat failure.

Thank you again gentlemen, I shall keep you informed of my progress.

[ukcol]

I plan to use this amplifier tonight after it not having been used a few days. I thought I would measure the contact resistances in the headphone jack before I linked them out. As you will see from the circuit attached to post #1 there are 3 contacts in the jack. Two are wired in parallel and feed one side of the speaker and the remaining contact is in series with the other side of the speaker. This last mentioned contact measured about 20 ohms whilst the other 2 were too low a resistance to resolve on my meter.

So it is definitely the headphone jack playing up. I have now fitted the links until I get a new jack.

Thank you for your help gentlemen.

[joebog1]

If it is your amp, and you use it regularly, I would suggest you change the output setup completely. By that I mean DO FIT a speakon socket. Without looking up the specs, from memory they are rated 50 volts @ 15 amps. Plus they have wiping contacts, so everytime you connect/disconnect they clean the contacts.

Its obviously a "modern" Marshall so it wont damage resale value, in fact I would suggest it would increase the value.
Im glad you got it sorted

Joe