The fact that you have to slap you amp suggests it is not a guitar problem.
Have a look at the volume pod, could be the problem!

Hi Jeremy

My guess is that your problem is typical of 90% of electronic problems: some bad connection/contact somewhere. Good news: Nothing is blown. Bad news: Your amp needs a service from an amp tech. The amp needs to be stripped and the pcb checked for bad solder joints and oxidised/broken contacts. Contact your local techie and get a quote 8)

I hope this helps...

Contact your local techie and get a quote

Jeremy, send Karel a PM. He has done magic on some of the forum members amps!

Hi

Yeah I did pull it apart and examined the joints (have my masters in electronic eng) and didnt find any bad joints. Maybe I missed one though. Thought it might be a bad pot as well, but havent tapped the pots yet. It is my spare amp so is used very little... Was just wondering if it could be valve related... or something wierd like that...

+1 to Karel

Either bad or dry solder joint somewhere. How old is the amp?

JeremyE wrote: Hi

Yeah I did pull it apart and examined the joints (have my masters in electronic eng) and didnt find any bad joints. Maybe I missed one though. Thought it might be a bad pot as well, but haven't tapped the pots yet. It is my spare amp so is used very little... Was just wondering if it could be valve related... or something wierd like that...

If it is an amp you don't use a lot it could be dirt in one of the pots.....Pots that stay in the same position for a long period os time can develop a spot where the wiper has damaged the carbon track below by it resting in the same place on that track.

Maybe start by changing the volume and gain pots in the amp first and see what happens.

TomCat wrote: +1 to Karel

How old is the amp?

Exact age I would not know... It is old and at times has sat for very long periods of time!!! Even over a year... I know something similar happened to my creative surround sound, where the pot wore out and the volume just kept changing...so wouldn't suprise me if its the same... Only thing is it does it on both channels... clean and overdrive...

JeremyE wrote: Only thing is it does it on both channels... clean and overdrive...

Might have answered your own question, Marshalls have individual volmumes for each channel and then a master, if it does it both channels then it'd point to the master volume? ?

Symptoms do not exclude the possibility of oxidised tube socket contacts. More likely than pot problem...

nick wrote:

JeremyE wrote: Only thing is it does it on both channels... clean and overdrive...

Might have answered your own question, Marshalls have individual volmumes for each channel and then a master, if it does it both channels then it'd point to the master volume? ?

The only problem is it doesnt have a master volume.... The volume controls are independant for both channels. The issue is worse on the overdrive channel though... Tapping the pots should show whether it is one of the pots... otherwise i can play with the amp open and gently tap the insides with something plastic in order to isolate what is causing the issue...

Gearhead wrote: Symptoms do not exclude the possibility of oxidised tube socket contacts. More likely than pot problem...

This sounds promising!!! I havent checked this out...will do this eve hopefully...

Since a rise in temp causes a drop in volume, it could mean a faulty component on the board or bad connection to a component that you are not able to see.

If you still have no luck after trying all the above (very good) suggestions, I would suggest running the amp until the fault is present and then use some freeze spray to rapidly cool the components one by one. The physical changes from freezing a component (or it's solder joint) can often be the difference between making contact, or not. Hopefully your volume will be regained when you freeze the right component and then your faultfinding is done!

I have used this often to faultfind TVs and amps:
http://za.rs-online.com/web/search/searchBrowseAction.html?method=getProduct&R=4954917

JeremyE wrote:

Gearhead wrote: Symptoms do not exclude the possibility of oxidised tube socket contacts. More likely than pot problem...

This sounds promising!!! I havent checked this out...will do this eve hopefully...

Promising? You must be an optimist!

Good news...opened the amp up and waited for it to start up its issues...then gently tapped/pushed/prodded and eventually found a resistor that when pushed caused the volume to come up again. So it looks like there's a bad joint on the resistor... so I am now sorted out. Thanks for all the suggestion guys. Really appreciated it!!

Absolute Pleasure....glad you came right..... 8) 8)