Uh oh, something is wrong...Need Help! - Amp Problems...

[7 Dying Trees]

OK, here goes: Yesterday I finally rescued my amp from the studio where it had been gathering dust for a couple of months, got it home, plugged my rig in and everything was cool, it sounded good, and I was happy.

Then this morning I couldn't resist the temptation to have a quickie on the guitar before heading of to 9-5 drudgery, only to discover that something was very very wrong...

Now my amp will only produce a low volume nast rasping distorted version of the input, and occasionally will blast back up to normal volume.

Right, so you can maybe help me figure out what is wrong I'll give you a rundown on what my signal path is:
Basically I run my guitar into my FX+triaxis and then out of the triaxis straight into the effects loop return of the amp, which is a marshall super 100w lead head, early 70's (like before they put the modern day switches on them). The effects loop was built in about 3 years ago, and hasn't been used since 2 years ago. It is simple send/return with a level control pot, which has snapped of (the pot is effectively set to a fixed level now). The effects loop has a bypass switch.

OK, before I had to rush oout this morning I tried two things:
->I plugged the amp through another cab, to make sure it wasn't a doodgy connection in that, but this yielded the same result.
->Set the effects loop to bypass and just plugged the guitar straight into the amp, which also yielded the same result.

I couldn't try anything else, and am probably going to check the speaker cable when I get home, but any other ideas I can try out would be welcome!

I also have a sneaky suspicion it's a tube that's gone, so I would also like to know if anyone knows where the FX loop is situated inside the amp. I am almost 100% sure it's between the pre and power amp. I am quite worried about the effects loop, as I am starting to think that that may have caused whatever is wrong, as this is the first time I have used this for years... The other thing I can think of is some dodgy connection inside.

I have had weird stuff like the power on/off switch catching fire happen before as well... Oh, and some idiot I lent it out to accidently blew all the transformers/tubes etc by connecting a dodgy load to it. I had it fixed and all the transformers rewound, so maybe it's something to do with that? As you can see I don't really know where to start looking...

Anyway, Suggestions please!

[bluenote]

first off, run your amp without anything in the effects loop.....just guitar, amp and speaker.....

second, effects loops ARE between the power amp and preamp

third, it sounds like the the tube for the effects loop has gone....try replacing it first before you go on and change everything else...if a tube change isnt the fix, then you may have to get your amp checked by a pro

[cerealk]

Sounds like a tube malfunctioning (at a guess).

I know this excellent amp tech in Chandler guitars in Kew. *He fixed my 1968 Laney 100 W head and has just serviced my Marshall 9200 that I am selling.

If I were you I would take it down to him. *I have always asked him to test it and let me know an estimate before proceeding. *He did however check my Laney while I waited and gave me a good idea of what needed to be done.

He REALLY knows his stuff.

BTW I am heading down there on Saturday morning (28th)

Cheers

Simon

[7 Dying Trees]

Basically the effects loop is a mod that's been put in, the guy that did it was very competent, basically it got sent from belgium->holland so this guy could work on it. but yeah, it does sound like valves which is a pain in the arse...

I have tried the guitar amp without the effects loop, was one of the first things, but still the same problem. So not fair! Grrrrrr! I haven't used this amp for ages...

Anyway, will try out some stuff this evening to see if I can deduce anything else...

[bluenote]

oh, i thought you just used some sort of bypass switch when you tested it without the loop.....if its the effects loop buffer tube, its no big deal....start with that tube and switch it out and see if it still makes the sound.....

if you dont have a tube driven effects loop( and it sounds like you may not) change all the tubes....then take it to a tech if it still has the problem......you can at least see if its a tube problem quickly that way, without bothering to bias.....if it is the tubes, then go back and bias the power tubes......if it isnt the tubes, then take it in to a tech

[7 Dying Trees]

Just when I was staring into a void of dispair yesterday, the amp suddenly kicked back in! Don't ask me why, but am going to open it up at the weekend to see if there are any connections which look dodgy.

Am still worried about it, but untill the problem resurfaces I am not sure what to do...

[Vaibanez]

Ummmm....2 years worth of sitting and not being used should be your first clue.

Dust, dirt and the ocasional fly dropping will reek havok on electronics.

My guess would be that a piece of fuzzy got lodge in an undesireable place and caused the problem. I'd take a can of compressed air and blow out the chassis. Then start looking for smoked tubes. It probably wouldn't be a bad idea to swap the tubes out anyway, being as they have sat for 2 years. I wouldn't trust them for long.

Good luck mate!

J>